Neutralization of soil acidity and release of phosphorus and potassium by wood ash

Wood ash is a residual by-product being produced at an annual rate of 1.5 to 3.0 million t by wood burning power plants in the USA. Currently, much of this ash is being landfilled. Landspreading of wood ash, which is an alternative liming agent, on both agricultural and forest soils is being proposed as a means of disposal that is both environmentally and economically attractive. As with any soil amendment, there is a potential of adverse environmental impact from the landspreading of wood ash. An experiment was conducted to evaluate the initial rate of soil acidity neutralization by wood ash and the release of P and K into suspensions of 10 acid soils. The wood ash had a CaCO3 equivalence of 480 g kg-1. Changes in soil suspension pH, which were used to quantify the rate of acid neutralization by wood ash, and concentrations of P and K of wood ash-amended soil suspensions were measured at reaction periods ranging from 30 to 300 s. Soil acidity neutralization rates were strongly correlated with soil pH and decreased 41% per unit pH increase in the pH 3.8 to 6.2 range. After the initial release of P into suspension, sorption of released P resulted in decreasing P concentration in the suspension with increasing reaction time for 7 of the 10 soils. Sorption of released P was greater at lower soil pH values, suggesting precipitation as Al and Fe phosphates. In contrast, the solubility of wood ash K was very high. The increase of suspension K levels resulted from the release of wood ash K as well as from the displacement of K on soil exchange sites by Ca and other exchangeable cations released directly from wood ash into the soil suspension. The increase in suspension K was equivalent to 55% of the total K content of the wood ash. The net K content of the suspensions remained near the same level during the reaction period suggesting that soil solution levels of K would immediately increase upon landspreading of wood ash.