ASH AS A FOREST FERTILISER

Transcription

1 ASH AS A FOREST FERTILISER 1

2 The properties of ash Approximately 600,000 tonnes of wood-, peat- and mixed-ash is generated in Finland annually as by-product of energy production. Ash has a good neutralising capacity (ph 10 13), and pure wood ash contains all the nutrients that trees need to grow in the right proportions, except nitrogen (N). Peat ash, on the other hand, releases a smaller quantity of potassium (K) for trees than wood ash. In addition to nutrients, certain heavy metals contained by wood and peat such as cadmium (Cd) and arsenic (As) are also concentrated in ash. The element concentrations of various mixed ashes differ considerably depending on the incinerated material and the incineration technique used. The fuel that is used affects the element composition and quality of the ash. Ash quality requirements and pre-treatment Ash that has been generated in the incineration of wood, peat or field biomass can be used as a forest fertiliser. In Finland, the utilisation of ash as fertiliser is regulated by the Fertiliser Product Act (539/2006) and related decrees (Ministry of Agriculture and Forestry Decree 24/11). The Finnish Food Safety Authority Evira is in charge of the supervision of fertiliser products. Ash that is used as a forest fertiliser must have a combined concentration of phosphorus (P) and potassium (K) exceeding 2% and at least 6% of calcium (Ca). The decree (Ministry of Agriculture and Forestry Decree 24/11) also specifies the maximum concentrations allowed for 2 harmful heavy metals: the cadmium concentration of ash, for example may not exceed 25 mg/kg, and the arsenic concentration must not exceed 40 mg/kg. These regulations help ensure that ash fertiliser products are of consistent quality, safe and suitable for their intended use. Only granulated or self-hardened ash is permitted for use as forest fertilisation in order to minimise flying ash. Granulated ash has ideal properties and usability. Ash can be enhanced" during the granulation process by mixing together different types of ash or by adding nutrients. Boron, and occasionally phosphorus or potassium, is usually added into ash products that are intended for forest fertilisation.

3 Granulation is the most effective ash stabilisation method currently in use. Ash can be "enhanced" during the granulation process by adding the needed nutrients. In the right portions, ash fertiliser is equal to commercial PK-fertilisers, but the advantages of ash fertiliser include ecology and long-lasting effect. The effects of ash fertilisation on tree stands in different locations Drained peatland forests Ash fertilisation causes long-lasting improvements in a stand s nutrient condition. Factors that affect the nutrient condition of trees include the nutrient content of the site, the amount and quality of the ash spread, and the stand s nutrient condition prior to fertilisation. Ash has especially good fertilisation effects on nitrogen-rich drained peatlands, where tree growth is limited by lack of potassium and poor availability of phosphorus. These sites are typically Vaccinium vitis-idaea type (Ptkg II) and Vaccinium myrtillus type (Mtkg II) with a thick layer of relatively decomposed peat. Ash fertilisation increases tree growth in direct relation to the amount of nitrogen in peat. On nitrogen-rich sites the increase in tree growth is approximately 2 6 m 3 /ha/y and on nitrogen-poor sites 1 3 m 3 /ha/y during one rotation. Consequently, depending on the site and the spreading methods, ash fertilisation may bear interest at 6 10% over 20 years (Ramboll Finland Oy 2012). Depending on nutrient contents, approximately 3 5 tonnes/ha of ash needs to be spread in peatland forests in order to meet the nutrient quantities of fertilisation recommendations, P kg/ha and K kg/ha (Hyvän metsänhoidon suositukset turv le, Recommendations for good silviculture in peatlands 2007). Depending on the quantity of ash used, the nutrient condition of the stand has remained good for years. Due to the fast dissolution of potassium and its tendency to leach away, some sites may need two rounds of fertilisation during the forest stand s rotation in order for the stand s nutrient condition to remain good. Symptoms of potassium (K) deficiency: the pine needles (left) are yellow, especially the previous year s growth, whereas the spruce needles (right) are yellow throughout. Recovery from the potassium deficiency can often be seen as early as the first few years after ash fertilisation. 3

