IMPACTS OF USING MULCH TO CONSERVE SOIL MOISTURE

Transcription

1 IMPACTS OF USING MULCH TO CONSERVE SOIL MOISTURE Written by: ASHLEY RATCLIFF

2 Background The benefits of using mulch in vineyards are well documented. Both scientific and subjective observations have shown that mulch suppresses weed growth, increases soil fertility and conserves soil moisture. The Riverland, the largest wine growing region in Australia, has experiences several years of drought, which has significantly impacted grape growers throughout the region. Drought has led to water allocations being slashed, meaning growers have had to employ new strategies to enable them to continue to grow a commercial viable crop. One strategy has been to buy in temporary water on an annual basis to make up the difference between the water needed to grow a commercial crop and the permanent water a grower has left after water allocation restrictions have been applied. At a price of <$100/Ml, the purchase of temporary water is economically viable (viability is also dependant on grape prices and yield), but when the price of temporary water surges to $600-$1000/Ml the use of temporary water as a long-term strategy in times of severe water restrictions is not sustainable. While the use of mulch has been employed as a strategy to conserve water through reducing evaporation of water from the soil surface, its use in the Riverland has been limited. Banrock Station used mulch during the recent drought in their vineyards to help limit the amount of irrigation water that needed to be applied to maintain a viable crop. While on face value this strategy appeared to work, the amount of water actually saved through the application of mulch was not tracked or recorded. The use of mulch by growers in the Riverland may be limited due to availability of mulch, cost of mulch and the understanding of the actual benefits of using mulch, in particular the water savings that could potentially be achieved. If the water savings of mulch could be demonstrated to growers in a fashion that they can understand, then the inclusion of mulch in strategies to mitigate crop loss in times of severe water restrictions could be adopted. YEAR 1 Trial Description A Chardonnay vineyard located in Barmera (Riverland, South Australia) has been selected as the vineyard where this mulch trial will be established. The Chardonnay was planted in 2002 and is grafted on Schwarzmann rootstock. The soil description is; red clay loam over limestone. The vineyard is irrigated via drip irrigation and is currently supplied 5.5Ml/Ha/year of irrigation. An application of Jeffries soil conditioning mulch was applied in early October 2011 at a rate of 90 cubic meters per Ha. The drip line was covered by the mulch. The area of vineyard mulched is approximately 1.9Ha, while the control was 0.2Ha. GWRDC MULCH TRIAL Page 2

3 To measure the influence mulch has on soil moisture, two Adcon Soil moisture monitoring probes have been installed in the trial, one in the mulched area and one in the control area (see map below probes 1 & 2). Four other soil moisture monitoring probes have been installed (please note that all probes have the capacity to measure soil conductivity) in the vineyard. Probes 3 & 4 are located in a patch of Chardonnay grafted in Ramsey rootstock. Probe 3 is located in the control section, while probe 4 is located in the mulched section. Probes 5 & 6 are located in a patch of Chardonnay that grows on their own roots. Probe 5 is located in the mulched section while probe 6 is located in the control (unmulched). The purpose of the different sites is to determine; A) will mulch have an impact on soil moisture conservation, B) will the combination of rootstock and mulch improve soil moisture conservation and C) will the use of mulch influence soil salinity. To gain a better understanding of the impact of mulch on soil health, soil fertility will be evaluated through soil analysis. Samples will be taken during the winter months, eight months after the application of the mulch. Communicating the Results The soil probes used in this project capture data every 15 minutes. This data is then displayed via the Adcon website. To allow growers and other interested parties to monitor the impact of mulch on soil moisture conservation, access to live data from each of the six probes will be made available via the Adcon website. Other information about the trials (rainfall, irrigation applied and irrigation frequency) will also available on the website. GWRDC MULCH TRIAL Page 3

