Aggregate-formation of allophanic soils: Effect of drying on the dispersion of the soils

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Abstract
Some allophanic soils in Japan, developed from deposits of volcanic ashes have aggregates, the sizes of which are mostly those of silt and sand, and which are very stable against the ordinary soil dispersion treatment such as decomposition of organic matter with hydrogen peroxide and addition of deflocculants to the suspension, but can be broken to smaller particles by mechanical forces. Oba and Kono (1, 2) reported that there were aggregates stable even against deferration treatment in the soils, especially plenty in ones developed from basic volcanic ashes, and these aggregates could be broken up into clay of allophane mainly by a sonic-wave vibration technique. Miyazawa (3) recognized that the aggregates were only found in Andosols developed under warm-temperate climates, being concentrated in B horizon, but not so much in A horizon and few in gleyey subhorizon, and assumed that they were formed under a specific weathering like seasonal wetting and drying. Yasuo et al. recognized a high correlation between the degree of aggregation and free iron oxide content of the soils in Kanto Plain and suggested that sesquioxide and structure of allophane might associate with the aggregate fonnation. On the other hand, airdrying of volcanic ash soils sometimes reduces their dispersibilities through irreversible formation of aggregates. Kishita (5) found that this effect of drying was remarkable at subsoiLs, and similar results were obtained by TAPA et al. (6). The poor dispersibility and the irreversible change by drying are known also about volcanic ash soils of New Zealand (7, 8), and Latin America (10, 11), Hawaii (9).

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