Ash use in forestry
BackgroundPitman,R.M. 2006. Wood ash use in forestry - a review of the environmental impacts. Forestry, 79, 563-588.
The research of possible uses of wood ash and its composition mainly comes from Sweden, USA, Finland and Denmark.
Wood ash in Sweden is produced largely from combustion of tree materials at power and heat plants. Research on recycling such ashes to forest sites on peats and podzols was brought about in the 1970s. Lately research has also been directed at using wood ash to restore pH in acidified soils, because of its liming effect. For example, the WoodAsh database is a product of the Swedish research.
On a national level in the USA, wood ash is mostly used for landfills (about 90 per cent). This is not the case for all areas of the country, however, as only 15 per cent of the ash in the north-east states goes to landfills. There, the main part (about 80 per cent) is land applied and the remaining 5 per cent co-combusted together with sewage sludge.
In Finland, wood ash has been used to improve certain soils since 1935. Research on using ash to alter soil pH and restore biodiversity is currently taking place. In Denmark ash quality has been rather mixed with some high levels of heavy metals and dioxins, as a result of using mixed organic fuels as for example straw, green waste, woodchip and tree thinnings. Because of this uneven ash quality, much of the ash is used in landfills.
Issues in forestryKarltun,E., Saarsalmi,A. et al. Wood ash recycling - possibilities and risks.Andersson, S., Karltun,E. et al. Wood ash - properties and ecological consequences of recycling to forest.
The increased use of forest fuels will result in an intensified export of plant nutrients from the forest. A large part of the forest fuel consists of fractions (branches, tops, needles) that were earlier left to decay in the forest. Although they only amount to a small proportion of the total weight of the tree, these fractions have a much higher nutrient concentration per unit weight than the stemwood. Thus, the increased nutrient export following the increased use of forest fuel is not negligible.
Another undesired effect of the nutrient export is enhanced soil acidity. A relevant question is therefore if the withdrawal of nutrients from the forest will result in a decrease in the productivity of the forest by impoverishment and acidification of the soil.
Recycling of wood ashIbid.
Wood ash is produced when biomass and residues from tree products are used as fuel. Due to the increased use of forest fuels the production of wood ash is increasing. Much of this ash is today considered as waste and is deposited on landfills at a considerable cost. Wood ash contains all the major mineral plant nutrients except nitrogen and has a liming effect when returned to the soil. Recycling of wood ash to the forest is a possible way to close the nutrient cycle and counteract increased soil acidity.
Thus, there are two problems that have arisen due to the increased use of forest fuels, namely large amounts of waste that have to be handled in a environmentally safe way at an acceptable cost, and a potential risk of withdrawing nutrients from the forest, which may affect the long-term productivity of our forests. An obvious solution to both these problems is to recycle wood ash to forests.
Naturally, it is important to determine that the wood ash recycling will not negatively affect the environment, and studies are constantly performed with that purpose. What can be said however is that the absence of nitrogen in wood ash results in a significant growth increase on phosphor or potassium-limited soils (nitrogen-rich peat soils), whilst having no direct growth stimulation on trees in nitrogen-limited soils. There are indications on influence on some tree physiological processes by wood ash amendment, especially at high doses, but more studies need to be done to confirm these results.
The "WoodAsh" database presents an additional aid by letting the user compare ash contents and composition to a large selection of measurements collected from companies in the Nordic forest fuel industry.
Fig. 1. Spreading wood-ash at the Wood-En-Man field experiment in Kacergine, Lithuania. Photo: Iveta Varnagiryte