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ASH AS A FOREST FERTILISER

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The properties of ashApproximately 600,000 tonnes of wood- , peat- and mixed-ash is generated in Fin-land annually as by-product of energy pro-duction. Ash has a good neutralising ca-pacity (pH 10–13), and pure wood ash contains all the nutrients that trees need to grow in the right proportions, except ni-trogen (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 var-ious mixed ashes differ considerably de-pending on the incinerated material and the incineration technique used.

The fuel that is used affects the element composition and quality of the ash.

Ash that has been generated in the in-cineration of wood, peat or field bio-mass can be used as a forest fertiliser. In Finland, the utilisation of ash as fertilis-er is regulated by the Fertiliser Product Act (539/2006) and related decrees (Min-istry of Agriculture and Forestry Decree 24/11). The Finnish Food Safety Authority Evira is in charge of the supervision of fer-tiliser products. Ash that is used as a for-est fertiliser must have a combined con-centration of phosphorus (P) and potas-sium (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

harmful heavy metals: the cadmium con-centration of ash, for example may not ex-ceed 25 mg/kg, and the arsenic concen-tration must not exceed 40 mg/kg. These regulations help ensure that ash fertilis-er 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.

Ash quality requirements and pre-treatment

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Granulation is the most effective ash stabi-lisation method currently in use. Ash can be "enhanced" during the granulation process by adding the needed nutrients.

The effects of ash fertilisation on tree stands in different locationsDrained peatland forestsAsh fertilisation causes long-lasting im-provements in a stand’s nutrient condi-tion. Factors that affect the nutrient con-dition of trees include the nutrient content of the site, the amount and quality of the ash spread, and the stand’s nutrient condi-tion prior to fertilisation. Ash has especial-ly good fertilisation effects on nitrogen-rich drained peatlands, where tree growth is limited by lack of potassium and poor avail-ability of phosphorus. These sites are ty- pically 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 nitro-gen in peat. On nitrogen-rich sites the in-crease in tree growth is approximately 2–6 m3/ha/y and on nitrogen-poor sites 1–3 m3/ha/y during one rotation. Conse-quently, 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 nu-trient quantities of fertilisation recom-mendations, P 40–50 kg/ha and K 80–100 kg/ha (Hyvän metsänhoidon suositukset turvemaille, ”Recommendations for good silviculture in peatlands” 2007).

Depending on the quantity of ash used, the nutrient condition of the stand has remained good for 20–50 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.

In the right portions, ash fertiliser is equal to commercial PK-fertilisers,

but the advantages of ash fertiliser include ecology and long-lasting effect.

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 po-tassium deficiency can often be seen as early as the first few years after ash fertilisation.

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Afforestation of agricultural landsThe peat in agricultural lands usually con-tains substantial amounts of nitrogen and phosphorus for tree growth but may lack sufficient potassium, boron, and other mic-

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 peatlandsAsh fertilisation is also suitable for cut-away peatlands that will be afforest-ed for energy wood production or reveg-etated for environmental reasons. Tree growth on cut-away peatlands is mostly limited by lack of phosphorus and potas-sium and occasionally also by lack of bo-ron. Ash fertilisation is a good alternative

for commercial PK fertilisers. The recom-mendable dosage is: P 50 kg/ha, K 80–150 kg/ha and B 1.5 kg/ha (Issakainen & Huo-tari 2007). The effects of ash fertilisation on nutrient conditions and tree growth on cut-away peatland depend on the quanti-ty of ash spread and the tree species be-ing 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, Limin-ka. (Photos: J. Issakainen and N. Huotari / Metla)

ronutrients. The primary aim of fertilising afforestation areas in agricultural fields is maintaining nutrient balance and safe-guarding the undisturbed growth of trees.

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Mineral soil sitesIn 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.

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 slow-est 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 alka-linity of ash the heavy metals remain in a very slowly dissolving form. Ash fertilisa-tion speeds up soil decomposition, which in the long term promotes the decompo-sition of organic soil matter and the re-lease of nitrogen to plants.

