Effects of non-inversion tillage for different farming systems on soil structure,water retentionand nitrogen mineralization in the Netherlands

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1 Effects of non-inversion tillage for different farming systems on soil structure,water retentionand nitrogen mineralization in the Netherlands Hollemans, T.J. Organic Farming Systems Group Wageningen University Europees Landbouwfonds voor Plattelandsontwikkeling: Europa investeert in zijn platteland. Het Ministerie van Economische Zaken, Landbouw en Innovatie ( EL&I) is eindverantwoordelijk voor POP2 in Nederland. 1

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3 Effects of non-inversion tillage for different farming systems on soil structure, water retention and nitrogen mineralization in the Netherlands Tom Hollemans MSc Thesis Biological Farming Systems (BFS-80436) Scientific supervisor: Dr.ir. JohannesScholberg (Organic Farming Systems Group) Practical supervisor: Dr.ir. Bert Vermeulen (Plant Research International, Wageningen) Examiner: Dr.ir. Walter Rossing (Organic Farming Systems Group) Organic Farming Systems Group Department of Plant Sciences Wageningen University Droevendaalsesteeg PB Wageningen The Netherlands January 2012 iii

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5 Table of Contents Preface...vii Executive summary...ix 1. Introduction...1 Tillage management in a historic context...1 Outline of prevailing conservation tillage practices...1 Key advantages of conservation tillage...2 Disadvantages of conservation tillage...2 Considerations for adaption of conservation tillage in the Netherlands...2 A) Transition period: From ploughing to conservation tillage...3 B) Suitability of CsT for conventional vs. organic farming systems...3 C) Crop rotations in the Netherlands...3 D) PedoClimatic condition in the Netherlands...4 E) Effect of NIT on soil structure...4 F) Soil moisture dynamic under NIT...4 G) Nutrient-cycling...5 Farmers perception regarding conservation tillage...5 Research scope...5 Research question with expected outcomes...5 Purpose and program structure...6 Thesis structure Materials and methods Experimental design Cultural practices Weather conditions Soil physics measurements and calculations Soil texture Soil pits Penetration resistance (PR) Volumetric and gravimetric soil-water-air characteristics Saturated water-permeability (Ks) Soil aggregate stability N dynamics Statistical analysis Results and discussion Observations in soil pits A. Brooijmans A. Van Hootegem C. van Tiggelen C. Steendijk H. Westers Overall tillage effects on rooting depth and earthworm-activity over time Soil Structure Organic matter Dry bulk-density (P b d )...33 v

6 3.2.3 Macro-porosity (MP) Penetration resistance (PR) Soil water Saturated water conductivity (Ks) Water retention: gravimetric water content at field capacity Aggregate stability Differences between farms Tillage systems and soil-texture Interaction between ΔPw pf2 and ΔP b d Results and discussion mineralization incubation study Primary tillage: INV vs. NIT Secondary tillage: Weeding-effects N mineralization in literature Mineralization rates Temperature-effect on nitrogen mineralization Summary of findings Visual observations on soil structure, worms and roots Physical determination of the soil structure Aggregates stability of NIT vs INV soils The effect of primary and secondary tillage on N mineralization Synthesis Recommendations...55 References...56 Appendix I. Measurement locations and field drafts...60 Appendix II. General information and layout of the mineral incubation study...70 Appendix III. Daily weather data Appendix IV. Output of the statistical analysis...83 vi

7 Preface This thesis report was commissioned by DLV Plant to examine the effect of non-inversion tillage on soil structure. This thesis was part of Demonstratieproject Niet Kerende Grondbewerking and Praktijknetwerk Niet Kerende Grondbewerking. First measurements were part of the Demonstratieproject and final was part of the Praktijknetwerk.Measurements were performed on five Dutch farms applying a non-inversion tillage systemand this report describes the development of soil structure between 2009 and With this study the suitability and effects of non-inversion tillage in the Netherlands were being investigated. I very much appreciated working on this research topic, especially due to the fact that it allowed me to implement scientific research in an applied context by conducting on-farm research. Besides, noninversion tillage is a very current topic and is in my opinion one of the key pillars of more sustainable farming systems. Furthermore I gained a lot of experience and knowledge on implementing field monitoring studies, field and lab procedures, data analysis, and report writing. The report in front of you is the final result of my study to the effect of non-inversion tillage and I would like to thank Johannes Scholberg, Bert Vermeulen, Ben Verwijs and Sander Bernearts for the newly gained knowledge and experiences and the opportunity to work on this research. Tom Hollemans vii

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9 Executive summary Conservation agriculture (CA), including the use of non-inversion tillage (NIT) and green manure, is becoming more important in today s agriculture in Western Europe. Root crops are assumed to be less suitable in combination with the use of NIT, although they are often included in Dutch crop rotations. The aim of this research was to determine the effects of NIT on soil structure, soil-life and the soil physical properties. These include soil organic matter (SOM), dry bulk-density (P b d ), penetration resistance (PR), water retention (Pw pf2 ), macro-porosity (MP), saturated waterconductivity (Ks) and aggregate stability. Furthermore, the effect of primary and secondary tillage on nitrogen (N) mineralization was also assessed in a N mineralization incubation study. Measurements were performed on five Dutch farms, from which three are organically managed,while the other two are conventional farms. The latest soil inversion on these farms occurredbetween 2005 and 2007, while the measurements were carried out in 2009 and This study was a follow-up of a previous study (Vermeulen, 2009), with the report in front of you thus focussing on the conversion period. Soil types are varying from sandy loam with 9% lutum to silt loam with 25% lutum. Soil-columns for the incubation study were taken in the sandy soil on the Droevendaal farm, Wageningen, the Netherlands. Soil structure, root penetration and depth of earthworm-activity were assessed by soil pits. Field samples were collected to measure SOM, P b d, Pw pf2 and MP. Resistance-curves till a depth of 50 cm were obtained by use of a penetrologger. Aggregate stability was measured in terms of the stability of 1-2 mm aggregates using a wet sieving method, with samples being collected on three NIT farms and neighbouring farms applying conventional tillage. The effect of tillage on N mineralization was investigated by an incubation study with soil-columns taken from fields under NIT and inversiontillage (INV), on which different weeding regimes were tested. Results showed that non-inversion tillage combined with sub-soiling resulted in a well-structured soil profile, with porous soil diagonally above the subsoil-tracks and root penetration till deeper layers. The use of NIT increased rooting depth between 2009 and 2011, though no increase in the depth of earthworm-activity was observed during the visual observations in the soil pits. In fine-textured soils rooting seemed to be depended on vertically oriented cracks, though minimal rooting was also found within soil peds. In 2011 no compaction layers were observed in the soil pits and the absence of distinct resistance-peaks confirmed these observations.well-structured soil profiles werecommonly found, except for Steendijk s farm, on which soil had smeared spots, as a result of sub-soiling under (too) wet conditions.the water retention at field capacity increased at cm depth, by which the plant available water content also (AWC) increased, making NIT more suitable under dry weather conditions. In the top five centimetre of the soil profile SOMtended to increase more compared tothe cm depth layer. Samples taken to determine P b d, Pw pf2, MP and Ks on the loamy soils were mainly shrunken due to the very dry period in spring At times this interfered with measurements and resulted in a higher soil particle density and a corresponding increase in P b d. This increase was the reason for the decreased Pw pf2 and initially negative MP values found. Whereas pore-volume in the soil is required to conduct water, also Ks was reduced in the shrunken samples. Although soil aggregate stability at 0-5 and cm depth was not enhanced by the use of NIT compared to INV across all farms, on Westers farm the aggregate stability was greater in the top five centimetre compared to anadjacent conventional tilled field. An increase in aggregate stability with NIT was not observed, this may because secondary tillage, crop rotation and farm-management between both systems differed and all these factors also influence the stability of the 1-2 mm aggregates. ix

10 Based on the N mineralization incubation study it appears that NIT systems had higher N mineralization in the top fifteen centimetre. Furthermore, the N mineralization-rate tended to increase at an increased frequency of weeding, though this effect was not significantfor both NIT and conventionally systems.a higher N mineralization was expected in the conventionally tilled system, because soil sampled from the NIT-system was not inverted for only one year and an initial setback in N min was expected. However, the reverse occurred, which wasexplainedby the sampling depth of 15 cm and the accumulation of soil amendments near the soil surface with NIT. Although with conventional tillage N mineralization is expected to be higher below this depth, which offsets higher rates near the soil surface with NIT, total mineral nitrogen (N min ) thus may not be higher under NIT. Based on the resultsof the current study it can be stated that NIT can work with root-crop based cropping systems in the Netherlands and that its use can result in a well-structured and deep rooted soil profile.however, thesuitability of a NIT-system depends on a combination of factors including farming type, soil type, crop rotation and weather circumstances during soil cultivation and thus may differamong farms and years. Field cultivation with the subsoiler under slightly moist field conditions is required to successfully apply NIT and root crops and water infiltration can hamper its applicability. At the beginning of the transition period a deeper working depth for sub-soilingis recommended to loosen the compaction layer. This depth may be reduced after the application of NIT for several years, if the compaction layer and impeding layers have disappeared. Keywords:Conservation agriculture, non-inversion tillage, compaction layer, soil physical properties,soil structure,aggregate stability, weeding, N mineralization, Netherlands x

11 1.Introduction Tillage management in a historic context Tillage is an essential aspect of farm management and has become a standard practice in agricultural production systems. Conventional primary tillage operations aim to alleviate soil compaction and to incorporate plant residues, organic manures and weed seeds to deeper soil layers (Vakali et al., 2011; Van der Weide et al., 2008) and this typically entails mouldboard or disc ploughing (Schjonning and Rasmussen, 2000). Furthermore, primary tillage is used to improve soil workability in spring (Schjonning and Rasmussen, 2000). In addition to primary tillage, secondary tillage is practiced for weed removal and to enhance seed germinationand seedling development by creating a suitable seedbed for root growth (Cannell, 1985). Conventional tillage (CT) is perceived to be less sustainable compared to conservation tillage (CsT)since it greatly perturbs the soil. This is caused by the use of (mouldboard) ploughing, which results in soil inversion to a depth of approximately 25 cm, with the soil surface remaining bare and unprotected(table 1). Nowadays, one of the biggest problems of agriculture associated with CT is increased sensitivity to erosion and compaction (D Haene et al., 2008), which is threatening 157 million hectares in Europe (Garcia-Torres and Martinez-Vilela, 2000). Although hectare in the hilly area with loess soils in South-Limburg are being impacted by (the negative effects of) watererosion (Kwaad en van Mullingen, 1991), in the Netherlands erosion is not considered to be a serious problem. However, soils in the Netherlands are prone to compaction by natural internal slaking and mechanical loading by wheel traffic due to high water tables (Cannell, 1985). Outline of prevailing conservation tillage practices An integral part of conservation agriculture (CA) often iscst (Van der Weide, 2008) and becomes increasingly important in farm and soil management at different levels (D Haene et al., 2008) (Table 1). A requirement for CsT is that soil coverage by plant residues is at least 30% (Casa and Lo Cascio, 2008) (Table 1) and that NIT is being practiced. Soil degradation, erosion and water contamination are expected to be greatly reduced through use of NIT techniques (Holland, 2004). Table 1:Different tillage practices and related soil operations. Expected results, possible synonyms and any special variants of the different practices are also being listed (Van der Weide et al., 2008). MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 1

12 In CsT either reduced tillage (RT) and no tillage (NT) practices may be used (Holland, 2004; Van der Weide et al., 2008). With RT or minimum tillage (MT) techniques a superficial loosening of the topsoil (0-10 cm) is used, while NT does not include any soil operations at all. Synonyms of NT, direct drill (DD) and zero tillage (ZT), are also commonly used (Table 1). Crop requirements and inherent soil structure determine the success of DD, which is not suitable for root crops (Cannell, 1985). Above mentioned terms of the various tillage-types are often incorrectly used in literature. Key advantages of conservation tillage Most advantages of CsT are indirect in terms of enhancing production potential (e.g. soil quality) and/or only become more articulated after several years, while the conversion to CsT requires additional investments in terms of learning, management and/or equipment. However, a direct advantage may be the reduction in fuel consumption, because INV is not practiced anymore. Moreover, lower energy and labour requirements (Hoogmoed, 2010) combined with reduced maintenance of machines may result in an appreciable reduction in overall energy use (Van der Weide et al., 2008; Berner et al., 2010). Furthermore,CsT tends to be more environmental friendly since soil erosion is being reduced, due to improved aggregate stability and common use of cover crops (Stagnari et al., 2009; D Haene et al., 2008).Another advantage is that runoff-induced loading rates of nutrients, sediments and pesticides to surface water decreases between 15% and 89% (Askegaard et al., 2011; Holland, 2003), thereby improving surface water quality (Van der Weide et al., 2008; Stagnari et al., 2009; Smolen).Moreover, risks for sealing and/or crusting are being reducedand the carrying capacity and soil accessibility under wet conditions is increased by an improved drainage (by earthworms) and water retention (Stagnari et al., 2009; Krauss et al., 2010; Hoogmoed, 2010; Schjonning and Rasmussen, 2000). Finally, NIT also increases carbon sequestration, although benefits are mainly reported for the topsoil (Berner et al., 2010; Berner et al., 2008; Govaerts et al., 2010; Peigné et al., 2007; Stockfisch et al., 1999; Holland, 2004; Lal and Kimble, 1997). According to Baker et al. (2007) a review of multi-year experiments, with tillage as variable, showed a positive effect of carbon sequestration withnitbeing practiced. However, this review only presented data of studies of the upper most 30 cm. Deeper sampling mostly resulted in increased total carbon-sequestration with INV, which findings are supported by Emmerling (2007). Disadvantages of conservation tillage Despite its potential benefits in terms of reduced energy use and soil conservation, NIT also hassome (initial) disadvantages. Subsoil P b d and soil-aeration respectively in- and decreases (Schjonning and Rasmussen, 2000). Moreover, adecline in N mineralization and related N-supply may reduce crop N- uptake, especially during the initial five years of practicing NIT (Peigné et al., 2007; Vakali et al., 2011). According to Koepke (2003) soil tillage may be required to increase N-availability by improving microbial activity, rooting-density and nutrient absorption. However, lower N-application may be sufficient once a new soil equilibrium has been reached and occurs around 20 years after the last inversion-tillage (Van der Weide, 2008). Next to lower aeration and N-supply it was reported by Van der Weide et al. (2008) that hydraulic conductivity (Ks) decreases and penetration resistance (PR) increases in DD systems. Moreover, seedbed preparation was hindered due to plant residues being left on the topsoil, which hampers the use of standard planters (Geerse, 2010) (Fig. 1). Another problem arises in organic farming systems, in which weeds tends to become a more severe problem, with a higher weed coverage (Berner et al., 2010), more persistent grass weeds (Peigné et al., 2007) and a higher weed diversity withnit being practiced (Wortman et al., 2010). Considerations for adaption of conservation tillage in the Netherlands In the next section some key considerations and challenges associated with the introduction of NITsystems in the Netherlandsare being highlighted. This list is based on a review of the existing literature and, though rather comprehensive, it is far from being complete since each soil type x farm management system combination poses unique challenges. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 2

13 Fig.1.Three main principles of conservation agriculture with related practices (Stagnari et al., 2009). A)Transition period: From ploughing to conservation tillage Farm management changes may be required after introducing CsT, including modifications in crop rotation, weed control including improved use of cover crops (Fig. 1) and/or partly replacement of the machine park (e.g. purchase of suitable equipment capable of dealing with crop residues). Moreover, it is still unclear if CsT is suitable for Western Europe and thus for Dutch agriculture (Van der Weide et al., 2008). Current problems and uncertainties are the main cause for the slow adoption of CsT by local farmers. Moreover, implementing CsT is a process which requires patience, because most positive effectsare typicallyoccurring after several years, while initially many setbacks may occur (Stagnari et al., 2009). For this reason the first years of practicing a NIT-system probably will require sustained capital investments, due to higher risks and lower returns (van der Weide et al., 2008), caused by a higher weed pressure and lower N-availability. B) Suitability of CsT for conventional vs. organic farming systems Soil fertility in organic farming is managed using a more long-term and integrated approach, which entails increased use of legumes, leys, divers rotations and stricter restrictions compared to the short-term solutions in conventional farming (Watson et al., 2002). Weed control in organic fields tends to be more problematic (Trewavas, 2004; Berner et al., 2008), because weeds, pest and diseases cannot be suppressed by use of agrochemicals, resulting in a higher weed diversity and greater incidence of annual and perennial grasses. A key problem is that the soil can become compacted by repeated mechanical weeding required for weed control (Peigné et al., 2007). Moreover, cover crops should also be managed differently, since killing the cover crop cannot be achieved by spraying, implying that cover crops should be removed mechanically or by frost to ensure a good seedbed preparation (Geerse, 2010). Mechanical cover crop control may be attained by mowing, undercutting and rolling (Peigné et al., 2007). Nutrient management is also much more important in organic agriculture due to the higher dependency on soil organisms for nutrients release from organic amendments and SOM (Vian et al., 2009; Koepke, 2003; Peigné et al., 2007). C) Crop rotations in the Netherlands Crop rotation is an important aspect of CsT (Fig. 1), especially in organic farming, since it commonly features wider crop rotations for a number of reasons (Stagnari et al., 2009). A diverse crop rotation contains crops grown at different times of the year with different mechanical practices (Bond and Grundy, 2001). Soil-borne diseases thereby may be prevented more adequately, because the suitable host-plant for crop pathogens and/or parasites is absent several years in one cycle. Besides, weeds can be suppressed in a more effective manner (Govaerts et al., 2009) and nutrient managementalso depends on a well-structured rotation. Crops differ in terms of nutrient-requirements and rooting MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 3

