EFFECT OF SODIUM-MAGNESIUM AND SODIUM-CALCIUM SYSTEMS ON SOIL HYDRAULIC CONDUCTIVITY AND INFILTRATION

We compared the effect of exchangeable Mg on the hydraulic properties of three South African soils to the effect of exchangeable Ca using Na-Mg and Na-Ca systems with different levels of exchangeable Na. The Na-Mg system always maintained lower relative hydraulic conductivity (HC) values than the corresponding NaCa system when leached with distilled water, although the electrical conductivity of the effluent after leaching with similar volumes of distilled water was higher in the Na-Mg system. