ALLEVIATION OF SUBSOIL ACIDITY IN NONALLOPHANIC ANDOSOLS BY PHOSPHOGYPSUM APPLICATION IN TOPSOIL

Subsoil acidity is one of the most important factors limiting crop pro duction in nonallophanic Andosols. In this study, the effectiveness of application of phosphogypsum (PG) in the topsoil on the alleviation o f the subsoil acidity of nonallophanic Andosols was investigated. Two types of nonallophanic Andosols, light colored (Kitakami) and thick hi gh humic (Kawatabi) soils, were used as test soils. Vertical movement of solutes was studied using open end soil columns (diameter 10 cm, le ngth 45 cm). In the Kitakami and Kawatabi soils, 60 and 54%, respectiv ely, of PG moved into the subsoil (15-35 cm) after leaching by 300 mm of water, whereas about 95% of calcium carbonate (lime) remained in th e topsoil. The Kitakami soil showed remarkable reduction of both pH(H2 O) and exchange acidity y(1), in the 0-35 cm depth reflecting the down ward movement of PG. In contrast, the Kawatabi soil showed changes in pH(H2O) and y(1) were not appreciable. With lime treatment, only the s oil acidity of the topsoil was amended in both soils. Deep rooting of barley (Hordeum vulgare L. cv. Miyuki) into the subsoil was observed i n the Kitakami soil following treatment with PG, reflecting the reduct ion of the y(1) value. However, deep rooting did not occur in the Kawa tabi soil treated with PG or in the Kitakami and Kawatabi soils treate d with lime. The effects of PG on the alleviation of the subsoil acidi ty of Andosols (Al toxicity) may not be due to changes in the soil sol ution chemistry, but may be related to the decrease in the amount of e xchangeable aluminum.