14 depths, resulting in different nutrient-uptake dynamics (Stagnari et al., 2009). In the Netherlands, root crops (e.g. potatoes, carrots, sugar beets and turnip-rooted celery) are commonly included in the rotation and cover more than 50% of the cropped area since they tend to be most profitable. With root crops being included in the rotation every two or three year, CsT still has a positive effect on soil physical properties (D Haene et al., 2008), although effects of specific crops at CsT are not well-known (Vakali et al., 2011; Berner et al., 2008; Krauss et al., 2010). D) PedoClimatic condition in the Netherlands The Netherlands has a temperate rainy climate, as in the rest of North-Western Europe, with an average yearly temperature of 9.9⁰C (KNMI). Characteristic are the quit wet seasons and relative warm summers (Koppen/Geiger classification). In case the Troll/Paffen zones are taken into account, the Netherlands are part of the cool-temperate zone with an oceanic climate (Cannell, 1985). The average annual precipitation is approximately 850 mm (KNMI) and it exceeds evapo-transpiration, especially during the wintertime. In terms of soils, both sandy gleyicpodzols and fine-textured fluvisols with a sensitivity for compaction and high water tables are commonly found in the Netherlands (Cannell, 1985). The high risk for compaction is caused by poorly drained soils due to high water tables. E) Effect of NIT on soil structure Implementation of NIT will have a big impact on soil structure via changes in soil physical aspects. The field can be entered earlier under wet conditions, while compaction is being reduced as a result of an improved soil strength (Van der Weide et al., 2008; Holland, 2004). Schjonning and Rasmussen (2000) and Geerse (2010) reported reduced compaction of the subsoil with DD-systems. Lopéz et al. (1996) found a higher PR under NT, which is inversely related with soil water content. Another important change is more carbon being sequestered in the topsoil (Van der Weide et al., 2008; Stagnari et al., 2009) and according to Stagnari et al. (2009) soil porosity and aggregate stability improves. Although with DD-systems the MP of the topsoil was being reduced, an increased volume of macro-pores was found in the lower soil-layers, resulting in an improved soil aeration (Schjonning and Rasmussen, 2000). This may imply that the plough pan disappears with NIT, which is in agreement with findings of Peigné et al. (2007), who reported a lower P b d at cm depth in MT compared to INV. This decrease could be caused by an enriched soil life and corresponding increase in earthworm channels and pore-volume (Hermawan and Cameron, 1993). According to Balan et al. (2009) this decrease counteracts soil compaction on the longer term. F) Soil moisture dynamic under NIT Soil moisture properties are also affected under NIT. It was reported that water retention (Krauss et al., 2008; Casa and Lo Cascio, 2008), water infiltration and water holding capacity (WHC) are beingimproved (Holland, 2004). This could be due to improved porosity by earthworm-activity (Schjonning and Rasmussen, 2000; Stagnari et al., 2009; Geerse, 2010; Vakali et al., 2011).HigherWHC with the use of NIT could be especially beneficial under dry conditions,which can be an important yield- limiting factor in a conventional tillage system (Garcia-Torres and Martinez-Vilela, 2000). Though the Netherlands typically has excess precipitation, in the last decade prolonged droughts during late spring, when crops are most prone to drought stress, are becoming increasingly common. Next to this evaporation and runoff are being decreased by plant residues covering the soil surface with CsT being practiced (Casa and Lo Cascio, 2008). Increased water retention may also poses challenges since fields are remaining wet for a longer time. This may prevent farmers from entering the field with farm equipment, thus delaying cultural practices including seeding and harvesting(geerse, 2010). It should however be taken into account that water retention can be measured on a volumetric and on a gravimetric basis. An increase in volumetric water content at pf2 does not necessarily mean that the plant AWC increases, though automatically increases at higher bulk densities. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 4

15 G) Nutrient-cycling Although use of CsT should allow for more efficient nutrient utilization, N mineralization in spring is delayed due to lower soil temperatures and/or aeration (Berner et al., 2008; Owen et al., 2006). This may reduce crop nutrient supply, while N-leaching also decreases (Van der Weide et al., 2008; Geerse, 2010; Peigné et al., 2007). An increase in supplemental N-demand was also reported by Koepke (2003), although only during the first years after converting to a NIT-system. Supplemental N- demand decreases over time, since residues and cover crops combined with SOM accumulation enhance nutrient retention, N-cycling and overall N supply (Stagnari et al., 2009; Geerse, 2010; Holland, 2004; Ekeberg and Riley, 1996). Moreover, a reduction in tillage combined with reduced nutrient inputs results in stabilization of SOM, N 2 O-emissions and plant-available N. In the literature it was reported that N mineralization is influenced by mechanical weeding. According to Owen et al. (2006), soil NO 3 -N concentration was depressed in the short term after mechanical weeding, while Bond and Grundy (2001) reported an increased N mineralization after secondary tillage. Shortterm reduction in N mineralization may be due to soil drying by an increased aeration, resulting in immobilization of NO 3 -N (Franzluebbers, 1999; Owen et al., 2006). Farmers perception regarding conservation tillage Interest of farmers in CsT slowly increases over time, because farmers are looking for short-term solutions and direct benefits, which is not realised by employing CsT-based techniques. However, acontinuous decline in soil structure associated with INV or potential cost savings due to reductions in energy use with NIT are the main reasons for farmers to pursue CsT (Vakali et al., 2011). Especially in Europe there still is quite a bit of scepticism towards NIT (Stagnari et al., 2009). The perception of organic farmers related to soil tillage, especially soil inversion, is that it disturbs soil structure and life drastically. For this reason CsT closely matches the philosophy of organic farmers, because soil disturbances are being reduced, while it provides opportunities for more sustainable solutions to enhance inherent soil quality (Kouwenhoven et al., 2002; Geerse, 2010). However, CsT is not widely used yet and many problems prevail, which hampers the implementation of CsT on organic farms.effective use of CsT, mainly NT, will be the next agricultural revolution due to the risk for erosion and a decreasing SOC under INV(Krauss et al., 2009). Research scope During the past decade CsT has received more attention, although effects and suitability for the Netherlands are still poorly understood. The aim of this study therefore is to investigate the effect of NIT on soil structure and its suitability for organic and conventional agricultural cropping systems. In this study the suitability of NIT under Dutch climatic conditions and locally employed cropping systems is evaluated. Research question with expected outcomes In this report the effect on subsoil compaction, aggregate stability and N mineralization are being investigated with the corresponding research questions:i) Is NIT improving soil structure by enhancing soil-life and increasing soil-stability on organic and conventional farmsii) Is NIT counteracting compaction of the upper subsoil (plough pan)?; iii) Is NIT having an initial negative effect on the N mineralization of SOM?. It is hypothesized that after transition from INV to NIT the plough pan may disappears over time. For this reason P b d and PR are expected to be reduced in the compaction layer. Therefore depth of rooting and earthworm-activity andks and MPare expected toincrease. Pw pf2 is expected to be a constant factor (Kuipers, 1961). The stability of the soil is assumed to improve by more stable aggregates and increase in SOM-content over time.overall Nmineralizationis expected to be lower under NIT in the first year after its initial use, though mechanical weeding is expected to increase N mineralization, because it increases soil temperature and aeration. Root growth at greater soil depth MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 5

16 is assumed to be enhanced with the use of NIT, which advantage is may off-set by the reduction in N mineralization rate observed with NIT in the first years. Purpose and program structure Effects on the soil structure, compaction layer and N mineralization were investigated during spring and early summer of 2011 on five Dutch farms where NIT is being used for at least four years. Fields under NIT and differences over time were being investigated. Differences between INV and NIT are only directly taken into account in this study for aggregate stability. The aim of this study therefore is to investigate the effect of NIT on soil structure and its suitability for both organic and conventional farms in the Netherlands with root-crop based crop rotations, which are locally employed cropping systems. Moreover, an incubation study is examined to investigate the effect of primary and secondary tillage on nitrogen (N) mineralization. Thesis structure This first chapter of this thesis provides the overall context of this work, including a brief literature review and outlines the overall research objectives and program components. In the second chapter detailed information of the field-conditions, including weatherdata and farm management, and experiment, including methods, calculations and statistical analyses used, are provided. Resultsof the structural and stability measurements are shown in the third chapter, in which they are also related to the literature. The results and interpretation of the N mineralization incubation study are provided in chapter four, in which the effects of primary and secondary tillage and the N mineralization-rate in time are discussed intensively. In chapter five conclusions are drawn from the findings in literature, while in chapter six all the hypotheses with related results are summarized in the synthesis. Chapter seven finalizes this thesis by providing some recommendations based on field observations. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 6

17 2. Materials and methods 2.1 Experimental design Farm-based NIT monitoring studies were carried out on five farms throughout the Netherlands in 2009 and 2011, including three organic and two conventional farms (Table 2). Soil types varied from sandy/light loam to silt loam(table 2). Table 2. Outline of farm characteristics of the five farms investigated including farm system, soil characteristics and original ploughing depth before practicing NIT. General characteristics Soil Farmer Location Farm system Type 1 Lutum % A. Brooijmans Blijham Conventional Silt loam 15 A. van Hootegem Kruiningen Organic Sandy loam 9 C. van Tiggelen Halsteren Conventional Loam 25 C. Steendijk Kamperland Organic Silt loam 24 H. Westers Hornhuizen Organic Sandy loam 10 1 textural class according to USDA (1975). In 2011 measurements were taken earlier during the growing season compared to 2009, with crop development being less advanced. Only at the farm of Brooijmans canopy closure had been attained. The exact sampling-dates and present crops during 2009 and 2011, along with the date of the last inversion tillage and original ploughing depth can be found in Table 3. Investigated fields in 2009 were recorded with Google maps, after which sampling spots were marked using a sketch that included mapping locations (Appendix I). In 2011 the original sampling spots were traced, andmeasurements were carried out at a distance of five meter from spots investigated in This approach was taken, because the soil pits dug in 2009 were expected to have impacted the inherent soil structure and thus to interfere with future measurements. The gross plot size used for the measurements was 6 x 0,75 m, i.e. 4.5 m 2. The field study contained five blocks (fields), three plots per block and eight to ten repetitions per plot, varying for the measurements taken. The general and specific layout of the field studies can be found in Appendix I. Table 3. Specific field information with dates of measurements and crops present in 2009 and Latest soil inversion tillage and original ploughing depth on the measured fields are listed as well. Year Soil-inversion 3 Farmer Crop 1 Latest Depth (cm) A.. Brooijmans 12 August 20 May Wheat/ wheat 2 Autumn A. Van Hootegem 11 September 11 May Beans/carrot Autumn C. van Tiggelen 31 July 17 May Potato/beets Spring C. Steendijk 30 July 6 May Wheat/potato Autumn H. Westers 3 August 24 May Potato/carrot Spring Crop present on the field at sampling during the cropping season of and , respectively. 2 This was the only field with a closed canopy in Latest ploughing activity and original ploughing depth before initiation of NIT. 2.2 Cultural practices During 2011 different crops were being grown on the investigated fields compared to 2009, except on the farm of Brooijmans (Table 3). The latest soil-inversion, till cm depth, occurred between autumn 2005 and autumn 2007 on all farms, while in subsequent years loosening the soil occurred through use of a subsoiler rather than ploughing. The rotary harrow is often used in combination with the subsoiler and/or the sowing machine in a single pass. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 7

18 2.3 Weather conditions Weather data were recorded at meteostations of the KoninklijkNederlandsMeteorologischInstituut (KNMI) in Lauwersoog, NieuwBeerta and Wilhelminapolder (KNMI, 2011). Weather data from these stations is chosen, because they are the most nearby with respect to the investigated farms. For 2011, daily weather data of the three meteo-stations are presented till sampling days in Appendix III. Lauwersoog Lauwersoog is located 10 km from the farm of Westers (http://www.himbuv.com/pdif.htm). It is assumed that weather data from the KNMI station in Lauwersoog is representative for the farm of Westers. Average temperature in Lauwersoog between January 1 st and May at 10 cm was 3.5 C and total precipitation 112 mm. However, in March, April and May total precipitation only was 40,0 mm. Radiation was on average 1018 J cm -2 d -1. NieuwBeerta NieuwBeerta is located 10 km from the Brooijmans farm (http://www.himbuv.com/pdif.htm). It is assumed that weather data from the KNMI station in NieuwBeerta is representative for the farm of Brooijmans. Average temperature in NieuwBeerta between January 1 st and May at 10 cm was 1.3 C and total precipitation 117 mm. However, in March, April and May total precipitation only was 41.2 mm. Radiation was on average 998 J cm -2 d -1. Wilhelminadorp Wilhelminadorp is located in between the farms of Steendijk, van Tiggelen and van Hootegem with distances of 12, 27 and 14 km between farm and the meteostation, respectively (http://www.himbuv.com/pdif.htm). It is assumed that weather data from the KNMI station in Wilhelminadorp is representative for these three farms. Average temperature at 10 cm in Wilhelminadorp between January 1 st and May 6, 11 and (sampling dates at the farm of Steendijk, van Tiggelen and van Hootegem) was 3.3, 3.6 and 3.8 C and total precipitation 183.3, and mm, respectively. However, in March, April and May total precipitation till sampling was maximal 34.7 mm. Radiation was on average 957, 1002 and 1043 J cm -2 d -1, respectively. 2.4 Soil physics measurements and calculations Soil texture The SOM-content was determined for two soil layers, 0-5 and cm. Soil samples used for the soil texture analysis were collected randomly andseparated by layer. This procedure was repeated for three plots per field. Collected and air-dried soil was sent to the Blggagroxpertus soil lab inoosterbeek for further analysis. At this commercial laboratory the C-organic content in air-dry soil was measured by their own Cor6 method (BlggAgroxpertus) Soil pits Per field three soil pits of 50*50*50 cm were dug at a distant of five meters from the pits made in 2009, with sampling locations being at least ten meters separatedfrom field edges. With a sharp flat shovel one wall directed in the longitudinal direction of the field was smoothened to be the exposure face of the soil profile in order for making structure, colours and contours clearly visible. A series photographs for each pit were made before further analysis. Subsequently, soil structure was described by assessing colour differences, spots and compactness. Soil was removed from the pit walls to determine depth and intensiveness of rooting and inherent soil structure afterwards. Depth of rooting was defined as the depth till which living or dead roots were visible. This was practiced byscraping offsoil particles with a knife. Depth of soil live was characterized by the depth of visible earthworm traces, i.e. living earthworms or earthworm channels. Boundaries at which these factors were no longer obvious anymore were recorded. Finally soil peds from all depths were investigated MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 8

19 and divided into grainy (0.3-1 cm), rounded-blocky (1-10 cm) and sharp blocky (hooky and compact) elements. A description in words was formulated after the exact analysis Penetration resistance (PR) An electronic penetrologger was used to measure PR (EijkelkampAgrisearch Equipment, 2010). The penetrologger has a maximum resistance of 1000 Newton and a force resolution of one Newton. The conus with a top-corner of 60 and a base-area of 1 cm 2 was used, which translates to a nominal diameter of mm. First, a plan with a hierarchical order and number of plots and repetitions was saved in the penetrologger. On three fields existing ridges first were equalised in order to be able to place the depth referential plate horizontally on the soil-surface. The penetrologger was pressed manually into soil through the hole in the referential plate, with the equipment automatically recording the measuring depth via an ultrasonic sensor. The penetrologger was pressed minimally 50 cm into the soil with a speed of 4 cm/s and a maximum allowed deviation of the rod from the vertical position of 3.5. A larger deviation will result in a lost signal, corresponding with a flat line on the display. To ensure for reliable measurements these measurements were not saved. Average penetration resistances (in MPa) in the upper most 50 cm were reported, because effects of NIT were expected to be most pronounced till this depth. On each farm ten penetration readings were randomly takenin such a way that the whole width (75 cm) of the plot was investigated Volumetric and gravimetric soil-water-air characteristics Sampling Soil-water-air (SWA) content was investigated using ring samples taken at two soil depth layers, namely and cm.per plot eight 100 cc rings per layer were sampled, by pushing the rings in the soil with a sampler. Hammering was required in case the soil was too hard and compactedanda smaller more robust sampler was used instead. Sampled soil volume was larger than the volume of the core ring and therefore excess soil was removed by slicing it off with a sharp knife. Smearing was prevented by cutting small parts of the excess soil at once. Samples were placed immediately in a box after they were taken and cut of straight, to minimize evaporation from the rings Sand-box method A filter paper and nylon cloth were fixed at the bottom of the sampled rings with an rubber-band. Prepared samples were weighted and placed in the sand-box to equilibrate the samples to field capacity (pf2). In the sand-box the rings were saturated for 24 hours by adding one centimetre of water. To create a one meter water column below the sand-box, a tap was opened after the saturation time. This was achieved by a water barrel connected with the sand-box,ending in an outflow one meter below the surface of the box. This results is a suction of one meter, corresponding with pf2. Samples were left for 24 hoursand weighted, after which they were dried in an oven with forced ventilation at 105 C for 48 hours. Finally, the dry weight was also determined Calculations Dry bulk density (Pb d ) The dry bulk density (P b d ) was calculated as: P b d = m s /V (Mg/m 3 ) In which: m s = dry weight of the sampled soil (g) V = volume of the 100cc ring (100 cm 3 ) MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 9

