Abstract
The most commonly used method to extract sulfate ion (SO_4^<2->) from soils is probably the method by Soil Science Society of America (SSSA). However, the SSSA method appeared to be not directly applicable for volcanic ash soils. In the present study, we extracted SO_4^<2-> with calcium dihydrogen phosphate solution, based on the SSSA method from two soils derived from greywacke and volcanic ash, Typic Haplic Brown Forest soils and Typic Brown Kuroboku soils, respectively. It was shown that the 5 times extraction was capable to extract almost all of the SO_4^<2-> present in soils. Compared with the amount of SO_4^<2-> extracted with SSSA method, 5 times sequential extraction showed higher amount of SO_4^<2->. And different correlations were obtained between these two amounts extracted from the soils of volcanic ash and greywacke, suggesting that the SSSA method underestimated the total SO_4^<2-> present in volcanic ash soils. Moreover, the SSSA method did not reach reaction equilibrium as to the shaking time and the ratio of soil/solution. Instead, newly proposed condition of 4 hour shaking and the soil to solution ratio of 1:100 was good enough to reach equilibrium, and the amounts of SO_4^<2-> extracted under this condition were almost same as those of the 5 times sequential extraction, irrespective of the difference in parent materials.
