FREEZING POINTS OF A GROUP OF CALIFORNIA SOILS AND THEIR EXTRACTED CLAYS

Abstract

The influences of moisture content, texture, dissolved solutes, constituent clays, and mechanical alteration of structure were studied with regard to their effects on the freezing-point depression of soils. The corresponding moisture potentials were calculated by means of the freezing-point equation. The moisture potential at the moisture equivalent is influenced principally by the osmotic effects of dissolved solutes, for the soils investigated ; the pressure potential makes a relatively small contribution to the moisture potential at this soil-moisture content. At the permanent wilting %, wide variations were found to exist in the moisture potential from soil to soil. The evidence in this paper suggests that the energy of retention at the permanent wilting % is in the order of magnitude of [long dash]18 X 106 ergs per g., corresponding to an equivalent pressure of about 18 atmospheres. This is in general agreement with values reported by other investigators. Drastic mechanical treatment had only a slight influence on the energy curves. This was attributed to a high degree of stability of the microstructure[long dash]an effect especially emphasized in the case of the kaolinitic Aiken soil. Irrespective of the cause, puddling had a rather small effect on the energy curves, and abnormal moisture deficiencies of plants growing in puddled soils should be attributed to factors other than the intensity of retention of water. The clay content of the soils, among other influences, affects the moisture content corresponding to a given moisture potential. This influence becomes progres-ively less as the energy level diminishes. It appears that the higher conc. of the soil soln., which are necessarily associated with the lower soil-moisture contents, are not responsible in themselves, through their osmotic potentials, for masking the effect of the clay. The different soil clays display widely differing moisture potential curves. The curves for the clays lie between those for the natural soils on the one hand, and those for bentonite and kaolinite on the other. Kaolinitic types tend to behave as though they were coarser-grained, though there is no clear-cut relationship between the position of the energy curve and the type of predominating clay mineral present. Soils of related genetic origin tend to show similar energy curves. Unless this is due to a balancing of factors, it appears that the surface properties, extent of surface, and structural configuration are very much the same for colloids from soils of related origin.