20 Soil particle density (Ps) In order to calculate the density of the soil phase (P s in g/100cm 3 ) for each depth layer, the densities of mineral parts and SOM were estimated to be 2.65 and 1.47 Mg m -3, respectively.with this assumption the density of the soil phase was determined, to calculate pore-volume,as: P s = 100/((SOM/1.47)+(1-SOM)/2.65) (Mg /m 3 ) A correction in mineral particle density was made for samples from the farms of Brooijmans, van Tiggelen and Steendijk. Mineral particle density wasadjusted to respectively 2.95, 2.74 and 2.80 Mgm -3, because there were clear indications that the soil had been shrunken (Table 6). Gravimetric water content (PwpF2) The gravimetric water content during soil sampling (Pw samp ) was determined as: Pw samp = (((R fw -R-C)-(R dw -R-C))/(R dw -R-C))*100 --> ((S fw - S dw )/ S dw )*100 (in % m/m, dry base) In which: R = weight of the 100 cc ring (g) C = weight of the cover in case necessary (g) R fw = weight of the 100 cc ring with cover and sampled soil (g) S fw = field weight of the sampled soil from the 100cc ring (g) R dw = weight of the 100 cc ring with cover and dry soil (g) = dry weight of the sampled soil from the 100cc ring (g) S dw The weight-based (gravimetric) soil water content at pf2 (Pw pf2 ) is calculated almost the same, although the total weight of the sample at pf2 (R pf2 ) is taken into account instead of R fw : Pw pf2 = ((R pf2 -R-C)-(R dw -R-C))/(R dw -R-C)*100 --> ((S pf2 - S dw )/ S dw )*100 (in % m/m, dry base) In which: R pf2 = weight of the 100 cc ring with cover and sampled soil at field capacity (g) = weight of the sampled soil from the 100cc ring at field capacity (g) S pf2 Macro-porosity (MP) The volume percentage air at pf2, MP, is the difference between Ф pores and Фw pf2 : MP = Ф pores - Фw pf2 (% v/v) In which: Фw pf2 = volume percentage water at pf2 (% v/v) Ф pores = total pore-volume (% v/v) Saturated water-permeability (Ks) Sampling At the (original) plough-pan depth (25-30 cm) Ks was determined. Eight soil cores of 100 cm 3 were carefully pressed into the soil after excavating and removing the top twenty-two centimetres. Soil protruding outside the sampling ring was cut off straight. Ensuring that possible changes in Ks over the years can be attributed to changes in the number and continuity of bio-pores, rings had to be totally filled with soil. This was achieved by sampling between eventually present big cracks in the soil, although at the farm of Brooijmans this proved to be practically impossible, because cracks were present all over the soil surface Falling and constant head Using an Eijkelkamp soil water permeameter Ks was determined according to Klute and Dirksen (1986). Highly permeable soil samples were analysed with the constant-head method, while for the MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 10

21 relatively more impermeable samples the falling head method was employed (Fig.2). Per sample the appropriate method was chosen during analysis,which depends on the level of permeability.the maximal duration of storage and measuringwas 24 and 48 hours, respectively. Fig. 2. Global Ks-values of different soil-types. The suitable method to use, constant or falling head, is indicated for differentpermeability s Saturation A hydrophilic gauze and a sieve shell were placed at the bottom of the samples and fastened in a ring holder. Prepared samples were placed upside-down in the permeameter which was filled with water till just below the lower part of the sample. Water supply was opened slowly to raise the water level to saturation in approximately three hours. In the permeameter a maximum water level was set to prevent water from flowing into the ring holders via the top to the samples. After saturation the water level on the samples in the ring holder was measured and the appropriate method, constant or falling head, was defined Permeability Permeable samples were analysed by the constant head method, with a constantwater level in the ring holders and container, although these levels were not the same.a water-filled overflow leading to a burettewas placed on the permeable samples. Water conducted through the samples was transported via ring holder and overflow to the burette. Depth of the overflow determined the water height on the sample. The water level in the burette was measured in time. Using the constant head method the K-factor was calculated with a derived formula of the law of Darcy. Furthermore a correction was made for the constant water height difference (h) between the ring holder and container. K = (V * L ) / (A * t * h) In which: V = water volume floating through the sample (cm 3 ) K = permeability coefficient or K-factor (cm/d) h = water height difference in- and outside ring holder (cm) L = length soil sample in float direction water (cm) i = increase height gradient, h/l(-) MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 11

22 A = surface cross section of sample (cm 2 ) t = time (d) Using the falling head method implies that the change in water height in the ring holder (Δh) is measured in a certain time interval (Δt). Besides, the water level in the container also had to be measured to calculate the changing water height difference between the in- and outside of the ring holder, as was also done for the constant head method. The falling head method requires a modified formula, because the water height (difference) changes over time: K =( (a * L)/((A *(t 2 -t 1 ))) *(ln(h 1 /h 2 )) + ((x * a *L) / (A * (h 1 *h 2 )) In which: a = surface cross section of ring holder (cm 2 ) x = evaporation factor: (cm/d) t 1 = beginning of measuring (d) t 2 = end of measuring (d) h 1 = water level difference inside and outside the ring holder at t 1 (cm) = water level difference inside and outside the ring holder at t 2 (cm) h Temperature correction Viscosity of the soil solution is important for permeability and depends on the water temperature, for which also a correction had to be made. The dynamic viscosity of water at 10 C, average temperature of soil water, is 1.31 * 10-3 Pa s, which was used to correct for temperature by: K 10 = K t *h T /h 10 In which: K 10 = corrected K-factor at 10 C (natural circumstances) (cm/d) K t = K-factor at laboratory temperature (cm/d) h 10 = dynamic viscosity of water at 10 C (Pa s) h t = dynamic viscosity of water at T C (Pa s) Soil aggregate stability Sampling Aggregate stability of inverted and non-inverted soils was investigated for the lighter soils, since the risk ofsealing and crusting ofthe topsoilwas considered to be more relevant for these soils. These include soils fromthe farms of Van Hootegem, Van Tiggelen and Westers (Table 3). Besides, neighbouring fields,which were separated by a ditch,weresampled and included in the analysis.soil was sampled at ten meters from a ditch at 30, 60 and 90 meter from the field edge and aggregate stability determined for the 0-5 and cm layer of both the NIT and INV-fields. Wet sieve method Samples were sieved using one and two millimetre sieves, of which the 1-2 aggregate fraction was used in the wet sieve method (Eijkelkamp, 2008). Aggregate stability was determined by the breakdown rate of the aggregates in water. A total of 4,00 grams aggregates were moistened on 0,25 mm sieves with a surface of 10,2 cm 2. Eight minutes after moistening aggregates were sieved for three minutes in cups with distilled water, which was done with eight replications at once. Subsequently sieves were lifted, water percolated and cups with dispersing solution replaced the distilled water. The dispersing solution contained two grams of natriumhexametafosfaat per litre. Aggregates were sieved for a second time in the dispersing solution until all material finer than the sieve diameter was being dissolved and only stones and organic material remained. The cups with distilled water and dispersing solution with dissolved soil were placed in a convection oven at MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 12

23 110⁰C,as long as required to evaporate all the water. Remaining soil was weighed, with a correction of 0,2 grams made for cups with the dispersing solution. Aggregate stability index Aggregate stability index was calculated by the aggregates dispersed in dispersing solution (g) divided by the total aggregates dispersed (g), thus in water as well in the dispersing solution. Aggregate stability index = Wds / (Wds + Wdw) In which: Wds = Wdw = aggregates dispersed in dispersing solution (g) aggregates dispersed in distilled water (g) N dynamics Nitrogen mineralization was examined with relatively undisturbed soil samples incubated in PVCtubes. Samples were taken on field three of the Droevendaal farm, Kielekampsteeg 12, in Wageningen, the Netherlands on June the 10 th. The effect of NIT and weeding on N mineralization, primary and secondary tillage-operations respectively, were investigated Soil-column preparation Soil was sampled from ploughed and non-ploughed soil. Twenty-four six millimetre thick PVC-tubes of 22 by 30 centimetre were sampled per soil tillage treatment. Pressing the tubes till 25 cm into soil was practiced by an atlas crane, with a metal template placed on top of the tube. A metal ring was attached onto thistemplate and protected the PVC from breaking. Sampling took place twenty meters from the field edge within the crane s turning cycle, to minimize destruction of the wheatcrop present at sampling. The upper five centimetre of the tubes remained soilless and contained two holes, in which a temporarily solid stave was placed, used to lift the tubes. Sampling was repeated in case soil was lost at rising of the tubes. Once removed, a permeable cloth was attached to the bottom of the tubes and fasten with strips of an inner car tire around the tube, tight enough to hold the soil column into its place in the PVC-tubes. After transport an insulating radiation reflecting foil was placed over the whole PVC-surface, which prevented from extreme temperature changes and drying. Finally the soil-columns were placed on dishes in a barn Storage and maintenance Soil storage temperature in the barn wasapproximately 20 C (Fig.35), and soil temperature was monitored with one Watchdog-sensor per soil tillage treatment in additionally sampled tubes. The sensor in the non-inverted soilcolumn was placed on June 23 and in the inverted soil column on June 24. On June 27 they were temporally removed for a test. Water was applied to the dishes on June 14, June 15, June 19, July 6 and August 4 and capillary rise transported the water throughout the samples. Additionally, minimal topsoil wetting was applied on June 14, June 20, July 6 and August 4 to preserve a moisten topsoil and to prevent from drying. The layout of the incubation study can be found in Appendix II Weeding regimes This experiment consists of three weeding regimes with different simulated weeding practices, which were: - no weeding; soil-columns were left undisturbed - one weeding; on day five after initial establishment of thesoil-columns - two weeding s; on day five and seventeen after initial establishment of thesoil-columns. Simulated weeding consist of imitated weeding by intensive topsoil stirring of the upper five centimetre. Samples assigned to the second regime were weeded on June 14 (day five), the third regime included an extra weeding on June 27 (day seventeen). Soil-columns from the first regime MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 13

24 remained undisturbed, despite manual removal of small weeds in the first weeks, which was done in all soil-columns N min -sampling Samples to determine N min were taken on June 10, June 24, July 6 and August 4 at the 0-15 cm layer. Samplingon June the 10 th was done adjacent to where the PVC-tubes were obtained on the field and not from the soil-columns to prevent them from disturbingfrom the first day onwards. The later three N min -samples were taken from the column at minimal five centimetre from the side and other sampling spots. In aluminium trays the soil-samples were oven-dried at 40⁰C for at least 48 hours, sieved at two millimetre and analysed in the lab Segmented flow-analysis In the lab N min was analysed by a segmented flow-analysis (SFA). In terms of soil extraction, 0,01 M calciumchoride (CaCl 2 ) was added to a sample of three grams dry soil. Samples were placed in a shaker for two hours, after which the solution was left for fifteen minutes. Subsamples were takenand centrifuged at rotations per minute for ten minutes. Total N min extracted by the CaCl 2 was measured by determining both NO 3 -N and NH 4 -N. The sample was first subjected to dialysis in the SFA. At every series an internal reference sample was included for control, as was a series of standard solutions (Houba and Novozamsky, 1998). Nitrate was taken up in a stream of ammonium chloride and reduced to nitrite by means of cadmium. In the acid medium the solution was coloured by adding α-naphtylamine and sulphanilamide. This colour reagent is known as the Griess-Ilosvay reagent. The absorbance of the coloured nitrite was measured at a wavelength of 540 nm (Houba et al., 1999). Ammonium was determined by the Berthelot reaction, in which an indophenol derivative is formed by a phenol derivative (salicylate). This derivate is formed under catalytic action of nitroprusside in the presence of ammonia and hypochlorite. In alkaline medium the indophenol derivate has a greenblue colour, of which absorbance is measured at a wavelength of 660 nm (Houba et al., 1999). 2.5 Statistical analysis Eight to ten repeated measures per plot were averaged and used as input in Univariate Generalized Linear Models for comparisons between 2009 and 2011, with farm included as random factor to correct for farm-effects. Data on Ks were log-transformed for normal distribution. The main effects on aggregate stability (primary tillage) and on N mineralization (primary and secondary tillage) were analysed by a one-way Analysis of variance (ANOVA) and regression analysis were used to compare between different variables. The statistical package used for analysis was IBM SPSS Statistics 19, with significance being assumed at P<0.05. The complete output of the statistical analysis can be found in Appendix IV. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 14

25 3. Results and discussion In the first section a detailed description is provided of soil pits excavated at the five pilot farms and results for 2011 will be compared to observations made during This paragraph is followed by a discussion of the overall observed trends across the different farms, whicharelinked with the current tillage practices. Rooting depth and earthworm-activity are expected to be positively influenced by a disappearing compaction layer. In the second section a description of SOM, P b d, MP and PRis provided and values ofthese soil structure characteristics during 2009 and 2011are being compared. Thereafter, a discussion of the overall trends in time across different farmsis given, based on the premise that a build-up of SOM and increase in MP are assumed to decrease P b d, with a sequential lower PR and enhanced soil-life. In the third section a description ofks and Pw pf2 is providedfor 2009, 2011 along with calculated difference during the intermediate period. The overall observed trends of these soil-water characteristics across the different farms arealso being discussed.earthworm-channels and SOM are expected to increase Ks and pore-volume. In the last section a description of aggregate stability at the three farms with light soils is provided, including adjacent fields under INV. This is followed by discussion of the overall measured stabilities for both soil-tillage treatments at two soil-depth layers. It is expected that a higher fraction of stable aggregates, as a result of increased earthworm-castings or SOM-content,will reducesealing and crusting and thereby increase Ks. 3.1 Observations in soil pits Although changes in soil profiles were observed, in general observations made for soil-pits investigated in 2009 and 2011 were comparable. Some general farm information and changes over time are listed below and linked to current tillage practices A. Brooijmans Farm description The conventional farm of Brooijmans measures 120 hectare of silt loam soil and it is located 36 km South-East of Groningen. The main crop is wheat, which is complemented with sugar beets and rapeseed. A four yearcrop rotation is typically used, while green manure crops are not grown. Under dry conditions soil loosening till 20 cm is practiced with an Unia subsoiler and if unfavourable conditions for sub-soiling prevail, the soil is being ploughed till cm instead. Seedbed preparation is normally combined with sub-soiling and the rotary harrow in a single pass, after which the rotary harrow is used again in combination with the sowing machine during the second pass Soil pits A visual overview of soil structure and biological activity for the soil pits during 2009 and 2011 is shown in Fig.3. A brief description of individual soil pits for 2011, along with key changes and previous measurements, is provided in the narrative below. Photos are included at the end of the paragraph clarifying soil structure in 2011, with cracks, topsoil and subsoil-track being highlighted(figs. 4-6). Pit 1 The soil profile consisted of a dark silt loam up to 35 cm depth with a granular soil structure. In this layersome earthworm traces were seen and confined to the top 15 cm. Asharp blocky soil structure prevailed deeper down.below 35 cm a grey soil layer occurred. Large cracks were vertically orientated (Fig. 4). Roots mainly grew between soil cracks, although there was some root penetration into the compact soil peds as well. Roots penetrated the soil to the bottom of the pit MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 15

26 equally over the width of the pit, with a declining diameter in depth. The uppermost 27 cm contained egg shells, with very porous spots present at 18 cm. A comparable layer pattern was observed in 2009, with the granular structure, dark and grey layerbeing positioned at similar depths. Earthworm-activity was found till a depth of 25 cm compared with 15 cm in 2011, while roots penetrated till at least 50 cm depth in both years. The bluish colour at 30 cm depth and brown spots from 35 cm onwards were absent in Pit 2 The topsoil had a granular structure, below which a dark compact layer with a sharp blocky structure was present, with two loose spots 50 cm apart (Fig. 5). From 35 cmonwards the soil was layered with brown/grey soil and compaction gradually decreasing with increasing depth. Rooting was very intensive across the entire transect both in terms of quantity and diameter between 25 till 50 cm depth. Earthworms mainly occurred in the top 20 cm and the topsoil (<30 cm) contained egg shelves. In 2009 the same layers were observed, although the soil contained a smeared soil layer (25-35 cm). On the other hand, the well-structured layer below 35 cm observed in 2009 occurred to be sharp blocky in 2011.Evidence of earthworm-activity was recorded five centimetre deeper in 2009 (25 vs. 20 cm), while root penetration was similar in both years. 0 Pit 1 0 Pit 2 0 Pit 3-10 dark grey clay, crumble structure Dark clay, crumble structure -10 Clay, dark grey, crumbled structure crumble structure -10 Dark grey clay, crumble structure Dark grey clay, rounded blocky crumble structure dark grey clay Compact, sticky Very compact, bluish Dark clay Egg shelves Dark grey clay Compact Very compact, sticky Dark clay, compact, sharp blocky Egg shelves Dark grey clay Compact Very compact, sticky Dark clay, compact, sharp blocky Depth (cm) Grey clay, brown spots, good structure 2009 grey clay Dark grey clay, brown spots, good structure, peat spot at bottom 2009 Dark grey, compact brownish, sharp blocky Dark layer Dark grey clay, brown spots, good structure 2009 Light grey clay Dark clay 2011 Bottom of profile pit Depth of visible worm-activity traces Rooting depth Fig. 3.Schematic soil profile till 50 cm depth at three soil pits on the farm of Brooijmans in 2009 and Pit 3 The third soil pit contained similar layers in 2011 as the second pit, with the topsoil having a granularstructure near the surface followed by sharp blocky dark peds (Fig. 6). Between 35 and 45 cm soil turned grey, with underneath this layer a second dark layer occurring. Several earthworms were found in the top 20 cm of the profile. Only the top 15 cm was intensively rooted, while at lower layers roots mainly penetrated between soil cracks. At a depth of cm a porous spot was observed. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 16

27 During 2009, overall soil layers, rooting depth and earthworm-activity were also identical for pits two and three. Although in 2009 blocky soil peds with rounded surface occurred between 10 and 15 cm in pit three and worm-activity was observed up to 25 cm. Fig. 4.Overview of the soil profile on spot 1 on the Brooijmans farm with the three arrows pointed at large vertically orientated cracks. Fig. 5. Overview of the soil profile on spot 2 on the Brooijmans farm. Below a detailed transition from porous to compact topsoil is indicated (left) and a close-up of the track from the subsoiler at cm depth (right). MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 17