4 1 5 Row 5-8 Row Row Row 1-4 GWRDC MULCH TRIAL Page 4

5 Picture 1 Mulch prior to application Picture 2 Application of mulch GWRDC MULCH TRIAL Page 5

6 Picture 3 Pre-mulch application Picture 4 Post-mulch application GWRDC MULCH TRIAL Page 6

7 Picture 5 Mulching depth Picture 6 Soil moisture monitoring probe GWRDC MULCH TRIAL Page 7

8 Observations/Results These observations/results review general trends in data and do not interrogate the results using statistical analysis. Soil Moisture The graph below clearly demonstrates that the soil moisture levels are greater where mulch has been applied. While the volume of water saved cannot be determined, the intervals between irrigation were extended as a result of the mulch (see recommendations for future work to assess water saving benefits of using mulch). In all three treatments the mulching has led to higher levels of soil moisture. In stating this, the soil moisture levels in the un-mulched own-roots section are higher than the mulched section of the Schwarzmann. Considering that the lowest soil moisture of all treatments was in the un-mulched Schwarzmann, it may suggest that the soil in this part of the vineyard has a lower RAW than the other sections where the own rooted and Ramsey vines are located? The mulches ability to converse soil moisture actually encouraged weed growth where the mulch cover of the soil was minimal. This was mainly observed been the rows where a thin layer of mulch was deposited at the time of spreading. Where the mulch was applied at a heavier rate (under vine) weed growth was minimal, although regular applications of herbicide did not allow for the growth of weeds hence it cannot be concluded that heavy rates of mulch suppressed weed growth. Graph 1 Own Roots GWRDC MULCH TRIAL Page 8

9 Graph 2 Ramsey Rootstock Graph 3 Schwarzmann Rootstock GWRDC MULCH TRIAL Page 9

10 Graph 4 All Sites Maturity Analysis & Yield While the data below in table 1 shows trends between the control and mulched sections, these trends demonstrate not actual patterns. There is no difference in bunch weights between the own rooted section, while there is a positive relationship between the mulch and control for the Schwarzmann section and a negative relationship for the Ramsey. These results cannot confirm if the mulch or rootstocks are having an impact on yield. The actual yield results at harvest did deliver a positive response for the Schwarzmann section. The difference between the control and mulch was 1.5kg/vine. Extrapolating this data leads to a 2.5t/Ha increase in yield in the mulch section. The actual yield data at harvest did not show a difference for the own rooted or Ramsey sections. The impact on grape maturity showed a difference between the control and mulched sections of the Schwarzmann rootstock. The increase in yield in the mulched sections may have lead to the difference in Baume between the mulched and control sections. The other treatments showed no real difference in changes in grape maturity. GWRDC MULCH TRIAL Page 10

11 Table 1: Yield and Grape Maturity Description Treatment Weight (g/bunch) Be TA ph Own roots Mulch (sample 1) Own roots Mulch (sample 2) Own roots Mulch (sample 3) AVE own roots Control (sample 1) own roots Control (sample 2) own roots Control (sample 3) AVE Schwarzmann Mulch (sample 1) Schwarzmann Mulch (sample 2) Schwarzmann Mulch (sample 3) AVE Schwarzmann Control (sample 1) Schwarzmann Control (sample 2) Schwarzmann Control (sample 3) AVE Ramsey Mulch (sample 1) Ramsey Mulch (sample 2) Ramsey Mulch (sample 3) AVE Ramsey Control (sample 1) Ramsey Control (sample 2) Ramsey Control (sample 3) AVE Plant & Soil Nutrition Both soil and petiole samples were collected in February The analysis was completed by APAL. The table below outlines the results from the samples taken from the mulched and control sections in the Schwarzmann patch only. While there are some macro and micro nutrient imbalances in the vineyards, the petiole analysis do not display any significant differences between the control and the mulched sections. The only point of interest was the difference in nitrate. The mulch section is showing a lower level than the control which may indicate that nitrate levels may be less available to the vines in the mulched section due to nitrate assimilation. The soil analysis, like the petiole analysis, does not show significant differences between the mulched and control sections. The outliers are iron and zinc, which have had a negative impact as a result of the application of mulch. GWRDC MULCH TRIAL Page 11

