Application of hydroxy-aluminum and its effect on the improvement of physical properties of heavy paddy soils.

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The content of water-stable aggregates and the permeability of soil increased as the result of hydroxy-Al application, though its effect on the contents of either water held at pF 1.5 - 2 or available water and on soil consistency (liquid limit, plastic limit, and plasticity index) varied. For soils, in which the increase of water-stable aggregates by hydroxy-Al application was evident, the adhesion forces or the ratio of adhesion force to shear strength decreased and increased and the yield of barley and soybean also increased.The following factors affected the improvement of soil physical properties by hydroxy-Al application: A. Aluminum ion species, B. application rate, C. kind of soil (paddy vs upland, surface vs subsurface, clay content, clay mineral composition, cation-exchange capacity (CEC)), D. soil moisture (drying), E. removal of salt and correction of soil reaction, and F. cultivation of crops. It was found that A. drying the soil before and after application of hydroxyAl enhances its favorable effect, B. the content, but not the mineral composition, of clay influences over the immediate action of hydroxy-Al, and C. the effect of hydroxy-Al maintains or disappears depending on the type of cultivation (upland vs paddy) and possibly also depending on the major clay mineral species of soil.It was found to be appropriate to apply hydroxy-Al to soils containing smectite and/or vermiculite and having CEC >15 meq/100g. The addition of hydroxy-Al at 100 - 150 kg as Al/10 a during the period, in which soil can be dried most effectively, was recommended. Rain or irrigation amounting to 100 - 150 mm was found to be enough for the removal of salt, and the correction of soil reaction after, but not prior to, the removal of salt was recommended.