28 Fig. 6. An overview of the soil-pit on spot 3 on the Brooijmans farm. On the right intensive rooting and sharp-blocky soil peds in the uppermost 20 cm is detailed Main structural changes Soil structure of pit one more or less remained the same, though the compact layer between 25 and 35 cm disappeared. In pit two similar changes were found, with the bluish spot at 30 cm depth disappearing in The overall soil structure of pits two and three was comparable for both years Linking observations with tillage practices Using the subsoiler for loosening deeper soil layers is known to improve soil structure by creating porous spots below the topsoil. Moreover, the depth of root penetration depends on cracks due to the compact nature of the soil at this field site. However, it is hard to say to what extend observed cracks were caused by sub-soiling, because shrinkage as a result of drought before fieldmeasurements could also have been the cause for the cracking of the soil. In 2009 the bluish spot in pit one and the sticky spot in pits two and three were found within the plough pan layer. Over time soil structure tended to improve also for the very compact layer, which may indicate that NIT can improved soil structure, especially around 30 cm depth. Alternatively, soilinversion before transition to NIT may have been practiced under (too) wet circumstances. Based on the gradual disappearance of the compactness between 25 and 35 cm depth it appears that current sub-soiling till a depth of 20 cm is sufficient, as can be concluded from the observed soil structure in the soil pits. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 18

29 3.1.2 A. Van Hootegem Farm description The organic farm of van Hootegem measures 100 hectaresandy loam soil and is located 62 km South- West of Rotterdam. The crops cultivated are wheat, peas, bean, maize, chicory. Green manure crops are included in the rotation used and these are present in the field during the wintertime. The latest ploughing activityoccurred till 28 cm in 2007 on the particular field were measurements were taken. Seedbed preparation is normally combined with sub-soiling, which is practiced with a Kverneland CLI subsoiler Soil pits A visual overview of soil structure and biological activity for the soil pits during 2009 and 2011 is shown in Fig 7. A brief description of individual soil pits for 2011 along with key changes and previous measurements is provided in the narrative below. Photos are included at the end of the paragraph clarifying soil structure in 2011, with blue spots, subsoil-tracks and porous spots highlighted (Figs. 8-10). Pit 1 The soil profile consisted of a porous layer till 35 cm and a more compact structure with sandy spots and stones below this layer. On this boundary a track of the subsoiler was present, above which more porous soil occurred especially on the right side (Fig. 8). Earthworm-activity by earthwormchannels and living earthworms mainly occurred till 30 and20 cm respectively, while small roots penetrated till at least 50 cm. During 2009, a loose sandy loam was observed, which became more compact between 10 and 25 cm. Destruction of the soil profileoccurred below 25 cm, while earthworm-activity and roots were observed till 25 and 35 cm, respectively Light grey sandy loam, dry, loose Light grey sandy loam, dry, hard Pit 1 Porous at right side Light grey sandy loam, dry, loose Light grey sandy loam Pit 2 Good structure near track subsoiler, porous Blue spot Light grey sandy loam, dry, loose Pit 3 Rounded blocky, loose structure, especially round track subsoiler Blue spot Depth (cm) Disturbed, spots grey upper soil and lighter from subsoil 2009 Compact, sandy spot with stones on left, disturbed pit yellow/ brown loam with small brown and grey spots 2009 compact Sandy layer Light grey sandy loam 2009 Rounded blocky, no layering Sandy structure 2011 Bottom of profile pit Depth of visible worm-activity traces Rooting depth Fig. 7.Schematic soilprofile till 50 cm depth at three soil pits on the farm of Van Hootegem in 2009 and MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 19

30 Pit 2 The second soil pit contained intensive rooting and a well-structured soil diagonally above the track of the subsoiler (<35 cm), while a blue spot (20-32 cm) and compacted layer (32-35 cm) were observed in the soil profile (Fig. 9). Below 40 cm a sandy soil layer occurred, which was the boundary of earthworm-activity and root penetration. However, root-intensity declined with depth, though the quantity of earthworm-channels remained constant. In 2009 the same topsoil (<10 cm) was observed in pit one and two, below which a light grey and yellow/brownsubsoil (30-50 cm) occurred. Penetrated roots and earthworm-activity were five centimetre deeper in 2011 compared to 2009 (40 vs. 35 cm). Pit 3 The soil profile consisted of blocky soil peds with rounded surface up to 30 cm depth and a blue spot was observed at 25 cm (Fig. 10). The soil profile became less porous between 30 and 45 cm. Below 45 cm depth a sandy layer occurred. One single earthworm-channel was observed at 50 cm, though most earthworms and channels occurred till 40 cm. Penetrated roots were observed till 50 cm and very minimal rooting and several root-hairs occurred below 30 cm. In 2009 soil pit three was homogeneous, because in whole the pit a grey sandy loam occurred, with earthworm-activity till 30 cm, while the only difference was the compacter structure from 10 cm onwards. Fig. 8. Overview of the soil profile of spot 1 on the van Hootegem farm with a detailed view of the subsoil-track on the right photograph Main structural changes In all three soil pits on van Hootegem s farm rooting and earthworm-activity increased (Fig. 7; Table 4). More porous soil occurred in 2011, mainly near the track of the subsoiler. In pit two and three similar changes were found, with a blue spot observed in 2011, indicating a bad-structured andanaerobic soil. Layering in the uppermost 30 cm disappeared and a homogeneous topsoil occurred in all pits in MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 20

31 Fig. 9. Overview of the soil profile of spot 2 on the van Hootegem farm with a detailed view of the present blue spot (left). Fig. 10.Overview of the soil profile of spot 3 on the van Hootegem farm with a blue spot on the left (arrow) and porous soil circled in the middle of the soil-pit above the subsoil-track. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 21

32 Linking observations with tillage practices Evidence of a compaction layer was absent in 2011, indicating that loosening deeper soil layers with the subsoiler is enough to foster a good soil structure at this layer. However, it was observed that the pins of subsoiler have loosened the soil at the same distance from the track each year.removal ofthe blue spots observed in pit two and three may be achieved by changing the position of the pins. It would be recommended to continue with the subsoilerunder dry circumstances, because a loose, well-structured rounded blocky structure occurred. Although a homogeneous topsoil is already observed without a compaction layer, sub-soiling less deep may is sufficient after disappearance of the bleu spots at 20 cm C. van Tiggelen Farm description The conventional farm of van Tiggelen measures 80 hectare ofloamy soiland it is located 46 km South of Rotterdam. The main crop is potato, which is complemented with sugar beets, wheat and barley. A four year crop rotation is typically used, while green manure crops are grown to achieve a soilcoverage of 90%.Soil-inversion was normally practiced till 25 cm, though the plough was not used anymore since The subsoiler is used for loosening deeper soil layers (<30 cm), while a cultivator and rotary harrow are used for secondary tillage Soil pits A visual overview of soil structure and biological activity for the soil pits during 2009 and 2011 is shown in Fig. 11. A brief description of individual soil pits for 2011 along with key changes and previous measurements is provided in the narrative below. Photos are included at the end of the paragraph clarifying soil structure in 2011 (Figs ). Pit 1 The soil profile consisted of a dark layer up to 25 cm depth with porous soil in the top 10 cm and a transition layer in which compactness increased with depth (25-35 cm) (Fig. 12). Below the topsoil a compact grey and brown layer occurred. Generally, the pit was quit compact with intensive rooting and a few earthworm-channelsbeing observed in the grey and brownish layer (35-50 cm). Four layers occurred in the soil profile of 2009, with boundaries observed at 10, 28 and 40 cm depth. The topsoil had a dark grey layer and containedsoil peds with a rounded surface, while underneath a layer with a sharp blocky structure and cracks occurred. Two grey layers occurred at the bottom of the pit, with a brown mottled pattern observed at the lowest layer. Roots penetrated till 30 cm, while earthworm-activity was found till a depth of 20 cm. Pit 2 The topsoil (<10 cm) had a dark loose layer, below which a compact layerwith sharp blocky structure (<30 cm) and grey/brown-coloured soil (>30 cm) occurred (Fig. 13). Although many earthworms were observedin the top 15 cm, in deeper layers no earthworm-activity occurred. Rootingwas very intense across the entire transect in the top 25 cm. Although the same soil layers were observed for pit one and two in 2009, boundaries of pit two were located at 10, 25 and 35 cm. Earthworm-activity was observed up to 10 cm and roots penetrated till 25 cm. Pit 3 The third soil pit contained a loosetopsoil, below which a compacted layer was present and both layers weredark upto a depth of 25 cm (Fig. 14).Within the next grey/brown fine-textured soil that extended between 25 and 50 cm a second dark layer occurred between 35 and 45 cm. The top 20 cm MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 22

33 of the soil profile containedmany earthworms, with no earthworm-activity occurringin the deeper soil layers. Small roots penetrated till at least 50 cm, while new big roots mainly occurred till 25 cm. During 2009, the overall soil layers, rooting depth and earthworm-activity were identical for pits one and three, while the depth of earthworm-activity and root penetration were slightly different. In pit three these boundaries were observed at 10 and 27 cm depth, respectively Main structural changes Roots penetrated till the bottom of all pits in 2011, while in 2009 no root penetration was observed below 30 cm. Root penetration was not hampered by the compact soil profile and similar results were observed at Brooijmans farm, on which most roots occurred between large vertically orientated cracks. Earthworms were not observed below 20 cm in 2009 andwere observed deeper in all pitsin 2011 (Table 4), while a more compact soil profile occurred Linking observations with tillage practices Using the subsoiler for loosing deeper soil layers is probably required to create soil cracks, because root penetration depends on cracks due to the inherent compact nature of this field-site. Although the subsoiler would be required to break the compact soil at 20 cm depth it is hard to say to whatextend the observed cracks were caused by weather circumstances and/or tillage practices with the subsoiler. The mulch left on the field appears to promote soil-life, although this was mainly confined to the topsoil. It would be desired to enhance earthworm-activity in the deeper soil layers of the soil profile as well, though in each plot the depth of observed earthworms increased over time. For this reason it may be wise to continue with the farm-management as has be done in the previous years. The advantage is that the decomposition of crop-residues is assumed to be enhanced with a sequential higher SOM-content (Table 5) Dark grey clay, rounded blocky Dark grey clay, sharp blocky, crack surfaces Brown/grey clay, transition layer Pit 1 Dark, porous Dark, compact Transition layer Porous Compact Dark grey clay, rounded blocky Dark grey clay, sharp blocky, crack surfaces Brown/grey clay, transition layer Pit 2 Dark clay, loose Dark clay, compact Sharp blocky Dark grey clay, rounded blocky Dark grey clay, sharp blocky, crack surfaces Brown/grey clay, transition layer Pit 3 Loose topsoil Dark layer compact Grey/ brownish clay, crumble Depth (cm) Grey clay, brown mottled 2009 grey layer, compact Brownish Grey clay, brown mottled 2009 Grey/ brownish clay Grey clay, brown mottled 2009 Dark clay Grey/brown clay, crumbled 2011 Bottom of profile pit Depth of visible worm-activity traces Rooting depth Fig. 11.Schematic soil profile till 50 cm depth at three soil pits on the farm of Van Tiggelen in 2009 and MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 23

34 Fig. 12. Overview of the soil profile of spot 1 on the van Tiggelen farm with at the right a detail of the natural formed porous spot present. Fig. 13.Overview of the soil profile of spot 2 on the van Tiggelen farm. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 24

35 Fig. 14.Overview of the soil profile of spot 3 on the van Tiggelen C.Steendijk Farm description The organic farm of Steendijk measures 80 hectare ofsilt loamy soil and is located 64 km South-West of Rotterdam. The crops cultivated in a six year crop rotation are spinach, winter and summer wheat, onions and tinned vegetables, while green manure crops are also frequently grown during the winter fallow period. Grass-clover is included in the six year crop rotation for maintaining a well-structured soil. Since 2008 ploughingis abandoned for inverting the top 25 cm and soil loosening till 30 cm is practiced with a Kverneland CLI subsoiler instead Soil pits A visual overview of soil structure and biological activity for the soil pits during 2009 and 2011 is shown in Fig 15. A brief description of individual soil pits for 2011 along with key changes and previous measurements is provided in the narrative below. Photos are included at the end of the paragraph to clarify the soil structure in 2011, with the smeared spots highlighted (Figs ). Pit 1 The soil profile consisted of a compact sharp blocky structure with a smeared spot at23 cm (Fig. 16). In the top 10 cm many small pores and earthworm-activity were observed and small roots penetrated till 50 cm depth. A fine-textured grey topsoil of five centimetre occurred, below which soil-peds with a rounded surface sharp were observed (15-30 cm) in Below 30 cm the structure was sharp blocky. Earthworm-activity was observed till a depth of 30 cm, while roots penetrated at least till 50 cm in both years. Below 15 cm this pit contained vertically orientated cracks. Pit 2 The top of the profile had a dark compact layer with a bad-structured sharp blocky structure till 25 cm (Fig. 17). Below 20 cm the soil became more porous, except for two very smeared spots, and the colour turned from grey to mottled brown at 40 cm. Earthworms mainly occurred in the top 20 cm and small roots penetrated at least till 50 cm. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 25

36 In 2009 the same layers were observedat the same depths in pit one and two, although thesecond pit was only 45 cm deep. Pit 3 The third soil pit contained two layers, with a sharp blocky compact structure in the top 30 cm,which turned lighter below a depth of 30 cm. Loose soil was absent, while two smeared, blue, wet spots occurred between 20 and 25 cm (Fig. 18). Although only the top 15 cm contained earthwormactivity,roots penetratedat least untilat least50 cm depth. The soil profile in 2009 was quite similar compared to the first and second pit, only the transition from round soil-peds surfaces to sharp occurredfive centimetre deeper (35 vs. 30 cm) Dark grey clay, crumble structure Dark grey clay, rounded blocky, cracks Pit 1 Sharp blocky, compact Sticky spot, sealed Dark grey clay, crumble structure Dark grey clay, rounded blocky, cracks Pit 2 Dark layer, compact, sharp blocky Porous, sealed spots Dark grey clay crumble structure Dark grey clay, rounded blocky, cracks Pit 3 Dark, compact, sharp blocky Sealed and blue spot Light grey clay -30 Depth (cm) Brown/ grey clay, sharp blocky, cracks Brown/ grey clay, sharp blocky, cracks 2009 Mottled brown Brown/ grey clay, sharp blocky, cracks 2009 Light clay, mottled brown, compact, sharp blocky 2011 Bottom of profile pit Depth of visible worm-activity traces Rooting depth Fig. 15.Schematic soil profile till 50 cm depth at three soil pits on the farm of Steendijk in 2009 and Main structural changes Roots penetrated till 50 cm in both years, while the depth of earthworm-activity decreased (Table 4). Earthworm-activity was very high up to 15 cm, although the earthworms did not burrow deeper into the soil profile. Smeared spots occurredin 2011, while these were being absent in The overall soil structure deteriorated, because the soil structurecontained soil-peds turned to sharp blocky and very smeared spots occurred in Linking observations with tillage practices Using the subsoiler for loosening deeper soil layers resulted in smeared spots, regarded the distance at whichthey occurred from each other. It appears that it was quit wet during soil-tillage, whichdid result in tillage negatively impacting soil structure. However, more porous soil was found around the track of the subsoiler on other farms, the soil was more porous on spots were the subsoiler not has tilled the soil on Steendijk s farm. It appears that sub-soiling under too wet conditions deteriorates the soilstructure and it would be advisable to stop using the subsoiler when the fields are still too wet. Ploughing once again may be desirable or alternatively sub-soiling should be practiced when the soil is relatively dry. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 26

37 Fig. 16.Overview of the soil profile of spot 1 on the Steendijk farm with at the upper-right a detailed porous topsoil and below the smeared soil between cm depth. Fig. 17.Overview of the soil profile of spot 2 on the Steendijk farm. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 27

38 Fig. 18.Overview of the soil profile of spot 3 on the Steendijk farm with both arrows pointed towards smeared spots H. Westers Farm description The organic farm of Westers measures 85 hectare of sandy loamsoil and it is located 24 km North- Westof Groningen. Wheat, rye, chive, pumpkins, carrots, potatoes and oats are cultivated in a six year crop rotation, while green manure crops are grown to preserve a soil cover during wintertime and to decreasen-leaching. Since 2006 ploughing has not been practicedanymoreand deeper soil layers till cm are tilled with a Kongskilde subsoiler instead Soil pits A visual overview of soil structure and biological activity for the soil pits during 2009 and 2011 is shown in Fig 19. A brief description of individual soil pits for 2011 along with key changes and previous measurements is provided in the narrative below. Photos are included at the end of the paragraph clarifying soil structure in 2011, with a well-rooted spot highlighted (Figs ). Pit 1 In the first soil pit no distinct soil layers occurred, only a blue spot (15 by 15)at 15 cm depth, below which the soil profile contained three sandy spots, indicating that the profile was disturbed (Fig. 20). Although a single channel was found at 25 cm depth, only the top 15 cm contained many earthworms. Roots penetrated till 40 cm, whileonly the top 25 cm was intensively rooted. In 2009 the soil profilecontained three layers. The topsoil (<8 cm) had a loose structure, below which the compaction layer occurred and density increased with depth till 25 cm. Below the compaction MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 28

