Frenkel, H., Inst of Soils and Water, The Volcani Centre, PO Box 6, Bet Dagan, Israel
Fey, M.V.
Levy, G.J., Inst of Soils and Water, The Volcani Centre, PO Box 6, Bet Dagan, Israel

The critical flocculation concentration (CFC) of the clay fraction of three strongly structured clay soils from Natal (two dominated by smectite and one by kaolinite) was compared with that of smectite and kaolinite in Na- and Ca-saturated forms. The soil clays had much higher CFC than the reference minerals. The addition of humic acid and of citrate, formate, orthophosphate, carbonate, and silicate to the reference clays increased the CFC by as much as a factor of 50. The results suggest that natural levels of soil humus, and possibly of certain inorganic anions, may be sufficient to impact a high degree of dispersion on the soil clay fraction. There is, therefore, some doubt as to the quantitative importance of clay mineral composition in dispersion behaviour. -from Authors