12 Table 2: Petiole Results Element Mulch Comment Control Comment Target Nitrogen 0.84% Normal 0.78% Below Normal 1.03% Nitrate <113ppm Deficiency 135ppm Deficiency ppm Sulphur 0.13% Below Normal 0.13% Below Normal 0.41% Phosphorus 0.09% Deficiency 0.07% Deficiency 0.40% Potassium 1.99% Normal 2.04% Normal 3.00% Magnesium 1.04% Above Normal 1.21% Above Normal 0.68% Calcium 2.33% Normal 2.75% Above Normal 2.18% Sodium 0.30% Normal 0.26% Normal 0.25% Chloride 1.45% Above Normal 2.20% Above Normal 0.88% Iron 43.34% Normal 28.85% Below Normal % Aluminium 55.18% 39.58% Manganese 98.53% Normal 82.29% Normal % Boron 41.98% Normal 40.44% Normal 60.00% Copper 5.26% Below Normal 3.48% Deficiency 32.50% Zinc 92.36% Excessive 62.73% Above Normal 36.50% Molybdenum 0.29% Normal 0.27% Normal 0.36% Table 2: Soil Analysis Test/Element Mulch Comment Control Comment Target Total Exchangeable Capacity TEC Acceptable 22.18TEC Acceptable 12-25TEC Colloidal Organic Matter 1.80% Very Low 1.70% Very Low 4-6% PH Water (water) 8.4 High 8.2 High Nitrogen (kg/ha) 57 Low 55 Low Sulphate (ppm) 18 Acceptable 22 Acceptable Phosphorous (Olsen) (ppm) 27 Low 40 Acceptable Phosphorous (Bray 2) (kg/ha) 286 Excessive 304 Excessive Calcium (ppm) 7276 High 7482 High 3238 Magnesium (ppm) 842 Acceptable 801 Acceptable 375 Potassium (ppm) 1673 Excessive 1327 Excessive 774 Sodium (ppm) 275 Acceptable 165 Acceptable 122 Salinity (ds/m) 0.19 Acceptable 0.16 Acceptable <0.15 Boron (ppm) 0.77 Very Low 0.75 Very Low >1.5 Iron (ppm) Low Very Low Manganese (ppm) Acceptable Acceptable Copper (ppm) 7.07 High 3.61 Acceptable >2.0 Zinc (ppm) Acceptable Excessive >8.0 Aluminium (ppm) 0.08 Very Low 0.08 Very Low <2.0 GWRDC MULCH TRIAL Page 12

13 Conclusions Year 1 The trial so far is showing potential water saving when mulch is applied to vineyards in the Riverland. Further assessment will be required to determine the actual amount of water that can be saved. At this point in time it is too early to make any firm recommendation on the use of mulch, with the exception that there is apparent water saving. The cost of the mulch is still a consideration that does not work in favour of using mulch that is trucked in from Adelaide. A total 180 cubic meters of mulch was applied at a cost of approximately $5,000 (includes freight). At a rate of 90 cubic meters of mulch per hectare, or $2,500/ha, the cost equate to 113Ml/Ha of temporary water (calculated at a rate of $22/Ml). The impact that mulch has on yield, wine quality, vine and soil health is still very much unknown. The data presented in this report does not provide a clear insight into the positive or negative impact of mulch with the exception of potential water savings. More time is required to assess how the vineyard and the soil change via the introduction of mulch. There are some interesting differences in the soil analysis which will require further investigation. A holistic approach should be taken when considering the use mulch. The cost benefits associated with using mulch in a vineyard will be reduced irrigation demands, less pumping costs, less infrastructure wear and tear, and less reliance of water from the Murray Darling Basin. If the use of mulch can be linked to better soil health, increase yields and the production of superior wines, then the use of mulch in vineyard should be further encouraged. One of the key barriers to the adoption of mulch, irrespective of all the benefits, is the cost of transport from Adelaide to the Riverland. YEAR 2 While the trial outlined above in this paper has concluded that mulch applied directly under a vine will save water, the amount of water saved is still to be determined. To establish the potential volume of water to be saved over a growing season a new was established. This new trail was established to determine the water savings through changing the existing drip tube to a lower output drip tube. Irrigation time, mulch and climate will be constants throughout the trial, with the variable being dripper output. The change to irrigation outputs were as follows. The application of the reduced irrigation treatments was achieved by replacing the drip tube along a complete row of vines. 1L/hr 1.6L/hr 2.3L/hr The location of the vineyard in the Year 2 trial also changes, as did the addition of an additional variety and mulch. The vineyard is located in Loveday (Riverland, South Australia). The soil is best described as a sandy loam over clay. The varieties included in this trail include Chardonnay (grafted on Schwarzman) and Merlot (grown on own roots). An evaluation of forest mulch was also conducted. Forest mulch contains a higher proportion of wood chips than the compost applied in the previous trial (year 1). The assumption is that the forest mulch will persist longer in the vineyard GWRDC MULCH TRIAL Page 13

