We studied various properties of nonallophanic Andosols in Japan. Our results showed that, despite many similarities between nonallophanic and allophanic Andosols, significant dissimilarities also exist between the two groups of soils. The nonallophanic Andosols used for our study formed from rhyolitic, dacitic, or andesitic volcanic ash in a humid temperature climate. They had distinctive morphological properties common to allophanic Andosols in Japan: (1) very dark, thick humus horizons, (2) granular structure in the humus horizons, (3) friable to very friable consistence, and (4) abrupt or clear smooth boundaries between humus and nonhumus horizons. The clay mineralogy of the nonallophanic Andosols was very different from that of the allophanic Andosols. In nonallophanic Andosols the clay fractions are mostly dominated by chloritized 2:1 minerals and the active Al (acid-oxalate-ex-tractable Al) consists largely or wholly of Al complexed with humus (pyrophosphate-extractable Al). The nonallophanic Andosols had mostly medium to fine textures and considerable high water retention at 15 bars. We noted that these soils have characteristically low bulk density, which is closely correlated with the organic carbon content. A remarkable accumulation of humus took place in the nonallophanic Andosols and was closely correlated with the formation of Al-humus complex. The high CEC of these soils was ascribed to the high humus content. We noted that nonallophanic Andosols are strongly acid to very strongly acid, reflecting the abundance of chloritized 2:1 minerals and the very low base saturation. Therefore, they had high Al saturation and large amounts of KCl-exchangeable Al, which is a major deterrent to plant growth in the soils. The average values of Al saturation and KC1-ex-changeable Al were 74.4% and 5.51 meq/100 g for humus horizon soils and 59.8% and 3.50 meq/100 g for nonhumus horizon soils, respectively. Almost all the nonallophanic Andosol samples had pH (NaF) >10 and phosphate retention > 85%. These values were significantly correlated with the acid-oxalate-extractable Al (Alo). Soils containing > 1% Alo have pH (NaF) > 9.4 and phosphate retention >85%. Therefore, we noted that Al complexed with humus has a significant amount of OH groups and is highly reactive. The analysis of pyro-phosphate-soluble components indicated that the humus associating with Al and Fe consists largely or wholly of humic acid with the highest degree of humification.
ASJC Scopus subject areas
- Soil Science