EFFECTS OF PHOSPHOGYPSUM ON AMELIORATION OF STRONGLY ACID NONALLOPHANIC ANDOSOLS

Subsoil acidity is recognized as one of the most important yield-limit ing factors in strongly acid nonallophanic Andosols. Surface applicati on of phosphogypsum (PG) has the potential to ameliorate subsoil acidi ty in these soil. According to humus content, two types of nonallophan ic Andosols were selected as test soils. The Kitakami soil (Light-colo red Andosol) has low humus content and the Kawatabi soil (Thick High-h umic Andosol) has high humus content. The effects of PG on subsoil aci dity were investigated using open ended columns and the mechanism of t he reduction of exchangeable Al was investigated using cation exchange resin. After 300 mm of water were applied, the Ca applied to the tops oil (0-15 cm) as PC moved into subsoil (15-35 cm) in both soils. In th e Kitakami soil with PG application, both pH(H2O) and exchange acidity (y1) were greatly reduced in the 0-35 m depth. In the Kawatabi soil, the changes in pH and y1 by PG were not appreciable. Lime application amended only topsoil in both soils. Root elongation of barley into the subsoil was observed only in the Kitakami soil with PG application to topsoil. Extraction of Al with the cation exchange resin suggested th at the mechanism for the reduction of exchangeable-Al was due to the p olymerization of hydroxy-Al by PG application followed by adsorption s electively and irreversibly to clay minerals. From the distribution of nonallophanic Andosols in which topsoil application of PG is effectiv e, it is concluded that PG is an effective ameliorant for subsoil acid ity in nonallophanic Andosols.