THE RÔLE OF CLAY MINERALS IN THE FORMATION OF SOIL STRUCTURE

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Abstract
The ability of kaolinitic and montmorillonitic clays to form water-stable aggregates when treated with sand, organic matter, hydrous ferric oxide, or electrolytes was tested by puddling the mixtures and exposing them to cyclic wetting and drying. The residue of ground alfalfa remaining after one month of incubation in admixtures with clay and sand had no effect on the aggregation of either kaolinite or montmorillonite. Hydrous ferric oxide had no measureable effect on the aggregation of kaolinite but 1.5% of the oxide decreased the amt. of montmorillonite remaining on the screen, possibly through the mechanism of mutual flocculation. Small amts. of sand added to mixtures of kaolinite and montmorillonite increased the resistance of the mixtures to dispersion in water. Larger amts. of sand had a reverse effect. Gel-like globules of mixtures of sand and homoionic montmorillonite were found to be more water-stable in the order of H-clay, Ca-clay, Na-clay. It was apparent throughout the expt. that the bulk of material remaining on the screens wherever montmorillonite was present existed as large gel-like globules rather than as definite aggregates similar to those found in natural soils. The results were thought to have a relationship to true aggregation, however, in view of the assumption that clays must dry from the gel rather than the flocculated state in order to act as strong cements capable of binding granules such as those found in field soils. It was also assumed that cements formed from clays drying out of gels where the adsorbed cations are less hydrated would be more water-stable than those from gels where the cations are more highly hydrated.

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