Effects of phosphogypsum on amelioration of strongly acid nonallophanic Andosols

Subsoil acidity is recognized as one of the most important yield-limiting factors in strongly acid nonallophanic Andosols. Surface application of phosphogypsum (PG) has the potential to ameliorate subsoil acidity in these soil. According to humus content, two types of nonallophanic Andosols were selected as test soils. The Kitakami soil (Light-colored Andosol) has low humus content and the Kawatabi soil (Thick High-humic Andosol) has high humus content. The effects of PG on subsoil acidity were investigated using open ended columns and the mechanism of the reduction of exchangeable Al was investigated using cation exchange resin. After 300 mm of water were applied, the Ca applied to the topsoil (0-15 cm) as PC moved into subsoil (15-35 cm) in both soils. In the 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 that the mechanism for the reduction of exchangeable-Al was due to the polymerization of hydroxy-Al by PG application followed by adsorption selectively and irreversibly to clay minerals. From the distribution of nonallophanic Andosols in which topsoil application of PG is effective, it is concluded that PG is an effective ameliorant for subsoil acidity in nonallophanic Andosols.