The Swedish Forest Agency strongly recommends recycling of fly ash from combustion of wood-based fuel by applying it on forest soil (Skogsstyrelsen, 2008). The purpose of such recycling is to counteract acidification and nutrient depletion of forest soil after extraction of tree tops and branches (Olsson and Westling, 2006). Nitrogen is often the limiting nutrient for the biomass growth. Although ash may have some positive influence on N availability in soil by increasing soil pH and facilitating the formation of nitrate instead of N2 and NO2, it does not contribute N to soil. Additional sources are therefore needed in order to maintain productivity of the plants.