39 layer the soil was mixed,indicating that the first soil pit did not contain an original profile in 2009 already. Earthworm-activity was observed till 20 cm and root penetration was observed five centimetre deeper. Pit 2 The soil profile consisted of loose soil with light coloured soil layers between 25 and 30 cm and below 48 cm, while two porous holes occurred at 20 cm (Fig. 21). Although thick roots penetrated till 40 cm at the right side of the pit, at the left side some roots were only observed up to 20 cm. Earthwormactivity was observed up to at least 20 cm. Earthworms may occurred deeper in the soil profile, though channels between 20 and 40 cm were not definable as being from earthworms and/or roots. In 2009 the composition of the soil pit differed. The top 10 cm consisted of granular soil, below which a compaction layer with increasing density till 20 cm occurred. Grey loamy sandfrom 20 cm onwards completed the soil profile. Earthworm-activity and root penetrationwere observed till the lowest part of the compaction layer at 20 cm depth. Pit 3 The soil profile consisted of loose soilwith a porous layer between 10 and 20 cm, till which earthworm-activity was observed (Fig. 22). Most roots penetrated the uppermost 22 cm, although minimal rooting was observed till 40 cm depth. Pit two and three contained the same layers in 2009, only the light layer below 48 cm was absent in pit three. Earthworm-activity and root penetrationoccurred till 25 and 40 cm in pit three, respectively Main structural changes The transition layer with compaction gradually increasing with depth disappeared in 2009 and a loose structure occurred in 2011 instead. Moreover, the results of the PR-curves (Fig. 27) showed that soil compaction changed similar in depth in both years. Despite a blue spot observed in the first pit (Fig. 19 and 20), soil structure tended to improve to a loose and porous soil structure in all pits in Loose loamy sand Pit 1 Loose 0 Loose light sandy loam Pit 2 0 Loose light loamy sand Pit 3 loose Compaction layer, density increasing with depth Blue spot Compaction layer, density increasing with depth Loose soil Compaction layer, density increasing with depth Loose, porous Depth (cm) Mixed, grey loam with light loam from surface 2009 Loose structure, sandy spots, no layering Grey loam, no traces of deep cultivation Light layer 2009 Lighter layer Light spot on right-side, loose soil Light spot Grey loam, no traces of deep cultivation 2009 Loose soil 2011 Bottom of profile pit Depth of visible worm-activity traces Rooting depth Fig. 19. Schematic soil profile till 50 cm depth at three soil pits on the farm of Westers in 2009 and MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 29

40 Linking observations with tillage practices In all pits observed the compaction layer disappeared between 2009 and 2011 and a loose structure was observed instead. It can be concluded that over time soil structure tended to improve, implying that sub-soiling till a depth of 20 cm is sufficient. Although the subsoiler may not has to be used anymore in particular years depending on crop choice, porous spots were found above the subsoiltrack. At adequate management minimal tillage is sufficient, because the soil is very well-structured. Fig. 20. Overview of the soil profile of spot 1 on the Westers farm with the arrow pointed towards a blue spot present at 15 cm depth. The sandy spots circled indicate a disturbed soil structure. Fig. 21. Overview of the soil profile of spot 2 on the Westers farm. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 30

41 Fig. 22. Overview of the soil profile of spot 3 on the Westers farm with a detailed view of the porous well-rooted topsoil below the carrot-ridge Overall tillage effects on rooting depth and earthworm-activity over time In this section tillage effectsacross farms on root growth andearthworms are being presented. Rooting depth significantly increased in the soil pits of 2011compared with 2009 (P=0.004) (Table 4), despite an earlier sampling-date in Table 4:Depth of root penetration and visible earthworm-activity for five Dutch farms during 2009 and 2011 followed by the change in the intermediate period. Farm Root penetration a Traces of earthworm-activity b Increase Increase Brooijmans Van Hootegem Van Tiggelen Steendijk Westers Mean (S.E.) c 9.2 (4.3) -0.4(5.2) P d * ns a Average depth of roots penetrated in the soil pits of three pits per farm. b Average depth of earthworms and/or earthworm-channels observed in the soil pits of three pitsper farm c Average increase in depth of roots and earthworms on all farms with standard error of the mean included between brackets. d Significance is assumed at P<0.05. In the upper subsoil a less compacted soil profile was expected as a result of NIT, which was assumed to enhance root penetration (Johnson-Maynard et al., 2007; Olesen and Munkholm, 2007; MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 31

42 Munkholm et al., 2001) and earthworm abundance. However, since rooting depth depends on varying external factors, this made the comparison between 2009 and 2011 rather difficult. First of all, the sampling-date was two months earlier in the growing season of On the farm of van Hootegem, Steendijk and Westers no commercial crop was growing yet and rooting depth had to be judged on the basis of old and new roots and roots of weeds. Moreover, each crop has specific root characteristics and observed differences thus may not be attributed to tillage-induced soil structure effects. On the farm of Brooijmans winter wheat was grown in both years and a rooting depth of up to 50 cm was found (Table 4). Winter wheat is known to root deep, even if the prevailing soil structure is rather poor, so it is more resistant to compacted soil layers (Olesen and Munkholm, 2007). Reports in the literature show that rooting depth is restricted in case PRexceeds 3.0 MPa (Hermawan and Cameron, 1993). Based on results presented in section 3.2.4, it appears that on the farm of Brooijmans and Steendijk the depth were rooting ceased indeed coincided with zones were soil PR exceeded 3.0 MPa (Figs. 3, 15, 23 and 26). However, at Westers farm rooting also occurred at a higher penetration resistances in 2011 (Figs. 19 and 27). In this case it may be possible that soil biota induced root channels allowed root penetration, despite the overall PR exceeding 3.0 MPa.Based on this observations it appears that under long-term NIT root growth no longer berestricted by a PR above 3.0 MPa, the threshold being used for well-managed standard tillage systems(ekeberg and Riley, 1996). On the other hand, roots could have penetrated the soil when the soil was still moist and the PR relatively low. Penetration may have increased as a result of a decreased moisture content and roots scavenging for water in the deeper soil layers. Other studies have shown that long-term application of NIT was found to increase earthwormdensity and stable bio-pores (Vian et al., 2009; Tebrügge and Dürling, 1999). However, in the current study a decreased depth of earthworm-activity was observed on the Brooijmans and Steendijk farm (Table 4). This may be due to earthworm-activity being mainly restricted to the topsoil (Berner et al., 2010) or to the quit bad soil structure on these particular farms, associated with dry compact and smeared spots, respectively. Earthworms could thereby be impeded to penetrate deeper into the soil profile. Earthworm-activity depends on temperature, vegetation, and soil disturbance and did not change over time (Table 4). However, this may be influenced by weather as well, since earthworm counts drop after prolonged periods of drought (Johnson-Maynard et., 2006). At van Tiggelen s farm earthworms penetrated deeper into the soil profile and one of the practices to stimulate earthwormactivity is leaving straw (mulch layer) on the soil surface. This stimulates biological activity in the topsoil when practiced in combination with NIT (Tebrügge and Dürling, 1999).However, no traces of anecic species, extended vertical earthworm channels, were detected in this investigation. In literature it was shown that NIT decreases the killing of anecic species, which are most important in terms of making vertical pores in deeper soil layers and will also not penetrate soil at more than 1.6 Mg m -3 (Peigné et al., 2007). Sampling time as related to drought and high dry bulk-densities around the critical value (Table 6) could have hampered the traces and activity of anecic species to increase. 3.2 Soil Structure Organic matter The SOM content in the topsoil (0-5 cm) remained constant on the farm of van Hootegem and Westers, while a trend towards a gradual build-up of SOM was found on the farms with a fine textured soil (Table 4). Across all farms the overall SOM-content in the topsoilalso tended to increase. Based on reports in the literature an increase in SOM was expected with NIT-practices (Stagnari et al., 2009; Stockfisch et al., 1999; Tebrügge and Dürling, 1999). However, it may be possible thatincreased secondary tillage for soil loosening still result in SOMbreakdown, especially on farms with a loamy or sandy soil. This is related to secondary tillage increasingthe decay rate of SOM in the tilled layer due to enhanced aeration and increased soil temperature, thus improving microbial activity and SOM breakdown (Peigné et al., 2007). Thereby it may offset perceived benefits of NIT MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 32

43 regarding the build-up of SOM. Besides, the intermediate period could have been too short to obtain a statistically increased SOM-content The apparent decay of 0.1% on the farm of van Tiggelen at cm was an exception, although as such this effect was not significant (Table 4). Peigné et al. (2009) reported a decay in SOMin deeper soil layers due to reduced incorporation depth of surface applied residues with the application of NIT, while in this study the SOM increase tended to be half of that of the topsoil. Table 5: Soil organic matter content for five Dutch farms at the 0-10 and cm soil depths during 2009 and 2011, followed by the calculated build-up of soil organic matter between 2009 and SOM at 0-5cm a SOM at cm a Farm Build-up Build-up Brooijmans v. Hootegem v. Tiggelen Steendijk Westers Mean (S.E) b 0.16 (0.075) 0.08 (0.05) P c ns ns a In 2009 SOM is based on one mixed sample from three plots. In 2011 the average was taken from one sample per plot. b Average build-up of all farms between 2009 and 2011, with standard error of the mean between brackets. c Significance is assumed at P<0.05. Increases in SOM have been linked with enhanced formation of stable soil aggregates (Czyz and Dexter, 2009). As aggregate stability was not measured in 2009, its influence on SOM over time could unfortunately not be discussed in this study Dry bulk-density(pb d ) The P b d at cm depth increased (P=0.007), with the values for the van Hootegem farm in 2009 again not being measured (Table 6). However, it is questionable whether the observed P b d is an appropriate measure to indicate changes in soil structure, because measurements may be influenced by extreme weather circumstances (Smatana et al., 2010; Miller et al., 1998). Sampled soil on the Brooijmans farm was shrunken, with an increase of g cm -3 within two years being unrealistic. During sampling the soil was very dry at the upper subsoil. Notably, Pw samp was 4.14% point lower than Pw pf2 and a difference of maximal 1.5% point between Pw samp and Pw pf2 was found on the other farms. Drought could have caused chemically bound water to be lost from the soil matrix. These losses were assumed to result in an increased particle density of clay particles, which explains the extreme increase in P b d on especially the Brooijmans farm. The hypothesis that shrunken soil had been sampled from between cracks was confirmed by the fact that samples from the farm of Brooijmans (2011) showed considerable swelling when placed in the sand-box. This indicates that more soil was present in the sample rings as would be at pf2, when soil would not be swollen or shrunken. Specific density of the mineral parts is usually assumed to be 2.65 Mg m -3. However, under the extremely dry circumstances this could be an underestimation due to shrinkage. Corrections thus were made for the fine-textured soils by assessing the specific density of mineral parts to 2.95, 2.74 MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 33

44 and 2.80 Mg m -3 for the farms of Brooijmans, van Tiggelen and Steendijk, respectively (Table 6). Specific densities to a maximum of 2.9 Mg m -3 and higher were also recorded in literature (University of Akron). However, when possible, samples should be taken at approximately pf2 to make P b d a representative and a more reliable soil structure measurement, without values being affected by excessive swelling or shrinking. As a result of the methodological problems with the measuring procedure of P b d, no conclusions could be drawn from these measurements. Table 6: Dry bulk-density and macro-porosity at cm depth for five Dutch farms in 2009, 2011 and the change in the intermediate period. P b d (g cm -3 ) a MP (%) a Farm increase b increase Brooijmans v. Hootegem ND ND ND 6.99 ND v. Tiggelen Steendijk Westers Mean (S.E.) c (0.022) (0.57) P d * ns a Farm-averages based on three plots, with eight samples per plot. b A correction was made because the usual lab-procedure yielded unrealistic (negative) MP values for the farms with heavy soil (Brooijmans, van Tiggelen and Steendijk). Most probably this was due to the extremely shrunken condition of the fine-textured soils at sampling, caused by the very dry conditions. It appears that the particle density of clay increases due to loss of chemically bound water when shrinkage is extreme. This is consistent with the substantial increase in P b d found, particularly those at the farm of Brooijmans. Usually, lab calculations are made with a specific density of the mineral parts of 2.65 Mg m -3. The correction included the assumption of a particle density of mineral parts of respectively 2.95, 2.74 and 2.80 Mg m -3 for the farms of Brooijmans, van Tiggelen and Steendijk. c Average change on all farms with standard error of the mean included between brackets. d Significance is assumed at P< Macro-porosity (MP) Across farms and sampling dates MP fluctuated (Table 6), while on the farm of van Hootegem during 2009 no samples were collected. Although overall MP values did not change significantly (P=0.055), they decreased by 0.99% point in the upper subsoil. Negative values for macro-porosities were initially found on farms with a fine-textured soil (Brooijmans, van Tiggelen and Steendijk), which was caused by shrinkage, which was assumed to be absent in sandy soils. Although MP was positive in this case, values were still relatively low.a correction was thus made for the farms with a finetextured soil as discussed in the previoussection (Table 6). With these adjusted particle densities an increase was only found at the Brooijmans farm (0.32% point). With the data available it was difficult to determine whether sampled soils were shrunken and to what extent.macro-porosity is linked to bulk-density and waterretention and was thus also affected by the problems with the methodology. Therefore conclusions could notbe derived from the results Penetration resistance (PR) At Brooijmans farm soil penetration values below 10 cm were much higher in 2011 compared to 2009 (Fig.23). However, PR and moisture content are negatively correlated (Cassel and Nelson, 1984) and therefore higher resistance may be related to dry soil conditions during sampling in MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 34

45 Dry soil conditions were confirmed by the low Pw samp at cm depth, as mentioned in paragraph Thus, changes in PR do not automatically induce a heavier or compacter soil. In terms of interpretation of the graphs, relative changes with soil depth, especially below the tillage zone, are more relevant than differences in time. A comparison in time would only be representative if measurements are carried out at approximately pf2 in both years. Depth below soil-surface (cm) Penetration resistance (Mpa) Brooijmans 2009 Brooijmans 2011 Fig. 23. Penetration resistance till 50 cm depth on the farm of Brooijmansin 2009 and Error bars are included at every three centimetre and relate to the standard error. An indication of a compacted layer was found in the curve from Brooijmans farm in 2009, in which resistance increased particularly around 28 cm (till 1.99 MPa) (Fig. 23). This relative steep increase at this depth in the gradual curve was absent in This year a gradual increase in PR was found till 35 cm depth, after which a gradual decrease was observed. Depth below soil-surface (cm) Penetration resistance (MPa) v. Hootegem 2011 Fig. 24.Penetration resistance till 50 cm on the farm of van Hootegem in Error bars are included at every three centimetre and relate to the standard error. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 35

46 No penetration-measurement was carried out for the farm of van Hootegem in 2009 (Fig.24). Resistance between 30 and 45 cm almost doubled (1.5 to 2.91 MPa), below which a reverse trend occurred and PR decreased again till 50 cm depth. The increase could be a trace of a former compaction layer, though the curve is very gradual and the increase would be expected to occur just below the plough pan (e.g. around cm) in case derived from former ploughing activities. Depth below soil-surface (cm) Penetration resistance (MPa) v. Tiggelen 2009 v. Tiggelen 2011 Fig. 25.Penetration resistance till 50 cm depth on the farm of van Tiggelen in 2009 and Error bars are included at every three centimetre and relate to the standard error. Depth below soil-surface (cm) Penetration resistance (Mpa) Steendijk 2009 Steendijk 2011 Fig. 26. Penetration resistance till 50 cm depth on the farm of Steendijk in 2009 and Error bars are included at every three centimetre and relate to the standard error. In 2011 the PR was lower on the farm of van Tiggelen compared to 2009, except for the top 10 cm (Fig. 25). An increase in PR between 23 and 28 cm depth may be related to remnants of a compaction layer in However, the peak in the resistance-curve at 28 cm depth disappeared in 2011 and MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 36

47 could have been former indications of a compaction layer. The gradual disappearance of the indications of a plough pan layer due to continued use of NIT could explain the disappearance of the small peak in the curve of On the farm of Steendijk the PR in 2009 was much higher across the entire profile till 50 cm (Fig. 26), which is in contrast with results observed at the Brooijmans farm (Fig. 23). In 2011, the soil was still relatively wet at sampling on the Steendijk farm, which decreased PR. The more gradual resistancecurve was obtained in 2011 with a constant resistance till approximately 20 cm, below which resistance gradually increased.based on the gradual increase in PR values in both curves, no compaction layer appeared to be present in both years. Depth below soil-surface (cm) Penetration resistance (Mpa) Westers 2009 Westers 2011 Fig. 27. Penetration resistance till 50 cm depth on the farm of Westers in 2009 and Error bars are included at every three centimetre and relate to the standard error. A gradual increase in PR till cm was found at the Westers farm in both years(fig. 27). No indications of a former plough pan could bedistinguished in either curve, though an impeding layer was observed at 35 cm in both years. These gradual curves are indicative of a stable structure on this farm, except for the impeding layer (Tebrügge and Dürling, 1999). 3.3 Soil water Saturated water conductivity (Ks) Since Ks was not normally distributed, a log-transformation was used before statistical analysis. Overall Ks values were highest at the farm of van Hootegem and Westers, which is related to these soils containing a higher percentage of sand and silt than the other soils investigated in this study. The change in Ks in the upper subsoil was not significantly different between 2009 and 2011, which isprobably due to large variation within and between plots (Table 7; Fig. 28). Although the use of NIT was assumed to improve Ks in the upper subsoil by increasing earthworm-density and enhancing soil porosity (Stagnari et al., 2009; Miller et al., 1998), this was clearly not the case. By just looking at overall numeric trends, it was observed that values were not consistent across farms. On the farms with a fine-textured soil (Brooijmans, van Tiggelen and Steendijk) the conductivity decreased by 0.182, and cm d -1, respectively. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 37

