Alleviation of subsoil acidity in nonallophanic andosols by phosphogypsum application in Topsoil

Subsoil acidity is one of the most important factors limiting crop production in nonallophanic Andosols. In this study, the effectiveness of application of phosphogypsum (PG) in the topsoil on the alleviation of the subsoil acidity of nonallophanic Andosols was investigated. Two types of nonallophanic Andosols, light colored (Kitakami) and thick high humic (Kawatabi) soils, were used as test soils. Vertical movement of solutes was studied using open end soil columns (diameter 10 cm, length 45 cm). In the Kitakami and Kawatabi soils, 60 and 54%, respectively, 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 the topsoil. The Kitakami soil showed remarkable reduction of both pH(H2O) and exchange acidity y1 in the 0-35 cm depth reiecting the downward movement of PG. In contrast, the Kawatabi soil showed changes in pH(H2O) and y1 were not appreciable. With lime treatment, only the soil acidity of the topsoil was amended in both soils. Deep rooting of barley (Hordeum vulgare L. cv. Miyuki) into the subsoil was observed in the Kitakami soil following treatment with PG, reflecting the reduction of the y1 value. However, deep rooting did not occur in the Kawatabi soil treated with PG or in the Kitakami and Kawatabi soils treated with lime. The effects of PG on the alleviation of the subsoil acidity of Andosols (Al toxicity) may not be due to changes in the soil solution chemistry, but may be related to the decrease in the amount of exchangeable aluminum.