4 The lack of phosphorus and potassium usually limits tree growth in nitrogen-rich peatland. Ash fertilisation initiates a slow but strong and long-lasting growth reaction. An unfertilised (left) and 15 years earlier ash-fertilised (right) trial plot in Pelso peatland, Vaala. Cut-away peatlands Ash fertilisation is also suitable for cutaway peatlands that will be afforested for energy wood production or revegetated for environmental reasons. Tree growth on cut-away peatlands is mostly limited by lack of phosphorus and potassium and occasionally also by lack of boron. Ash fertilisation is a good alternative for commercial PK fertilisers. The recommendable dosage is: P 50 kg/ha, K kg/ha and B 1.5 kg/ha (Issakainen & Huotari 2007). The effects of ash fertilisation on nutrient conditions and tree growth on cut-away peatland depend on the quantity of ash spread and the tree species being grown. Ash fertilisation is a good alternative for afforestation or vegetation of cut-away peatlands. An unfertilised (left) and 5 years earlier ash-fertilised (right) cut-away peatland in Hirvineva, Liminka. (Photos: J. Issakainen and N. Huotari / Metla) Afforestation of agricultural lands The peat in agricultural lands usually contains substantial amounts of nitrogen and phosphorus for tree growth but may lack sufficient potassium, boron, and other mic- ronutrients. The primary aim of fertilising afforestation areas in agricultural fields is maintaining nutrient balance and safeguarding the undisturbed growth of trees. 4

5 Mineral soil sites In mineral soils, tree growth is limited by lack of nitrogen, and therefore ash fertilisation does not usually increase tree growth. However, in mineral soils ash fertilisation can be used to prevent growth disorders caused e.g. by nutrient imbalances or lack of boron. The sites that are best suited for ash fertilisation are nitrogen-rich drained peatland forests where peat layer is at least 30 cm thick and tree growth is limited by lack of phosphorus and potassium. How does ash fertilisation affect the environment? The effects of ash fertilisation on soil Ash fertilisation decreases soil acidity and causes a long-lasting increase in the total nutrient stores of the surface soil. In fact, ash can be used to replace nutrient losses caused by wood biomass harvesting and to prevent soil acidification. Of the main nutrients of ash, phosphorus is the slowest to dissolve, whereas potassium and boron are quick to dissolve in soil water. Soil heavy metal concentration also rises after ash fertilisation, but due to the alkalinity of ash the heavy metals remain in a very slowly dissolving form. Ash fertilisation speeds up soil decomposition, which in the long term promotes the decomposition of organic soil matter and the release of nitrogen to plants. The effects of ash fertilisation on vegetation Spreading ash may initially decrease the site s original moss cover. There is usually an increased coverage of grasses and a decreased coverage of shrubs. The quantity of species in the site may even increase because new and old species appear side by side. In non-vegetated areas, such as cut-away peatlands, ash fertilisation significantly accelerates the formation of vegetation. Ash fertilisation increases the quantity of grass and shrub species especially in nitrogenrich peatland forests, making the site appear more luxuriant. 5

6 No transfer of the heavy metals has been observed from ash to vegetation, berries or mushrooms. In mosses, heavy metal concentrations have occasionally increased slightly after ash fertilisation, but no substantial deviations from the natural variation has existed. The heavy metal content of berries and mushrooms may increase temporarily due to ash dust accumulation on their surfaces. Hence, picking berries and mushrooms the summer immediately after ash fertilisation should be avoided. Fire-favouring moss species are quick to spread to ash fertilised areas. In the picture: Golden Thread moss. (Photo: N. Huotari / Metla). Photo: K. Kylmänen Berries and mushrooms are edible after ash fertilisation, but due to ash dust, picking them the summer immediately after ash fertilisation should be avoided. The effects of ash fertilisation on watercourses and animals There are no observations of increased accumulation of cadmium in bird eggs or in various insects, rodents or fish due to ash fertilisation. There has been very little leaching of heavy metals into watercourses from ash-fertilised areas, unless ash has got directly into ditches. Leaching after ash spreading in the winter could be prevented by leaving a buffer zone of 1 2 metres on either side of a ditch. A buffer zone of metres should be left on the banks of streams and a buffer zone of at least 50 metres on the banks or shores of lakes, ponds and rivers. It is recommended that 50-metre buffer zones are used in aerial spreading of ash. However, in order to minimise long-term risks, it 6 Ash fertilisation has little effect on watercourses as long as the ash quantities follow regulations and sufficient buffer zones are observed. (Photo: E. Oksanen / Metla). is important to have clearly defined limit values for the heavy metal concentrations of ash used in forest fertilisation.