48 Table 7: Soil water characteristics of five Dutch farms in 2009, 2011 and the change in the intermediate period. Saturated water conductivity was measured at depth and weight percentage water at field capacity additionally at cm. PwpF2 (%, w / w ; dry base) a Ks(cmd -1 ) a cm cm Farm increase increase increase Brooijmans Hootegem ND ND v. Tiggelen Steendijk Westers Mean (SE) b (0.850) (0.34) (0.85) P c ns * ns a Farm-averages are based on three plots with eight samples per plot. b Average change on all farms with standard error of the mean between brackets. c Significance is assumed at P<0.05. Increased Ks values on the van Hootegem farm may be explained by sampling in very dry soil 2009, thus resulting in a lower Ks measures in this particular year. As discussed, sampling in shrunken soil and swelling in the laboratory have interfered with the measurements. Notably, swelling at saturation may have caused blockage of the larger pores. Besides, primary or secondary tillage within two or three months before sampling could also have been the cause of a lower Ks (D Haene et al., 2008). To examine the effects of spatial variability on changes in Ks over time, results for separate plots within farms will also be presented in the following section (Fig. 28) Ks (cm d -1 ) BRO HOO TIG STD WES plot 1 plot 2 plot Location Fig. 28.Change in saturated water conductivity (ΔKs) per plot on the farm of Brooijmans (BRO), van Hootegem (HOO), van Tiggelen (TIG), Steendijk (STD) and Westers (WES), with all fine-textured soils. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 38

49 The most extreme changes in Ks values were obtained in one particular plot on the farms of van Hootegem, van Tiggelen and Westers (Fig. 28), in which ΔKs wasrespectively8.84, -5.17, and 3.73 cm d -1. These deviations greatly affected overall farm results and similar pronounced difference among replicates were found in literature as well (D Haene et al., 2008). With this, the question may be raised how reliable/representative the data is and whether the use of the current method for measuring Ks makes sense (Miller et all., 1998); larger inherent variation in Ks values interferes with trying to establish statistically significant difference, within and among farms.either the method as such or its execution or both have been inadequate and results thus not be representative. The effect of soil disturbance at sampling, such as due to cracking, and the extreme response of a continuous channel is immense due to preferential flow (Miller et al., 1998). In addition to this, the labprocedures such as saturation and preparing samples could have also affected results due to small air-pockets remaining in the samples during saturation, which block water flow and thus decrease Ks. Furthermore, the measuring period on the permeameter (<2 days) may have beeninsufficient, because samples may not always have been fully saturated. To ensure more reliable data, it is to be recommended that samples should only be taken at or near pf2, without the soil being excessively wet or dry, since this may result in smearing or cracking. In addition to this, Ks values may become more constant only after a longer measuring period. Thetimesamples should be kept on the permeameter clearly depends on overall P b d and soil compactness, because in some cases steady state conditions may not be attained within two or three days. Finally, bigger samples are more representative for field conditions, since a single flow channel or small crack would not impactresults drastically. Alternatively, more replicates and/or standard statistical practices may be used to more effectively screen for outliers and artifacts. It should be taken into account that actual Ks values are higher than measured in the lab, because cracks and big pores were avoided during sampling. Without including such cracks and pores, measured Ks decreases drastically compared with actual field conditions. However, the purpose of the research was to assess longterm tillage effects on Ks as related to bio-pores and inherent soil structure at the soil-pedscale (<five centimetre).the variability in the data, despite sampling in between the cracks, showed that the effectiveness of the method is even doubtful for the purpose mentioned (Fig 28) Water retention: gravimetric water contentat field capacity When examining the claim that water retention improves when NIT is adopted, it appears that the data concern the volumetric water content at field capacity (Schjonning 2000, Hudson, 1964). However, an increase in volumetric water content at field capacity does not necessarily mean that the amount of water in the total quantity of soil available to plant roots increases. Notably, the volumetric water content always increases when the bulk density of the soil increases, except when the soil is saturated and water is pressed out of the soil. Therefore, the gravimetric water content (Pw pf2 )seems to be a better characteristic to indicate the water retention in terms of plant available water (Kuipers, 1961). According to the literature review of Hudson (1994) about available water capacity, the AWC increases linearly with SOM, except for clay. Thoughin this study the volumetric water content at pf2 was also measured instead of Pw pf2, which also increases with SOM. Without changes in soil texture and SOM, Pw pf2 is reasonably constant for a certain soil as long as soil handling is not extreme, tilled to dust or deformed in a wet state, and as long as the soil is not in a very compacted condition. When soil handling is extreme Pw pf2 increases and in case soil is very compact Pw pf2 decreases. The intention of soil sampling for bulk density and related soil characteristics in the current project was to get an indication of the development of the compaction state of the soil, the related aeration of the soil and the (gravimetric) water retention. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 39

50 As stated before, the determination of these characteristics was hindered by the very dry sampling conditions, resulting in shrunken soil on the farms of Brooijmans, van Tiggelen and Steendijk. Therefore no conclusions can be drawn from these samples. The unsuspected data from the farms of van Hootegem and Westers show small changes in Pw pf2 (Table 7), which provide no prove for differences in water retention due to the application of NIT.Shrinkage is not assumed to have influenced the cm soil depth layer, in which the volume of pores smaller than 30 µm significantly increased by 1.53 % point.however it was expected that Pw pf2 would not change, Casaand Lo Cascio (2008) did found a higher gravimetric water content under NIT.The increase in Pw pf2 for this depth was may due to enhanced soil-life at this depth or an increased SOM-content (Kuipers, 1961) and results in a higher plant AWC, because it does not concern a volumetric water content at pf2. Due to the higher plant AWC the drought-resistance of plants cultivated also improves, as was reported by other authors as well (Stagnari et al., 2009; Olesen and Munkholm, 2007; Garcia-Torres and Martinez-Vilela, 2000). According to Miller et al. (1992), changes in pore-size distribution may be related to the lag time between sampling and latest tillage-activity. Soil tillage may increase the fraction of macro-pores, but after rainfall the soil may become more consolidated and this will result in a shift from larger to smaller-sized soil pores. In the current study the lag time between sampling and soil tillage was between one and three months, except for the Brooijmans farm. However the relative number of smaller pores (<30 µm), which determine the water retention, are usually not affected by tillage (Kuipers, 1961). In terms of the relationship between soil water retention and crop yields, the literature indicates that use of NIT did not result in higher yields under normal weather circumstances. However, reports in the literature show that when precipitation values were below 300 mm year -1 overall water use efficiency and related yields were higher under NIT (Olesen and Munkholm, 2007; Stagnari et al., 2009; García-Torres and Martinez-Vilela, 2000). Therefore use of NIT may be an essential factor in more resilient/ sustainable crop-production in the context of more erratic rainfall patterns associated with climate change, due to NIT potentially increasing water infiltration and plant AWC as a result of an increase in SOM and, therefore, Pw pf Aggregate stability Tillage had no significant effect on aggregate stability at 0-5 and cmsoil depth (P=0.516 resp )(Table 8). However, the numeric aggregate stability values with NIT tended to be slightly higher in the topsoil, while the reverse was true for the subsoil. Table 8: Aggregate stability for three Dutch farms with NIT and adjacent fields with INV at the 0-5 and cm soil depths. Differences between the systems are also included. Aggregate stability 0-5 cm a Aggregate stability cm a Farm INV NIT Increase INV NIT increase v. Hootegem v. Tiggelen Westers Mean (S.E.) b 0.02 (0.04) (0.03) P c ns ns a Averages are based on three plots, with eight samples per plot. b Average increase in aggregate stability of all farms with standard error of the mean between brackets. c Significance is assumed at P<0.05. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 40

51 3.4.1 Differences between farms Use of conventional tillage on an adjacent field of Westers farm resulted in significantly lower (P=0.026) aggregate stability values in the topsoil compared to non-inverted soil (Fig. 26). Although for the farm of van Hootegem values were only numerically higher with the use of NIT, the reverse was true for van Tiggelen and in both cases differences were not significant (Fig. 29). In the cm depth soil layer on the farm of van Tiggelen and Westers aggregate stability was numerically lower compared to values from a conventionally tilled adjacent field (INV), while the reverse was true for the van Hootegem farm, though on none of the farms the difference was significant (Fig. 30). The interaction between tillage treatments and soil layers was not significant either (P=0.376). It appears that observed trends at cm soil depth tended to be the reverse of those observed in the top layer. However, differences were not statistically different. It may be speculated that NIT would increase SOM accumulation and earthworms activity in the topsoil, and thus increase aggregate stability, because both factors provide binding agents/mechanisms responsible for the formation of water-stablemacro-aggregates (Johnson-Maynard et al., 2007; Amézketa, 1998). Fig. 29.Aggregate stability in the topsoil (0-5 cm) of farms applying NIT and INV. Error-bars relate to standard error of the mean Tillage systems and soil-texture Higher aggregate stability values for the 1-2 mm aggregates on Westers farm with the use of NIT compared to INV appears to be indicative of improved soil quality of the topsoil as proposed by Stagnari et al. (2009) and Amézketa (1998). This is especially relevant to soils that have a low clay content andthereby inherently lack aggregate stability. These soils are also prone to crusting as is the case for the van Hootegem and Westersfarm. With the use of root crops in the crop-rotation, andespecially potato having a very strong perturbing effect on soil structure, an increased aggregate stability under NIT was reported in literature as well (D Haene et al., 2008; Carter and Sanderson, 2001). Besides, the different primary tillage, crop rotation and secondary tillage also differed with adjacent fieldsthat were being compared in the current study and this could have also affected aggregate stability. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 41

52 Overall differences in soil aggregate stability among farms is probably related difference in soil texture, since values were shown to be positively correlated with the fractionclay (Tebrügge an Dürling, 1999; Amézketa, 1998).Indeed the percentage clay was9% (van Hootegem), 10% (Westers) and 25% (van Tiggelen) and overall stability values increased according to soil claycontent (Figs. 29 and 30). Stability of the soil on Westers farm was however much higher as would be expected for a clay percentage of 10%, when compared with the soil from the farm of van Hootegem and van Tiggelen. The lack of significant effects of tillage on aggregate stability may be related to soil not having reached an equilibrium yet. The method used to determine the stability index also may have been of great importance, because different wet-sieving procedures were shown to result in different stability-fractions between specific studies (Amézketa, 1998). Fig. 30.Aggregate stability at cm depth on farms applying NIT and INV. Error-bars relate to the standard error of the mean. 3.5 Interaction between ΔPwpF2 and ΔPb d In the statistical analysis of all data, the development of soil-physical factors appeared to be correlated with farm-effects. Therefore farm was included as random factor in the statistical analysis to correct for farm-specific conditions. With this correction included only one interaction remained significant. The change in Pw pf2 was negatively correlated with the change in P b d (P=0.015), in which an increase in P b d with 0.1 g cm -3 resulted in a decrease in Pw pf2 of 3.6% point(fig. 31). It is evident that the soil was very dense in 2009, which can be concluded from the low macroporosities (Table6).In this case nearly all water-filled pores were smaller than 30 mµ, because macropores were rarely present. It should be noted that smaller pores are not sensitive to soil compaction and in general they will not be compressed due to tillage operations, provided the soil is not excessively wet during tillage. However, at the upper subsoil a decrease in volume of the small pores was found (Table 7), which supports the assumption that a shrinkage-induced decrease in Pw pf2 caused an increase in P b d (Fig. 31). In fact, the strong correlation of ΔPw pf2 and ΔP b d suggests that the MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 42

53 measured development in Pw pf2 and P b d is almost entirely induced by the shrink-swell state of the soil during sampling, because Pw pf2 is usually more or less a constant, for a certain soil, when changes in SOM are small (Kuipers, 1961) y = x R² = Plinear = ΔPw pf2 (%, w / w ; dry base) ΔP bd (g cm -3 ) Fig. 31. The relationship between the change in dry-bulk density and the change in weight percentage water at field capacity at cm depth in the period Farm-effects were included in the model. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 43

54 4.Results and discussion mineralization incubation study 4.1 Primary tillage: INV vs. NIT Use of NIT as primary tillage-practice resulted in a significantly (P=0.007) higher N mineralization compared to INV, with overall N mineralization values being 0.96 and 0.69 kg N min d -1,respectively (Fig. 32). The corresponding increases in N-NH 4 + and N-N0 3 - were 0.16 and 0.12 kg d -1 higher for the NIT-treatment (P=0.002 resp ).In the literature it is stated that Nmineralization decreases during the first years after transition to NIT, which will increase external N-requirements (Hansen et al., 2011; Kong et al., 2009; Vian et al., 2009; van der Weide et al., 2008; Peigné et al., 2007; Koepke, 2003). However, in the current study an opposite effect of primary tillage in the topsoil thus occurred. Initial N min did not differ between both tillage-treatments (P=0.381) and an initial N min -flush at ploughing thus not being the explanation for the higher N mineralization-rate with NIT.Increased microbial biomass N content or total N could have enhanced N mineralization in the topsoil (0-15 cm) under NIT, because mixing of the soil was absent. However, from 15 cm onwards columns with a NITtreatment wereassumed to have a lower N mineralization due to a lower amount of crop-residues and organic manures being inserted at lower soil depths. Cassmann and Munns (1980) found that 58% of N mineralized occurred below 18 cm under INV. With NIT most of the added soil amendments were remaining near the soil surface, resulting in total N mineralization being higher than the amount measured in the uppermost 15 cm of the INVtreatment. In this treatment mixing of the soil layers and organic amendments would have resulted in a dilution of N min. This mixing and layering effect may mask potential difference in N mineralization between both soil-tillage practices as was also reported in the literature (Vian et al., 2009; D Haene et al., 2008). Higher N min -contents were also found by Hofmeijer (2010) on the Droevendaal farm in the top 15 cm of non-inverted soil. At cm depth the N min -content of inverted soil was more than double the N min found at non-inverted soil. In the columns the difference may even more pronounced due to higher temperatures, increasing the difference in present N min between both layers even more (D Haene et al., 2008). Fig. 32. Mineralization at NIT and INV. Error bars relate to standard error of the mean of total increase in Nmin. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 44

55 Although more N min in the topsoil promotes the initial plant growth (Hofmeijer, 2010), this could also be a disadvantage because root growth is restricted to the topsoil. However, NIT is assumed to improve the plant AWC, which decreases drought-stress (Olesen and Munkholm, 2007; Stagnari et al., 2009; García-Torres and Martinez-Vilela, 2000) and counteracts the negative effects of restricted root-growth. 4.2 Secondary tillage: Weeding-effects The overall trend of the effect of weeding intensity on N mineralization rate appeared to be quadratic, but the effect was not significant (Fig. 33, P=0.441). The three weeding regimes being no, single and double weeding resulted in daily mineralization rates of 0.72, 0.88 resp kg N min d -1. Total N min was calculated by summing up soil the N-NH 4 + and N-N0 3 - content, which individually were also not influenced by weeding regime (P=0.780 resp ) kg Nmin day ns N-NH4 N-NO weeding intensity (#) Fig. 33.Increase in N min (N-NH N-NO 3 - ) at a zero (0), single (1) and double (2) weeding treatment. Error bars relate to the standard error of the mean of daily N min increase. Fig. 34.Increase in N min (N-NH N-NO 3 - ) at a zero (0), single (1) and double (2) treatment for NIT and INV. Error bars relate to the standard error of the mean of daily N min increase. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 45

56 One of the reasons for conducting this study was that contradicting effects of weeding on N mineralization were reported in the literature. In the current study weeding effect at both primary tillage-practices combined were not significant (Fig. 33), and they were also not significant for both primary tillage-treatments separately (Fig. 34). However, non-inverted soil tillage tended to show a linear increase in N mineralization (P=0.678), while for INV an optimum in N mineralization was found at one single weeding, though this apparent quadratic effect was not significant as well (P=0.103). 4.3 N mineralization in literature In an incubation study of D Haene et al. (2008) Nmineralization with INV and NITin the uppermost 15 cm of undisturbed soil-columns varied from 52 to 73 kg N ha -1 year -1, respectively. In the current research N mineralized was 43 and 51 kg N ha -1 in the first 54 days after initial soil-column establishment, but it should be noted that the study was conducted during the warmest part of the year and the columns were kept moist all the time. Therefore lower N mineralization will be found in the field with NIT, due to commonly observed lower temperatures and either excessively dry or wet soil moisture conditions (D Haene et al., 2008). Values from the study of D Haene were some lower, because a corrections for temperature and moister conditions were made in this study and the temperature was kept constant at 15⁰C. A N mineralization rate of 80 kg N ha -1 year -1 was reported in a study of Johnsson et al. (1987) in a fertilized (120 kg N ha -1 year -1 )barley crop, in which the topsoil consists of clay and 2.2% SOM, comparable with this current study. Relative high N mineralization-rates were found in this study, with more than half of the N mineralized within 54 days, compared to the yearly N mineralization-rate in the studies of D Haene et al. (2008) and Johnson et al. (1987). WithINVapproximately the same amount of mineralized N min is found within 54 days compared to N mineralized within a whole year in the study of D Haene et al. (2008). Comparing N mineralization with other literature is hampered by the fact that studies are carried out at different temperatures, moisture contents, soil types, measuring methods (drying and/or sieving can differ), conditions and the periods. 4.4 Mineralization rates For both N-NO 3 - and N-NH 4 + a linear increase was found during the first 54 days of the incubation study, in which a slight decrease in N min -contents was found on day 26 after initial columnestablishment for all weeding intensities at INV and NIT (Fig. 35). Especially with INV moren- NO 3 appeared to bemineralized on day 14 compared to day 26. Although this trend was not observed for the double weeding treatment, cumulative N-NO 3 decreased from 14.3 to 9.4 kg ha -1 in the columns without weeding (Fig. 35A). Values of N-NH 4 also decreased after day 14, with this trend being most pronounced for the double weeding treatment (Fig. 35C), in which kg N min ha -1 was mineralized on day 54 compared to kg N min ha -1 onday 14 already. The effect of weeding frequency on total N min for INV are presented in Fig. 35E. On day 14 all weeding treatments had higher total N min present compared to the amount expected based on the linear trend, whereas for day 26 the reverse was true. Overall N-NO 3 and N-NH 4 -mineralization was higher for NIT, with an average increase of 0.67 kg N-NO 3 ha -1 d -1 and 0.29 kg N-NH 4 ha -1 d -1 (Figs. 35B and 35D). In Fig. 35F total changes in N-NO 3 - and N-NH 4 + were summed, with a decrease in total N min at the single weeding treatment and a small increase at the other treatments between day 14 and 26. Total increase in N min was the highest with NIT, with 0.89, 0.94, and 1.05 kg N min ha -1 d -1 for the different weeding intensities, respectively (Fig. 35F). A reduction in N min between day 14 and 26 could not have been due to plant uptake, because the columns were left bare during the study and the few weeds that emerged were removed just after germination. Leaching from the soil-columns was also unlikely, because columns were placed in a barn with water applied in a dish. Therefore, water entered the columns mainly via capillary (upward) flux. Although exceptional topsoil wetting prevented the topsoil from dehydration, this was MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 46

