Effects of crushed wood ash on soil chemistry in young Norway spruce stands

Abstract

The use of logging residues as a fuel in district heating plants has led to regular wood ash recycling to forest soil to prevent soil nutrient depletion. Wood ash with a low content of unburnt matter reacts with water and air in a so-called self-hardening process. Hardening of the ash is a way to reduce its reactivity and thus increase the time for its dissolution. In four field experiments, changes in soil chemistry were analyzed 6 years after application of hardened crushed wood ash to 8–10-year-old Norway spruce ( Picea abies (L.) Karst.) stands. The 0–5, 5–10 and 10–20 cm layers of the soil were sampled, including the disturbed humus layer. Logging residues were collected at the preceding harvest. At each site, hardened crushed wood ash of two types, Nymölla and Perstorp, was applied in a randomized block design ( n=4). The dose applied was 3 Mg ha −1. Exchangeable Ca, Mg, K and effective cation exchange capacity (CEC eff) on a per hectare basis in the 0–20 cm soil layers were significantly higher in wood ash-treated plots than in the control plots in analyses across all study sites. Total exchangeable acidity was significantly lower after wood ash application. There was a significant effect on pH in the 0–5 cm layer of the soil in three of the sites, where pH increased on average by 0.5 units in wood ash Perstorp-treated plots and by 0.7 units in wood ash Nymölla-treated plots compared to control plots. Further down in the soil profile, no differences were detected at any site. Wood ash application resulted in a higher level of base saturation (BS), especially in the 0–5 cm layer. BS increased on an average by 39 and 31% units in the 0–5 cm soil layer, and by 13 and 10% units in the 5–10 cm soil layer, in wood ash Nymölla- and Perstorp-treated plots, respectively, compared with the control. The results show that hardened wood ash recycling can compensate for base cations lost at whole-tree harvesting.