Active aluminum of cultivated Andosols and some related soil chemical properties in Tohoku district were examined with special reference to soil colloidal composition : allophanic Andosols and non-allophanic Andosols. The major form of active aluminum of allophanic Andosols was derived from the allophane fraction, whereas those of non-allophanic Andosols were derived from Al humus complex with a small amount of exchangeable Al. The Al/Si ratios of the allophane fraction of soil found in Tohoku district were close to 2, suggesting the presence of Al-rich allophane. Pyrophosphate-extractable Al had a close linear correlation with organic carbon contents. Non-allophanic Andosols adsorbed more phosphate per unit oxalate extractable Al than allophanic Andosols, suggesting Al from humus complex are more active than that from allophane in phosphate absorption. Judging from soil pH and soil exchange acidity, liming of plow layer soils was well done in both allophanic and non-allophanic Andosols, but, the subsoil of non-allophanic Andosols still showed high amounts of exchangeable Al, indicating the presence of Al toxicity of crops. Both phosphate fixation and soil acidity were fairly reclaimed in cultivated Andosols compared to non-cultivated Andosols.
