DISPERSION AND AGGREGATION OF SOILS AS INFLUENCED BY ORGANIC AND INORGANIC POLYMERS

In this study, the influences of three organic polyanions (a soil humi c acid, a soil polysaccharide, and a commercial anionic polysaccharide ) and hydroxy-Al polycations (Al-p) on soil clay dispersion, aggregati on. and subsequently hydraulic conductivity were investigated. It was found that the organic polyanions, especially humic acid, were not flo cculating agents, but were effective dispersing agents for the Na-clay s and Na-soils. Removal of soil organic matter reduced soil clay dispe rsibility, whereas addition of small amounts of the polyanions increas ed soil clay dispersion. The Al-p effectively screened the surface neg ative charge on clay colloids and, in addition, acted as bridges betwe en negatively charged colloids and the anionic polysaccharide, and thu s prevented soil clay dispersion. Hydraulic conductivities of soil col umns treated with Al-p or its combinations with the anionic polysaccha ride or humic acid were about two orders of magnitude higher than thos e without addition of Al-p throughout the 5-d leaching period. Althoug h treatment of the Na-soils with the anionic polysaccharide increased soil aggregate stability as measured by the wet-sieving method, it did not result in an increased soil hydraulic conductivity, due to redisp ersion of clay colloids after leaching with distilled water. This stud y indicates that soil colloidal stability is strongly influenced by so il organic components in addition to factors such as pH, ionic strengt h, and composition. In the absence of polyvalent cations, negatively c harged humic substances, especially humic acid, may not contribute to stable soil aggregation, whereas the presence of both polyvalent catio ns and polyanions may be additive in preventing soil clay dispersion.