57 approximately one millimetre at once, while leaching would only occur if irrigation application rates exceed 75 mm (Pang et al., 1997). Considering that the columns were placed in an aerobic environment, not compacted and uncovered, N min depletion by denitrification was also assumed to be absent or at least minimal. In case denitrification still would have been occurred in the columns, this was expected at higher quantity in non-inverted soils, with more anaerobic zones due to the compacter soil. Mineral nitrogen (kg N-NO 3 ha -1 ) Mineral nitrogen (kg N-NH 4 ha -1 ) Mineral nitrogen (kg Nmin ha -1 ) INV_0 INV_1 INV_2 y = x y = x y = x Sampling day y = x y = x y = x Sampling day y = x y = 0.82x y = x Sampling day A C E Mineral nitrogen (kg N-NO 3 ha -1 ) Mineral nitrogen (kg N-NH 4 ha -1 ) NIT_0 NIT_1 NIT_2 y = x y = x y = x Sampling day y = x y = x y = x Sampling day y = x y = x y = x Sampling day Fig. 35.Effect of weeding and soil tillage treatment on N mineralization in the topsoil (0-15 cm) on the first 54 days after initial soil-column establishment. Three weeding regimes were applied; zero, one and two weeding s. On day 0, 14, 26 and 54 after initial soil-column establishment samples for N-NO 3 (A and B), N-NH 4 (C and D) and total N min (E and F) were taken and analysed from INV and NIT tillage-treatments, respectively. Mineral nitrogen (kg Nmin ha -1 ) B D F 4.5 Temperature-effect on nitrogen mineralization Temperature is positively correlated with N mineralization (Dessureault-Rompré et al., 2010; Anderson and Jensen, 2001)and is maximal on day eighteen, with soil and air temperature higher than 24⁰C (Fig. 36), while in this period with a decrease in cumulative N min was observed (Fig. 35). These results are contrasting, because the high air and soil temperature were expected to increase N MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 47

58 mineralization and related cumulative N min in the soil-columns. However, a depression in N min was found at day 26 instead (Fig. 35), after the warmest period during the incubation study Temperature (⁰C) day soil air Fig. 36.Soil and air temperature during the incubation study indicated per day after initial soilcolumn establishment in the soil-column with the NIT-treatment. Data from the logger in the soilcolumn with the INV-treatment could not be obtained. A better fitted linear trend should be obtained with temperature sum plotted against cumulative N min (Fig. 37) instead of days after initial soil column establishment (Fig. 35). At low temperatures N mineralization declines strongly, with the base-temperature (Tb) being 0 ⁰C (Andersen and Jensen, 2001) or near freezing according to Jarvis et al. (1996). To determine the temperature sum a Tb of 0 ⁰C was also used in this current study. As can be concluded from Fig. 37 N mineralization on day 14 remains higher compared to the amount expected based on the linear trend, whereas for day 26 the reverse was true. The fit of cumulative N-NO 3 -, N-NH 4 + and total N min against days after initial soil-column establishment (Fig. 35) is poorer compared to the fit against temperature sum (Fig. 37), with NIT_0 and INV_2 being theexceptions (Table 9). This can be concluded from the r-squared values obtained from the linear trend lines, listed in Table 9 for all tillage-treatment combinations. Although temperature sum was expected to be more accurate, it increased around day eighteen as a result of high temperatures (Fig. 36), while the N min -content did not differ. A higher N min -content would have been expected at an increasing temperature sum, because N mineralization and temperature are positively correlated (Dessureault-Rompré et al., 2010; Anderson and Jensen, 2001). The correlation between N mineralization and soil temperature is given by the Q 10 -value, which represents the factor increase in N min at an increase of temperature by 10⁰C. This value increases with temperature, because the breakdown of recalcitrant substances is more limited at low temperature than that of the more easily degradable substances (Anderson and Jensen, 2001) and varies between 2 and 4 in the range 5⁰C to 35⁰C (Gilmour and Mauromoustakos, 2011; Dessureault- Rompé et al., 2010; Sierra, 2002; Kladivko and Keeney, 1987). MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 48

59 Mineral nitrogen (kg N-NO 3 ha -1 ) Mineral nitrogen (kg N-NH 4 ha -1 ) Mineral nitrogen (kg Nmin ha -1 ) INV_0 INV_1 INV_2 y = x y = x y = x Temperature sum (⁰Cd) Fig. 37.Effect of weeding and soil tillage treatment at different temperature-sums on N mineralization in the topsoil (0-15 cm).three weeding regimes were applied; zero, one and two weeding s. Samples for N-NO 3 (A and B), N-NH 4 (C and D) and total N min (E and F) were taken and analysed from respectively INV and NIT tillage-treatments. Table 9: R-squared values of N-NO 3 -, N-NH 4 + and total N min mineralization at zero, single or double weeding, fitted against time and temperature sum. Mineral N Weeding Time a (d) Temperature sum (⁰Cd) INV NIT INV NIT - N-NO N-NH Total N min Mean R a Days after initial soil-column establishment A y = 0.01x y = x y = x Temperature sum (⁰Cd) C y = x y = x y = x Temperature sum (⁰Cd) E y = x y = x y = x MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 49 Mineral nitrogen (kg N-NO 3 ha -1 ) Mineral nitrogen (kg N-NH 4 ha -1 ) Mineral nitrogen (kg Nmin ha -1 ) NIT_0 NIT_1 NIT_ Temperature sum (⁰Cd) B y = x y = x y = x Temperature sum (⁰Cd) D y = x y = x y = x Temperature sum (⁰Cd) F

60 5. Summary of findings The monitoring of the development of soil structure over time was quite difficult, mainly due todifferent soil and crop management conditions during2009 and 2011 and various soil-types responding differentially to prevailing weather conditions. Weather conditions influenced the soil conditions to a much larger extend then was expected beforehand.moreover,the sampling date and crops being grown differed between 2009 and 2011, thereby increasing the overall variability of the results. Especially soil shrinking caused inaccuracies at in situ sampling and ex situ measurements. Despite the varying circumstances between 2009 and 2011 mentioned above, some conclusions can still be drawn. 5.1 Visual observations on soil structure, worms and roots Although soil compaction was observed, no distinct compaction layers were observedin the soil pits that were excavated. Changes in soil compactness tended to bevery gradual or located far too deep to be causedbyformer ploughing activities. Resistance values recorded with the penetrologger confirmed these visual observations (Figs ). These figures showed no distinct spikes in soil resistance beingindicative for acompaction layerand in most cases increases were rather gradual instead.on the van Hootegem farm blue spots were located below previous permanent wheel tracks, where the subsoiler never passed to loosen the soil. The most deteriorated soil structure was found on Steendijk s farm, with very pronounced smeared and/or sticky spots present at a depth of 20 cm in all pits. At these spots soil structure was damaged by use of the subsoiler, because tillage had to be practiced under excessivelywet conditions. Except for this impediment in soil structure, no negative impacts of NIT on soil structure where observed in any ofthe soil pits dugin2009 and Furthermore, well-structured soils were observed with porous spotsdiagonally above tracks where the subsoiler was being used. Root penetration was especially enhanced by soil cracks, which may have resulted from sub-soiling, though shrinkage may contributed even more to their formation. Roots penetrated till a soil depth of at least 50 cm depth in the three most fine-textured soils. Earthworms were intensively present in the uppermost fifteen centimetre on the van Tiggelen farm. Presence ofpartly decomposed straw residues combinedwith the sugar beets grownmay have been conducive to earthworm growth. The observed earthworms were most likely Lumbricusterrestris, which species mainly remained just below the soil surface, while deeper penetration by anecic species would be desirable with NIT practiced. 5.2 Physical determination of the soil structure In the top five centimetre the increase in SOM tended to be higher compared to the build-up in the cm layer. This effect was not significant, nor were the corresponding changes in SOM for the individual soil-layers. It appears that the intermediate period between the two measuring-points may have been too short to have caused a significant increase. The tendency of SOM build-up could have been derived from crop-residues left on the soil surface (Tebrügge and Dürling, 1999) and/or a decreased breakdown. An increase in Pw pf2 have been found for the cm soil depth, which was may due to an enhanced soil-life or the slightlyhigher SOM-content (Kuipers, 1961). Higher Pw pf2 values improve water retention and plant AWC, which increases the drought-resistance of plants cultivated. Penetration resistance showed high fluctuations between sampling times, which could be ascribed to the soil-moisture content at sampling. However, across the overall soil profile depth, only very gradual changes in the curves were obtained. The lack of distinct peaks provides evidence of the absence of (former) compaction layers (Figs ). This corresponds with observations in the soil pits (Figs. 3-19), in which no distinct compaction layers were visually observed at the former ploughing of around 25 cm depth. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 50

61 To test whether soil structure improved and a compact layer was indeed not a part of the soil profile at cm depth, measurements of different soil parameters including P b d, MP, Ks and Pw pf2 were carried out. Unfortunately not all measurements of these soil-characteristics mentioned could be compared with previous measurements as a result of shrinkage of samples taken in The sampled volume contained more soil particles as would have been present when sampling moist soil at pf2 without shrinking. As a result, apparent bulk-densities showed relatively high values, which was caused by water losses from the soil and related increase in specific particle density. The increase in P b d is expected to have reduced both the volume of large pores (MP) and intermediate pores (<30 µm) that can still being filled at pf2 (Pw pf2 ). An associated tendency in decreasing Ks was observed, because especially larger pores are effective in conducting soil water and facilitating rapid drainage. The sampling of shrunken soil caused swelling during saturation in the laboratory, in the permeameter used for Ks measures or the sand-box used for P b d, Pw pf2 and MP determination, and it is uncertain whether this swelling of the soil samples was complete. Most of the soil-characteristics investigated were correlated, because they depended on specific farm- and field-conditions. However, with a correction made for farm-effects it was observed that only Pw pf2 was negatively correlated with P b d (both at cm depth). This correlation indicates that the measured change in Pw pf2 and P b d is induced by the shrink-swell state of the soil,because normally Pw pf2 is constant for a certain soil, when changes in SOM are small (Kuipers, 1961). As a result of the methodological difficulties, limited conclusions could be drawn about the change in P b d, Pw pf2,mp and PR. 5.3 Aggregates stability of NIT vs INV soils Despite findings of other researchers who reported an increased aggregate stability after several years of practicing NIT (D Haene et al., 2008; Carter and Sanderson, 2001; Amézketa, 1998; Hermawan and Cameron, 1992), this increase was not observed for both the 0-5 and cm soil layer in the current study. However, soil aggregates obtained from the top five centimetre on Westers s farm were considerably more stable than aggregates obtained from the same layer in the conventionally tilled adjacent field. Besides this, aggregate stability tends to increase with clay content (Tebrügge an Dürling, 1999; Amézketa, 1998), though the stability on the Westers farm was higher as would be expected based on the relatively low clay-content of this soil (10%). The stability could have been increased by earthworm-castings, because NIT is less destructive and could have decreased the mortality-rate of earthworms, which provide binding-agents to increase the stability of macro-aggregates. Moreover, SOM accumulation also increases the aggregate stability. 5.4 The effect of primary and secondary tillage on N mineralization Based on the results of the incubation study with sandy soil it, appears that weeding had no effect on N mineralization for both primary tillage practices. However, primary tillage did influence N mineralization, with a higher apparent N mineralization-rate of the topsoil occurring under NIT. The soil used for this incubation study was not ploughed for only one year, while increased N mineralization would only be expected five years after the latest ploughing-activity. This is related to sampling being confined to the uppermost fifteen centimetre, which combined with an accumulation of added soil amendments in the upper fifteen centimetre of the soil, probably have resulted in higher apparent N mineralization rates for NIT systems. In the literature it was reported that approximately 50% of the N min is present below fifteen centimetre when conventional tillage is being used. For this reason N mineralization rates at lower soil layers at conventional systems may thus be relatively high compared to NIT. Additionally, soil temperatures tend to be relatively high near the soil surface in the soil-columns, which combined with higher accumulation of applied organic amendments could have resulted in higher N mineralization-rates in the top fifteen centimetre with NIT. In addition to the total N min present in the columns on day 54 after initial column establishment, changes in N min over time were determined. Based on simple linear regression, deviations from the MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 51

62 trend line occurred on day 14 and 26, with values being either relatively high and low. In order to account for temperature effects, the temperature-sum was calculated and used to plot N min instead. However, this also failed to explain the deviation in linearity and in some cases the linear trend became even less clear (Table 9). The high temperatures occurring between day 14 and 26 were assumed to increase the overall N mineralization-rate. However, with the temperature sum approach being used even lower N min -values were measured on day 26 as would be expected compared to the linear trend. An explanation for this dip in the cumulative N min -content on day 26 could not be given. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 52

63 6. Synthesis Although the long-term effects of NIT, with root crops included in the rotation, on soil structure are not well-understood, there is increased interest in the use of NIT for both conventional and organic production systems in the Netherlands. Perceived benefits of NIT are often based on expectations rather than rigorous and/or long-term scientific studies, which are especially needed since the complex interactions between inherent soil properties, management practices and climatic conditions hampers development of standard recommendations that provide instant solutions. This thesis aimed at assessment of the effects of NIT on soil structure for a select group of farms throughout the Netherlands, while additionally examining the effect of secondary tillage on N mineralization. Based on current literature about effects of NIT a number of hypotheses were formulated, which are summarized in Table 10. Most hypothesis could not be confirmed by the results of this study, which was mostly due to significant sampling-errors related to rather prolonged droughts prior to sampling. Moreover, the overall inherent variations were typically rather large and changes in soil structure tend to be sluggish. The summary table not only outlines significant effects, but also lists the overall tendencies (non-significant trends). Thesetendencies are compared with expected changes in soil structure over time as far as the data are considered to represent the change of soil in time (Table 10). Hypotheses that were being confirmed showed that rooting depth increased across all farms, as was expected. Soil water retention was assumed to remain equal for both the and cm soil layers, while this only occurred in the upper subsoil. Other hypotheses were not fully confirmed since changes were mapped by tendencies rather than by significant effects. At both soil depths SOM tended to increase as related to increased soil layering and reduced SOM breakdown. Though the overall increase was lower at cm soil depth compared to the topsoil, differences were not significant for both layers. In terms of aggregate stability, the initial hypothesis that NIT would enhance inherent soil structure stability was only confirmed for the uppermost five centimetre of the soil profile on Westers farm, while the results at cm depth were less conclusive. As discussed before, prolonged dry periods appeared to have influenced soil-conditions, which explains some tendencies conflicting with initial hypotheses for P b d, MP and Pw pf2 at cm soil depth. Shrinkage by these dry periods in spring increased P b d and related decrease in MP, Ks and Pw pf2 values on the fine-textured soils. Therefore, it is quite difficult to come up with generic recommendations that hold across all soil types. Shrinkage may also have hampered earthworms to penetrate deeper into the soil profile. Nevertheless, sampling date and crops grown are main factors influencing the presences of earthworms. Since these factors differed between 2009 and 2011 in the current study, this complicates the interpretation of the results. The N mineralization incubation study was originally designed to focus on secondary tillage based on conflicting findings reported in the literature. Meanwhile, a significant higher N mineralization under NIT was found compared to INV. As discussed earlier, the sampling depth used (0-15 cm) may hampers correct interpretation of the results. Firstly, with the use of NIT applied soil amendments tend to be more concentrated in the uppermost 15 cm. Moreover, a combination of nutrient and/or SOM accumulation near the surface, with higher soil temperatures, resulted in higher apparent N mineralization rates. Whereas when looking at total N mineralization across the entire soil profile, the reverse may be true and during subsequent studies sampling should thus include deeper layers as well. Although no effects of secondary tillage under NIT as well INV were observed in this study, weeding tended to increase N mineralization. This effect was however not significant and either a longer incubation period or more replicates should be used to assess potential (long-term) effects. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 53

64 Table 10:Hypothesis and results as related to the effects of the use of NIT on the development of soil characteristics between 2009 and 2011 at five selected farms in the Netherlands Hypothesis, use of NIT will result in: Results, use of NIT actually: Soil pits Deeper root penetration over time Increased rooting depth between 2009 and 2011 Deeper earthworm-activity over time Soil structure Higher soil organic matter (SOM)-content at the 0-5 cm soil depth in 2011 (increase over time) Higher SOM-content at the cm soil depth in 2011 (increase over time) Larger accumulation of SOM- in the topsoil (0-5 cm) compared to cm soil depth layer Decreased bulk density (P b d ) at the cm depth Increase in macro porosity (MP) at the cm soil depth layer Decrease in penetration resistance (PR) at the cm soil depth layer Soil-water Increase in hydraulic conductivity (Ks) in soil peds at the cm soil depth layer No change in gravimetric soil water content at field capacity (Pw pf2 ) values at cm depth No change in Pw pf2 at cm depth layer Aggregate stability Increased aggregate stability at the 0-5 cm soil depth layer compared to INV Increased aggregate stability at cm soil depth layer compared to INV N mineralization Lower N mineralization in the top 15 compared to INV Higher N mineralization at the top 15 cm with increased intensity of superficial weeding Did not affect depth or traces of earthworm-activity SOM tended to increase Tended to enhance SOM Tended to result in an accumulation of SOM in the topsoil Increased in P b d, as far as results were unsuspected Tended to decrease MP, as far as results were unsuspected Tended to decrease PR values while it only increased on Brooijmans farm Tended to increase Ks values, as far as results were unsuspected Increased Pw pf2 values at cm, with farms with heavy soils showing the highest increase Tended to decrease Pw pf2 values slightly on unsuspected soil of Westers Tended to increase aggregate stability Tended to reduce aggregate stability Higher N mineralization under NIT than INV. However, this effect may be an artefact due to higher accumulation of readily mineralizable nutrients near the soil surface with NIT Overall N mineralization tended to be enhanced as the frequency of weeding was increased MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 54

