THRESHOLD APPLICATION RATES OF WOOD ASH TO AN ACIDIC FOREST SOIL

Changes in soil and soil solution chemistry were studied for 2 yr afte r application of wood ash to an acidic forest Spodosol. Application ra tes to four 50 m(2) plots were: 0 (control), 6, 13, and 20 Mg CaCO3 eq uivalents per hectare (Mg ha(-1)). In soil solution, the 13 and 20 Mg ha(-1) treatments caused large but generally transient increases in ba se cations, pH, and anions. At 20 Mg ha(-1), solution concentrations o f H, acid neutralizing capacity (ANC), K, and SO4 remained elevated fo r 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 d ecreased 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 h orizon, exchangeable Ca, K, Mg, and percent base saturation were highe r than In the control. Responses in soil chemistry were not generally proportional to the ash application rate. Furthermore, the responses i n soil chemistry al different application rates were generally not dis tinguishable from each other after 25 mo. The treatment rate of 6 Mg h a(-1) ash was sufficient to favorably alter soil exchange chemistry, w ithout seriously affecting solution chemistry. Above 6 Mg ha(-1), soil exchange sites were unable to buffer ash amendments, resulting in sub stantial changes in solution chemistry.