EFFECTS OF HYDROXY-ALUMINUM ON FLOCCULATION AND PERMEABILITY OF SILT AND CLAY FRACTIONS SEPARATED FROM PADDY SOILS

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Flocculation tests and permeability measurements were carried out for slit and clay fractions separated from surface and subsurface horizons of four paddy soils. There was not much difference in the flocculating power between Al(OH)_<2.2>Cl_<0.8> and CaSO_4. Their flocculation value, 0.3 to 3 mM, was attained by adding the reagents at the rate of 0.24 to 2.4 mmol/100 g of the slit and clay. The flocculation with Al(OH)_<2.2>Cl_<0.8> or CaSO_4 did not affect the sediment volume of the slit and clay. This sediment volume was smaller for the surface than for the subsurface horizons of each soil, suggesting that soil organic matter and repetition of wetting and drying contribute to the stable aggregation of the slit and clay in the surface horizon. The permeability was higher for the slit and clay fractions separated from the surface horizon than that from the subsurface horizon, and the addition of 2.5 to 4.2 mmol of Al(OH)_<2.2>Cl_<0.8> or CaSO_4/100 g of the slit and clay increased its permeability by 1.5 to 4 times. The effect of Al(OH)_<2.2>Cl_<0.8> was more lasting than that of CaSO_4 and was recognized even after 200 ml of water passed through 1 g of the slit and clay. The effect of the flocculating reagent on the permeability was more or less enhanced by air-drying the treated slit and clay.
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