Threshold application rates of wood ash to an acidic forest soil

Changes in soil and soil solution chemistry were studied for 2 yr after application of wood ash to an acidic forest Spodosol. Application rates to four 50 m(2) plots were: 0 (control), 6, 13, and 20 Mg CaCO3 equivalents per hectare (Mg ha(-1)). In soil solution, the 13 and 20 Mg ha(-1) treatments caused large but generally transient increases in base cations, pH, and anions. At 20 Mg ha(-1), solution concentrations of H, acid neutralizing capacity (ANC), K, and SO4 remained elevated for 20 mo. There was minimal response in soil solutions at 6 Mg ha(-1), suggesting that this rate may represent a threshold below which there is only minor response in soil solution chemistry. Loss of soil N into soil solution (as NO3) at the two higher application rates may have decreased available-N in the already N-limited soil. In the O horizon, all treatments resulted in increased pH and higher exchangeable Ca, K, Mg, CEC, and percent base saturation; Mn and Al decreased, In the B horizon, exchangeable Ca, K, Mg, and percent base saturation were higher than In the control. Responses in soil chemistry were not generally proportional to the ash application rate. Furthermore, the responses in soil chemistry al different application rates were generally not distinguishable from each other after 25 mo. The treatment rate of 6 Mg ha(-1) ash was sufficient to favorably alter soil exchange chemistry, without seriously affecting solution chemistry. Above 6 Mg ha(-1), soil exchange sites were unable to buffer ash amendments, resulting in substantial changes in solution chemistry.