14 over time, but the nutritional values and water saving advantages may be less than the compost (For the remainder of this report the two different mulches will be referred to as forest mulch and compost). Application Rates, Costs & Observations (Chardonnay) In the Chardonnay trial only compost was applied. The rate of compost applied at approximately 450 cubic metres per hectare (five times the rate of the applied rate for Year 1 project). At a cost of $30 per cubic metre this equates for $13,500/Ha (does not include freight of spreading costs). The reason for applying the higher rates was to extend the life of the compost. The aim is to get a minimum of seven years of value from the compost (hence a price of $1,928/yr). It is a fair assumption to make that at a rate of 450 cubic metres per hectare the cost of applying compost is prohibitive. As of middle of December 2012, a total of 250hrs of irrigation had been applied. The total Ml/Ha/treatment applied to date (9/12/2012) is outlined below 1L/hr drip tube: 1.6L/hr drip tube: 2.3L/Hr drip tube: 1.2Ml/Ha applied (55% less than 2.3L/hr) 1.9Ml/Ha applied (27% less than 2.3L/hr) 2.7Ml/Ha applied The soil moisture graph below shows that the 2.3L/hr treatment stays wetter for longer that the other treatments. The 1.6L/hr treatment rate of drying appears more rapid than the 2.3 and 1L/hr treatments. This graph, plus the appearance of the vineyard suggests that the irrigation frequency and amount of irrigation could be reduced. None of the three treatments are showing and signs of water stress. Crop level and pruning weights still need to be evaluated. Visually the 1L/hr treatment has produced a smaller canopy than the other treatments. Despite this the canopy is adequate to provide the fruit protection from the sun. GWRDC MULCH TRIAL Page 14

15 Application Rates, Costs & Observations (Merlot) In the Merlot trial both the forest mulch and compost were applied. Both the mulch and the compost were applied at substantially less rate than the Chardonnay, (90 verses 450 cubic metres per hectare). This equates to $2,700/Ha. It is assumed that the beneficial life of the mulch and compost will differ (mulch lasting longer than the compost due to a higher proportion of fibrous material see pictures below). It is also assumed the compost in the Merlot trial will have a shorter beneficial life than the compost applied to the Chardonnay due to the difference in application rates. Observations from the Year 1 project suggest that the compost should last at least three years. Therefore the cost per year/ha for the mulch and compost, at a rate of 90 cubic metres per hectare, is $675 - $900 (working on a 3-4 year of beneficial life). As of middle of December 2012, a total of 225hrs of irrigation had been applied. The total Ml/Ha/treatment applied to date (9/12/2012) is outlined below 1L/hr drip tube: 1.6L/hr drip tube: 2.3L/Hr drip tube: 0.95Ml/Ha applied (57% less than 2.3L/hr) 1.5Ml/Ha applied (32% less than 2.3L/hr) 2.2Ml/Ha applied (Note the current application rates for the entire vineyard was checked with CIT. As of the 9/12.12 the water consumption for the 6.5Ha vineyard was 14.8Ml. The calculation for the 2.3L/Hr is 14.3Ml, hence supporting the volumes outlined above). G-Dots were used to measure the soil moisture in the Merlot trial. A total of two G-Dots per treatment were used (placed at soil depths of 20 & 40cm) in total 18 G-Dots were installed. G-Dots are an electrical resistance blocks attached to a reader that displays the soil moisture via yellow dots (see picture below). G-Dots were used as they were cost effective and the provided a visual display of the soil moisture in the vineyard. Observations in the vineyard suggested that the control (no mulch or compost) dried out the quickest. Of the two treatments, the compact retained soil moisture for a longer period of time. The canopy growth of the 1Ltr/hr treatment appears less than the 1.6 and 2.3Ltr/hr treatment. While there was less canopy growth the growth was not detrimental to crop protection from the sun. No heat stress was observed in any of the treatments. Discussions/Strategies While temporary water prices remain low (current price of $40/Ml) it is very difficult to justify the use of mulch/compost to save water. But this trial was developed to gain a better understanding of using mulch to mitigate the increased cost of temporary water prices during extended period of drought. During the recent drought serve water restriction drove the price of temporary water past $1,000/Ml. Those irrigators who took the approach of managing their vineyard through limiting GWRDC MULCH TRIAL Page 15

