The influence of mineralogical composition, saturating ions, and electrolyte concentration on the flocculation and dispersion behavior of various clay minerals was used to explain reductions in the hydraulic conductivity observed in many sodic soils containing little or no expandable clay minerals. Sodium-saturated clay minerals were found to differ in their sensitivity to flocculation by NaHCO3 in the decreasing order: illite, vermiculite, smectite, and kaolinite. Critical salt concentrations (CSC) were 185, 58, 28 to 60, and 8 me/liter, respectively, for the above clays.
Very low concentrations of CaCl2 or MgCl2, on the order of 1 me/liter or less, were required to flocculate Ca- or Mg-saturated clays. At a sodium adsorption ratio of 15, CSCs were several times greater than those for CaCl2 or MgCl2. A small amount of smectite, possibly deposited on positively charged edges of kaolinite, was responsible for large increases in CSC values of kaolinite. The flocculation behavior of soil clays extracted from four California soils was influenced by their mineralogical composition, similar to the patterns observed for reference clay minerals.