Evaluation of nonexchangeable potassium content of agricultural soils in Japan by the boiling HNO3 extraction method in comparison with exchangeable potassium

摘要

The nonexchangeable potassium (neK) content of 178 agricultural soils in Japan was determined by subtracting the amount of K extracted with 1mol L-1 ammonium acetate, i.e., exchangeable K (exK) from that extracted with boiling 1mol L-1 HNO3. The statistical relationships between the neK content and physico-chemical properties of the soils were examined to investigate the factors controlling neK content. The neK content of agricultural soils in Japan ranged from 0 to 1120mg kg(-1) with an arithmetic mean and median of 303 and 255mg kg(-1), respectively. It showed a significant positive correlation with the total K content, fixed ammonium content, and silt content (p&0.01) and a significant negative correlation with Alo+1/2 Feo content and total carbon content (p&0.01). These results suggest that the controlling factors of neK are mainly the total K content and 2:1 type phyllosilicates such as mica and vermiculite, with the indirect negative influence of organic matter and amorphous materials. Terrestrial Regosols, Brown Lowland soils, and Dark Red soils had relatively high neK contents. In contrast, Andosols, Wet Andosols, and Volcanogenous Regosols had relatively low neK contents. The neK content showed no significant correlation with exK content, suggesting that neK is a moderately to slowly available fraction of soil K, which is independent of exK. In conclusion, evaluation of nonexchangeable K in combination with exchangeable K would enhance the rational management of agricultural soils in terms of K fertility by taking account of longer term K-supplying power of soils.