CLAY DISPERSION AS RELATED TO SOIL PROPERTIES AND WATER PERMEABILITY

Seal formation is affected by clay dispersion, and both parameters depend on the same soil and soil solution properties. Since clay dispersion can be determined easily in the laboratory, its use for predicting soil susceptibility to seal formation was evaluated. Laboratory clay dispersion tests were conducted on soil samples from semi-arid (northern Cameroon and Israel) and humid (Georgia, USA) regions. Clay dispersion (presented as percent of total clay) in distilled water (to simulate rainwater) was significantly correlated with silt and clay content. Clay dispersion was severe (>40%) in soils with silt and clay content >15%, which were the kaolinitic soils from the humid region and the predominantly smectitic soils from semi-arid regions. Kaolinitic and illitic soils from the semi-arid regions, whose silt and clay content was <15%, were less dispersive, Deflocculation of kaolinitic clay by decomposed organic matter present in humid conditions was suggested as an additional cause for the dispersivity of the kaolinitic soils from Georgia. In stable soils, clay dispersion was affected by exchangeable sodium percentage (ESP) at all levels. In dispersive soils, only high ESP levels increased clay dispersion. Clay dispersion decreased in electrolyte solution (EC = 0.8 dS m-1) and was significant mainly in the kaolinitic soils from both northern Cameroon and Georgia. The degree of clay dispersion depended also on the energy applied; different soils needed different amounts of energy for reaching their utmost dispersion. The correlation obtained between clay dispersion and the final infiltration rate or percent runoff explained only a relatively small proportion of the variance measured and, hence, prevented an accurate prediction of these two variables from percent dispersed clay.