Acidity of Japanese Cultivated Andosols and Significance of Exchange Acidity y_1 in Their Classification

Abstract

The acidity of Japanese cultivated Andosols and significance of exchange acidity y_1 in their classification were examined with special reference to soil colloidal composition: allophanic Andosols and non-allophanic Andosols. The mean values of the soil pH for topsoil and subsoil in allophanic Andosols were 6.0 ± 0.6 and 5.9 ± 0.7, respectively, and for topsoil and subsoil in non-allophanic Andosols, 5.8 ± 0.6 and 5.5 ± 0.8, respectively. The soil pH of topsoil for both types of Andosols was higher than that for subsoil values, ranging from 0.1 to 0.3. Out of 610 cultivated allophanic Andosols, 28 soils with an exchange acidity of y_1≧6 were found. On the other hand, out of 317 cultivated non-allophanic Andosols, 239 soils with an exchange acidity of y_1 < 6 were found. Compared with the database of The Soil Information System, the ratio of strong acid soil to non-cultivated Andosols in each prefecture was relatively high, ranging from 3 to 93%. The liming of the plow layer soil was done well for both allophanic Andosols and non-allophanic Andosols. The 292 cultivated Andosols were divided into two groups by exchange acidity y_1 in the subsurface soil following The Classification of Cultivated Soils (The 3rd approximation): Group 1 (y_1 < 5) and Group 2 (y_1 ≧ 5). In Group 1, there were 242 soils: 202 soils, or 83%, were allophanic Andosols; and 40 soils, or 17%, were non-allophanic Andosols. In Group 2, there were 50 soils: 21 soils, or 42%, were allophanic Andosols; and 29 soils, or 58%, were non-allophanic Andosols. The exchange acidity y_1 of the cultivated Andosols was not closely related to the clay mineral composition. We conclude here that a criterion for non-allophanic Andosols in cultivated soils should reflect soil colloidal compositions using analytical date for amorphous materials.