7 The implementation of ash fertilisation Ash fertilisation should be planned and carried out as a joint project for several forest properties to minimize costs in the transportation and spreading of ash. Pretreated ash is transported to the forest by lorry, which means considering the bearing capacity, and in the winter, the snow clearance and gritting of the road. The fertiliser storage requirements for contractors must also be checked and taken into account separately. Ash fertilisation can be implemented aerially or on the ground. Ground spreading is usually executed using a forest tractor or a farm machinery. The forest floor needs to have sufficient bearing capacity for ground spreading, and therefore peatland forests are fertilised in the winter when the ground is frozen. The timing of silvicultural operations is also important in ground spreading of ash. First, the forest is thinned and tracks for the spreading machines are harvested. After the thinning, the ash fertilisation is carried out, followed by maintenance of ditch network. More information on ash fertilisation Photo: N. Huotari / Metla The spreading of ash using a helicopter is not tied to a certain time of year or silvicultural treatment, so it can be carried out any time of the year and also in unthinned forests. Aerial spreading is a good alternative in areas with poor accessibility due to large rocks, peatlands or watercourses. Aerial spreading is more effective than ground spreading but also more expensive. That is why the fertilised area should be no smaller than ha for helicopter spreading. Hyvän metsänhoidon suositukset turv le (2007) (Recommendations for good silviculture in peatlands) Forestry Development Centre Tapio. Metsäkustannus Oy, 50 pages. Issakainen J & Huotari N (2007) Suopohjien metsittäminen (Afforesting of cut-away peatlands). Leaflet, 11 pages. The Finnish Forest Institute, Muhos. PDF version (in Finnish): metla.fi/julkaisut/muut/ suopohjien-metsittaminen/suopohjien_metsittaminen.pdf Ministry of Agriculture and Forestry (2011). Ministry of Agriculture and Forestry Decree on fertiliser products. Decree 24/11. Helsinki, 13 September PDF version (in Finnish): fi.pdf Ramboll Finland Oy (2012). Tuhkan rakeistaminen Pohjois-Pohjanmaalla (The Granulation of Ash in North Ostrobothnia). Report, 47 pages + appendices. 7

8 This brochure has been produced as part of The Granulation of Ash in North Ostrobothnia project administered by the Finnish Forest Institute (Metla) and funded by the Council of Oulu Region (ERDF), Metla, and local enterprises. This brochure is a summary of the guidebook Tuhkan käyttö metsälannoitteena ( The use of ash as a forest fertiliser ), which was published in June The brochure s information is based on the research publications listed in the guidebook. The guidebook s information Huotari, N (2012) Tuhkan käyttö metsälannoitteena. (The use of ash as a forest fertiliser) The Finnish Forest Institute (Metla), Oulu. 48 p. ISBN (bound) ISBN (PDF) The guidebook and brochure have also been published online: (in Finnish). 8 Text: Original Finnish text Noora Huotari / Metla Cover: Sanna Korpela / Metla. Back cover, small photographs in descending order: Erkki Oksanen, Asko Kaikusalo, Noora Huotari and Noora Huotari / Metla Photography: Jorma Issakainen / Metla (unless otherwise stated) Layout: Irene Murtovaara / Metla Publisher: The Finnish Forest Institute (Metla), Oulu. The Granulation of Ash in North Ostrobothnia project (Tuhkan rakeistus Pohjois-Pohjanmaalla) Printed in: Kopijyvä Oy 2012