GRANULATED WOOD ASH AND A N-FREE FERTILIZER TO A FOREST SOIL - EFFECTS ON P-AVAILABILITY

Addition of wood ash to forest soils has been suggested as a measure to counteract decreases in pH and amounts of cations caused by whole-tree harvesting or acid deposition. Four thousand kilograms per hectare (3200 kg dry weight) of granulated wood ash containing (kg ha(-1)) 25 P, 129 K, 448 Ca, 47 Mg and 9 S were added to a 22-year-old spruce forest stand with a low concentration of P in needles. The effects of the additions on phosphorus availability to trees and microorganisms in the humus layer were investigated 18 months (two growing seasons) after application. The ash treatment was compared with a 'N-free' fertilizer treatment, containing (kg ha(-1)) 30 P, 50 K, 210 Ca, 47 Mg and 80 S, all in readily available forms. Amounts of P in microorganisms and amounts of labile P in the soil did not differ between treatments. In the humus layer, the control contained 21.5 kg P in microorganisms, the ash treatment 17.6 kg and the 'N-free' fertilizer treatment 26 kg ha(-1). Labile P varied from 3.3 kg ha(-1) in the ash treatment to 4.4 kg ha(-1) in the 'N-free' fertilizer treatment. Acid phosphatase activity in soil, expressed per surface area, decreased significantly in the 'N-free' fertilizer treatment. In the ash treatment, the situation was reversed, with a numerically higher mean value for acid phosphatase activity, but the mean was not significantly different from that of the control. For individual plots, acid phosphatase activity was negatively related to the P content in needles of the previous year. The acid phosphatase activity in the humus was elevated in plots where the P concentration in the previous year's needles was below 1.3 mg g(-1) dry weight, compared with plots with higher concentrations. None of the P in the granulated ash was water-soluble, while around 20% was extractable with ammonium acetate pH 4.2. An amendment with soluble P should be considered, if ash is added to forests with low P status.