65 7. Recommendations Positive aspects of sub-soiling under NIT were observed in this study in terms of favouring the formation of porous spots diagonally above the subsoil-tracks, provided that sub-soiling is implemented at suitable (dry) soil conditions. It seems that rooting depth is not dependent on sub-soiling, because roots occurred well-below the sub-soiling depth on all farms. Although a plough pan typically presents a clearly distinct zone that hampers root growth, below this zone the impeding actions of tillage gradually decreases. Once the roots break through this zone, their growth may no longer be confined on the condition that oxygen is available below the plough pan. So in the absence of a distinct compaction zone, the use of the subsoiler may be discontinued and/or its depth may be reduced after several years of practicing NIT. However, the question remains whether this is desirable, since no reliable values of Ks were obtained. Regardless, sub-soiling may stay an important tillage practice to enhance depth percolation and drainage. Moreover, the effect of sub-soiling on soil structure was found only directed diagonally towards the soil-surface and not across the entire working-depth. Reducing the depth of sub-soiling may results in the same effect as would be obtained with superficial topsoil loosening. In this study it seemed that sub-soiling till 20 cm depth was in general sufficient and depending on the farmers perception sub-soiling may be continued or replaced by another secondary tillage practice. In case permanent wheel-tracks are being used, it is recommended to vary the position of the shanks in the subsoiler sometimes. This should be done in such a way that existing bad-structured spots are also more effectively being loosened so that they will disappear as well over time. The results from this study clearly shows that the use of NIT, including regular sub-soiling, can maintain or improve well-structured soil in practise. It is recommended to apply a deeper working depth initially to remove pre-existing compaction layer, while sub-soiling on the longer term may not be required anymore as biological processes increasingly start contributing to inherent physical soil fertility, including the formation of a well-structured soil and deep rooted cropping system. It is also recommended to avoid sub-soiling under wet soil circumstances to avoid smearing, as was clearly seen in the soil pits on the farm of Steendijk (Figs ). It should be kept in mind that this study did not entail a comparison between NIT and INV, but that the main focus of this study was to examine the development of soil structure of NIT systems over time. Based on this, it is concluded that NIT can result in a well-structured soil, regardless whether the use of NIT is better or not compared to conventional tillage. In addition to enhancing inherent soil structure, many more and potentially more compelling arguments could be given to apply NIT, but the overall scope of the current study has been to show that use of NIT is physically feasible. MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 55

66 References Anderson, M.K. and Jensen, L.S. (2001). Low soil temperature effects on short-term gross N mineralisation-immobilisation turnover after incorporation of a green manure. Soil Biology and Biochemistry, vol. 33, p Askegaard, M., Olesen, J.E., Rasmussen, I.A. and Kristensen, K. (2011). Nitrate leaching from organic arable crop rotations is mostly determined by autumn field management. Agriculture, Ecosystems and Environment, article in press (available online). Baker, J.M., Ochsner, T.E., Venterea, R.T. and Giffis, T.J. (2007). Tillage and soil carbon sequestration: what do we really know? Agriculture, Ecosystems and Environment, vol. 118, p.1-5. Balan, A., Raus, L. and Jitareanu, G. (2009). Effects of tillage managemetn on soil porosity and bulk denisity on rape (BrasicaNapus). Bulletin UASVM agriculture, vol. 66, issue 1, p Berner, A., Hildermann, I., Fliessbach, A., Pfiffner, L., Niggli, U. and Mäder, P. (2008). Crop yield and soil fertility response to reduced tillage under organic management. Soil and Tillage Research, vol. 101, p Berner, A., Messmer, M., Dierauer, H. and Mäder, P. (2010). Reduced soil tillage in organic farming for improvement of soil fertility and mitigation of greenhouse gases. Research Institute of Organic Agriculture (FiBL), powerpoint institute of Botany, University of Basel. BlggAgroxpertus (2011). https://blgg.agroxpertus.nl/over-blgg-agroxpertus/documents/overzicht % 20geaccrediteerde%20analyses.pdf,p.2. (Viewed on ). Bond, W. and Grundy, A. C. (2001). Non-chemical weed management in organic farming systems. Weed Research, vol. 41, p Cannell, R.Q. (1985). Reduced tillage in North-West Europe A review. Soil and Tillage Research, vol. 5, p Carter, M.R. and Sanderson, J.B. (2001). Influence of conservation tillage and rotation length on potato productivity, tuber disease and soil quality parameters on a fine sandy loam in eastern Canada. Soil and Tillage Research, vol. 63, p Casa, R. and Lo Cascio, B. (2008). Soil conservation tillage effects on yield and water efficiency on irrigated crops in central Italy. J. Agronomy and Crop Science, vol. 194, issue 4, p Cassel, D.K. and Nelson, L.A. (1985). Spatial and temporal variability of soil physical properties of norfolk loamy sand as affected by tillage. Soil and tillage reseach, vol. 5, p Cassman, M. G. and Munns, D. N. (1980). Nitrogen mineralization as affected by soil moisture, temperature and depth. Soil science of America journal, vol. 44, issue 6, p Czyz, E.A. and Dexter. (2009). Soil physical properties as affected by traditional, reduced and notillage for winter wheat. International Agrophysics, vol. 23, p Dessureault-Rompré, J.D., Zebarth, B.J., Georgallas, A., Burton, D.L., Grant, C.A. and Drury, C.F. (2010). Temperature dependence of soil nitrogen mineralization rate: Comparison of mathematical models, reference temperatures and origin of the soils. Geoderma, vol. 157, p D Haene, K., Vermang, J., Cornelis, W.M., Leroy, B.L.M., Schiettecatte, W., De Neve, S., Gabriels, D. and Hofman(2008). Reduced tillage effects on physical properties of silt loam soils growing root crops. Soil and Tillage Research, vol. 99, p EijkelkampAgrisearch Equipment (2010). Penetrologger users manual. PenetroViewer version 5.08, March. Eijkelkamp (2008). Nattezeefmethode ,July Ekeberg, E. and Riley, H.C.F. (1996). Effects of mouldboard ploughing and direct planting on yield and nutrient uptake of potatoes in Norway. Soil and Tillage Research, vol. 39, p Emmerling, C. (2007). Reduced and conservation tillage effects on soil ecological properties in an organic farming system. Biological Agriculture and Horticulture, vol. 24, issue 4, p MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 56

67 Franzluebbers, A.J. (1999). Potential C and N mineralization and microbial biomass from intact increasingly disturbed soils of varying texture. Soil Biology and Biochemistry, vol. 31, p Garcia-Torres, L. and Martinez-Vilela, A. (2000) Conservation agriculture in Europe: Environmental and economic perspectives. Man and soil at the third millenium. Proceedings International Congress of the European Society for Soil Conservation, Valencia, Spain, 28 March 1 April Geerse, T. (2010). Seedbed preparation in an organic minimum tillage system. Thesis farm technology, 30 April. Gilmour, J.T. and Mauromoustakos, A. (2011). Nitrogen mineralization from soil organic matter: a sequential model. Nutrient Management and Soil and Plant Analysis, vol. 75, issue 1, p Govaerts, B., Verhulst, N., Castellanos-Navarrete, A., Sayre, K.D., Dixon, J. and Dendooven, L. (2009). Conservation agriculture and soil carbon sequestration: Between myth an farmer reality. Critical Reviews in Plant Science, vol. 28, p Hermawan, B. and Cameron, K.C. (1993). Structural changes in a silt loam under long-term conventional tillage. Soil and Tillage Research, vol. 26, p Hofmeijer, M. (2010) The effects of non-inverted tillage on weed diversty and nutrient cycling in organic agriculture on sandy soil. Bachelor thesis, Biological Farming Systems Group, Wageningen University. Holland, J.M. (2004). The environmental consequences of adopting conservation tillage in Europe: reviewing the evidence. Agriculture, ecosystems and environment, vol. 103, p Hoogmoed, W. (2010). Conservation agriculture, in FTE March 18. Houba, V.J.G. and Novozamsky. (1998). Certification of an air-dry soil for ph and extractable nutrients using one hundredth molar calcium chloride. Communications in Soil Science and Plant Analysis, vol. 29, issue Houba, V.J.G., Temminghoff, E.J.M., Gaikhorst, G.A. and van Vark, W. (1999). Soil analysis procedures ectraction with 0.01 M CaCl 2. Wageningen agricultural university, department of environmental science. Hudson, B.D. Soil organic matter and available water capacity. Journal of Soil and Water Conservation, vol. 49, issue 2, p Jarvis, S.C., Stockdale, E.A., Shephard, E.A. and Powlson, D.S. (1996) Nitrogen mineralization in temperate agricultural soils: processes and measurements. Advances in Agronomy, vol. 57, p Johnsson, H., Bergström, L. and Jansson, P.E. (1987). Simulated nitrogen dynamics and losses in a layers agricultural soil. Agriculture, Ecosystems, and Environment, vol. 18, p Johnson-Maynard, J.L., Umiker, K.J. and Guy, S.O. (2007). Earthworm dynamics and soil physical properties in the first three years of no-till management. Soil and Tillage Research, vol. 94, p Jordan, D., Stecker, J.A., Cacnio-Hubbard, V.N., Li, F., Gantzer, C.J. and Brown, R. (1997). Earthworm activity in no-tillage an conventional tillag systems in Missouri soils: A preliminary study. Soil Biology and Biochemistry, vol. 29, issue 3-4, p Kladivko, E.J. and Keeney. (1987). Soil nitrogen mineralization as affected by water and temperature interactions. Biology andfertility of Soils, vol. 5, p Klute, A. and Dirksen, C. (1986). Hydraulic conductivity and diffusivity: laboratory methods. Agronomy, issue 9, p Koepke, U. (2003). Conservation agriculture with and without use of agrochemicals. II world congress on conservation agriculture: producing with harmony with nature, Foz do Iguassu, Brazil, August. Kong, A.Y.Y., Fonte, S.J., Kessel, C. and Six, J. (2009). Transition from standard to minimum tillage: Trade-offs between soil organic matter stabilization, nitrous oxide emissions, and N availability in irrigated cropping systems. Soil and Tillage Research, vol. 104, p MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 57

68 KoninklijkNederlandsMeteorologischInstituut (KNMI). selection.cgi (Viewed on ). Koninklijk Nederlands Meteorologisch Instituut (KNMI). (Viewed on ). Koopmans, C., Zanen, M. and ter Berg, C. (2008). De kuil, bodembeoordeling aan de hand van een kuil. Louis Bolk Instituut, version 3, November Kouwenhoven, J.K., Perdok, U.D., Boer, J. and Oomen, G.J.M. (2002). Soil management by shallow mouldboard ploughing in the Netherlands. Soil and Tillage Research, vol. 65, p Krauss, M., Berner, A., Burger, A., Weiemken, A., Niggli, U. and Mäder, P. (2010). Reduced tillage in temperate organic farming: implications for crop management and forage production. Soil Use and Management, vol. 26, p Kuipers, H., Water content at pf2 as a characteristic in soil cultivation research in the Netherlands. Neth. Journal of Agric. Science 9, Kwaad, F.J.P.M. and van Mulligen, E.J. (1991). Croppings systems effect of maize on infiltration, runoff and erosion on loess soils in South-Limbourg (The Netherlands): a comparisson of two rainfall events. Soil Technology, vol. 4, issue 3, p Lal, R. and Kimble, J.M. (1997). Conservation tillage for carbon sequestration. Nutrient cycling in Agroecosystems, vol. 49, p Leblanc, M.L. and Cloutier, D.C. (2001). Susceptibility of dry edible Bean (Phaseolus vulgaris, Cranberry bean) to the rotary hoe. Weed Technology, vol. 15, p Lopéz, M.V., Arrúe, J.L. and Sánchez-Girón, V. (1996). A comparison between seasonal changes in soil water storage and penetration resistance under conventional and conservation tillage systems in Aragón. Soil and Tillage research, vol. 37, p Miller, J.J., Larney, F.J. and Lindwall, C.W. (1998). Physical properties of a Chernozemic clay loam soil under long-term conventional tillage and no-till. Canadian Journal of Soil Science, vol. 79, issue 2, p Munkholm, L.J., Schjonning, P. and Rasumssen, K.J. (2001). Non-inversion tillage effects on soil mechanical properties of a humid sandy loam. Soil and Tillage Research, vol. 62, p Olesen, J.E. and Munkholm, L.J. (2007). Subsoil loosening in a crop rotation for organic farming eliminated plough pan with mixed effects on crop yield. Soil and Tillage Research, vol. 94, p Owen, J., Leblanc, S. and Fillmore, S.A.E. (2006). Short-term effect of soil disturbance by mechanical weeding on plant available nutrients in an organic vs conventional rotations experiment. Aspects of Applied Biology, vol. 79, p Pang, X.P., Letey, J. and Wu, L. (1997). Irrigation quantity and uniformity and nitrogen application effects on crop yield and nitrogen leaching. Soil Science Society America Journal, vol. 61, Jan- Feb. Peigné, J., Ball, B.C., Roger-Estrade, J. and David, C. (2007). Is conservation tillage suitable for organic farming? A review. Soil use and management, vol. 23, p Schjonning, P. and Rasmussen, K. J. (2000). Soil strength and soil pore characteristics for direct drilled and ploughed soils. Soil and Tillage Research, vol. 57, p Sierra, J. (2002). Nitrogen mineralization and nitrification in a tropical soil: effects of fluctuating temperature conditions. Soil Biology and Biochemistry, vol. 34, p Smatana, J., Macák, M. and Demjanová, E. (2010). The influence of different tillage practices on soil physical characteristics. Research journal of agricultural science, vol. 42, issue 3, p Smolen, M. Effects of no-till on water quality. Chapter 3, p Soil survey staff. (1975). Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys. USDA Handbook 436, US govt, Print. Off, Washington, p Stagnari, F., Ramazzotti, S. and Pisante, M. (2009) Conservation Agriculture: A different approach for crop production through sustainable soil and water management: A review. Sustainable Agriculture Reviews, vol. 1, p MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 58

69 Steinmann, H.H. (2002). Impact of harrowing on the nitrogen dynamics of plants and soil. Soil and Tillage Research, vol. 65, p Stockfisch, N., Forstreuter, T. and Ehlers, W. (1999). Ploughing effects on soil organic matter after twenty years of conservation tillage in Loer Saxony, Germany. Soil and Tillage research, vol. 52, p Tebrügge, F. and Düring, R.A. (1999). Reducing tillage intensity a review of results from a long-term study in Germany. Soil and Tillage Research, vol. 53, p Trewavas, A. (2004). A critical assessment of organic farming-and-food assertions with particular respect to the UK and the potential environmental benefits of no-till agriculture. Crop Protection, vol. 23, p University of Akron, specific gravity of soils. Vakali, C., Zaller, J.G. and Köpke, U. (2011). Reduced tillage effects on soil properties and growth of cereals and associated weeds under organic farming. SoilandTillageResearch, vol. 111, p Van der Weide, R., van Alebeek, F. and Van den Broek, R. (2008). En de boer, hij ploegde niet meer? Literatuurstudie naar effecten van niet kerende grondbewerking versus ploegen. Praktijkonderzoek Plant en Omgeving B.V. Vermeulen, B., Van Velde, P. and Verwijs, B. (2009). Bodemfysische waarnemingen niet-kerende grondbewerking op praktijkbedrijven. Meetrapport plant research international Wageningen UR, nota 648. Vian, J.F., Peigné, J., Chaussod, R. and Roger-Estrade, J. (2009). Effects of four tillage systems on soil structure and soil microbial biomass in organic farming. Soil Use and Management vol. 25, p Wang, XB., Cai, DX., Hoogmoed, W.B., Oenema, O. and Perdock, U.D. (2006). Potential effect of conservation tilllage on sustainable land use: A review of global long-term studies. Vol. 16, issue 5, p Watson, C.A., Atkinson, D., Gosling, P., Jackson, L.R and Rayns, F.W. (2002). Managing soil fertility in organic farming systems. Soil Use and Management, vol. 18, p Wortman, S.E., Lindquist, J.L., Haar, M.J. and Francis, C.A. (2010). Increased weed diversity, density and above-ground biomass in long-term organic crop rotations. Renewable Agriculture and Food Systems, vol. 25, issue 4, p MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 59

70 Appendix I. Measurement locations and field drafts A) Brooijmans Location measurements Farm Brooijmans Location measurements MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 60

71 MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 61

72 B) Van Hootegem Farm van Hootegem Location measurements Farm van Hootegem Location measurements MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 62

73 MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 63

74 C) Van Tiggelen Location measurements Farm van Tiggelen Location measurements MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 64

75 MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 65

76 D) Steendijk Farm Steendijk Location measurements Farm Steendijk Location measurements MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 66

77 MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 67

78 E) Westers Farm Westers Location measurements Farm Westers Location measurements MSc Thesis Plant Sciences, January 2012 Tom Hollemans Page 68

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