How does ash fertilisation affect the environment?The effects of ash fertilisation on soil

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.

The effects of ash fertilisation on vegetation

Spreading ash may initially decrease the site’s original moss cover. There is usual-ly an increased coverage of grasses and a decreased coverage of shrubs. The quan-tity of species in the site may even in-crease because new and old species ap-pear side by side. In non-vegetated are-as, such as cut-away peatlands, ash fertili-sation significantly accelerates the forma-tion of vegetation.

Ash fertilisation increases the quantity of grass and shrub species especially in nitrogen-rich peatland forests, making the site appear more luxuriant.

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No transfer of the heavy metals has been observed from ash to vegetation, berries or mushrooms. In mosses, heavy met-al concentrations have occasionally in-creased slightly after ash fertilisation, but no substantial deviations from the natu-ral variation has existed. The heavy met-al content of berries and mushrooms may increase temporarily due to ash dust ac-cumulation on their surfaces. Hence, pick-ing berries and mushrooms 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 lit-tle leaching of heavy metals into water-courses 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 10–15 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 recom-mended that 50-metre buffer zones are used in aerial spreading of ash. Howev-er, in order to minimise long-term risks, it

is important to have clearly defined limit values for the heavy metal concentrations of ash used in forest fertilisation.

Ash fertilisation has little effect on watercours-es as long as the ash quantities follow regula-tions and sufficient buffer zones are observed. (Photo: E. Oksanen / Metla).

Berries and mushrooms are edible after ash fertilisation, but due to ash dust, picking them 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

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The implementation of ash fertilisationAsh 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. Pre-treated ash is transported to the forest by lorry, which means considering the bear-ing capacity, and in the winter, the snow clearance and gritting of the road. The fer-tiliser storage requirements for contrac-tors must also be checked and taken into account separately.

Ash fertilisation can be implemented aerially or on the ground. Ground spread-ing is usually executed using a forest trac-tor or a farm machinery. The forest floor needs to have sufficient bearing capacity for ground spreading, and therefore peat-land 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 for-est is thinned and tracks for the spreading machines are harvested. After the thin-ning, the ash fertilisation is carried out, followed by maintenance of ditch net-work.

The spreading of ash using a helicopter is not tied to a certain time of year or sil-vicultural treatment, so it can be carried out any time of the year and also in un-thinned forests. Aerial spreading is a good alternative in areas with poor accessibili-ty due to large rocks, peatlands or water-courses. Aerial spreading is more effec-tive than ground spreading but also more expensive. That is why the fertilised area should be no smaller than 30–40 ha for helicopter spreading.

More information on ash fertilisationHyvän metsänhoidon suositukset turvemaille (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): http://www.metla.fi/julkaisut/muut/ suopohjien-metsittaminen/suopohjien_metsittaminen.pdf

Ministry of Agriculture and Forestry (2011). Ministry of Agriculture and Forestry Decree on fer-tiliser products.

Decree 24/11. Helsinki, 13 September 2011. PDF version (in Finnish): http://www.finlex.fi/data/normit/37638–11024fi.pdf

Ramboll Finland Oy (2012). Tuhkan rakeistaminen Pohjois-Pohjanmaalla (The Granulation of Ash in North Ostrobothnia). Report, 47 pages + appendices.

Photo: N. Huotari / Metla

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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 2012. The brochure’s information is based on the research publications listed in the guidebook.

The guidebook’s informationHuotari, N (2012) Tuhkan käyttö metsälannoittee-na. (The use of ash as a forest fertiliser) The Finnish Forest Institute (Metla), Oulu. 48 p.ISBN 978-951-40-2370-5 (bound)ISBN 978-951-40-2371-2 (PDF)

The guidebook and brochure have also been published online: www.metla.fi/hanke/7464/(in Finnish).

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