16 water without the assistance of water saving tools (mulch, rootstocks.) paid the price via lower crops and declined vine health. Most irrigators during the recent drought adopted strategy number 1. For a 6.5Ha vineyard, irrigating with 5.5Ml/Ha the cost of buying temporary water for a 4 year period was equate to $63,000 (does not take into consideration interest on borrowings This was equate to $80,640) Vineyard observations from the mulch/compost trial outlined in this report suggests that the use of compost at 90 cubic metres per hectare with a beneficial life of 4 years and using 1.6Ltr/hr dripper could be a strategy that maintains yield while providing a financial benefit over a drought that lasts 4 years. It must be noted that the impacts on yield and vine health through the use of lower irrigation and application of compost still needs to be determined. The cost of applying this strategy is $54,270 (with interest $69,465) (a financial benefit that is $11,175 ahead of strategy 1). Strategy 3 reduces irrigation rates to more than 55%. Observations from this trial have already shown that this low irrigation rate has restricted canopy growth, although the growth at this stage does not pose a problem. The impact on yield also needs to be determined at harvest, but vineyard inspections of the lowest irrigation treatment have not caused any concerns regarding significant yield decline. If yield and vine health is not significantly impacted by the use of compost and mulch and the application of 2.3Ml/Ha/Yr then this strategy provides a real cost saving opportunity. The cost of applying this strategy for a 6.5Ha vineyard for a 4 year period is $37,390 ($47,859 after interest). This provides a financial relief of $32,781 over strategy 1 and $21,606 over strategy 2 (these financial reliefs assume there is no change in crop level). The performances of these results could possibly be enhanced through the use of drought tolerant rootstocks (Schwarzman and own rooted vines have been used in this trial, both which are not drought tolerant). Soil and petiole test have been collected from all the trial sites. At this point in time there is not nutritional difference between the mulch/compact treatments and the control areas of the vineyard. Due to the dry growing seasons for the past two years it has been difficult to determine if the mulch suppresses weed growth. One problem observed has been the growth of vineyard cuttings under the vines. This is only a problem in the heavily composted trial (450 cubic metres per hectare). So what are the limitations of using mulch in the Riverland? 1. Cost of freight from Adelaide the freight costs are nearly one third of mulch price (Solution = source mulch from a closer location) 2. Contamination of mulch this was not an issue with the mulch/compost supplied from Jeffries. GWRDC MULCH TRIAL Page 16

17 STRATEGY 1 - BUY WATER Vineyard Area 6.5 Ha Existing Water Allocation 35 ML Irrigation Allocation 0.25 Usable Allocation 8.75 ML Irrigation Rate/Ha (Normal Year) 5.5 ML Required Water to be Purchased to Achieve 5.5Ml/Ha Temporary Water Prices (During Drought) $ 600 Years of Drought 4 Total Water Purchase During Drought $ 63,000 STRATEGY 2 - REDUCE IRRIGATION (1.6Ltr/Hr) Required Water to Achieve 3.7Ml/Ha (1.6Ltr/Hr) ML Required Water to be Purchased to Achieve 3.7Ml/Ha 15.3 ML Temporary Water Prices (During Drought) $ 600 Total water Purchase During Drought $ 36,720 Price of Compost (Year 1 Only) $ 17,550 Total Water Purchase During Drought (+ Compost) $ 54,270 STRATEGY 3 - REDUCE IRRIGATION (1Ltr/Hr) Required Water to Achieve 2.3Ml/Ha (1Ltr/Hr) ML Required Water to be Purchased to Achieve 2.3Ml/Ha 6.2 ML Temporary Water Prices (During Drought) $ 600 Total water Purchase During Drought $ 19,840 Price of Compost (Year 1 Only) $ 17,550 Total Water Purchase During Drought (+ Compost) $ 37,990 Further Work The monitoring and evaluation of the mulch verses the control will continue throughout the 2013/14 growing season. In addition, petiole and soil samples will be collected and tested for nutrients. The nutritional make-up of the mulch will also be tested. An extra soil probe will be installed in a control/non-mulched section of the vineyard to confirm the water saving in year two of the project (Chardonnay). Acknowledgements This project would not have been possible without the assistance of the Riverland Viticulture Technical Group, Jeffries, Riverland Wine, GWRDC, Woolworths and Landcare. GWRDC MULCH TRIAL Page 17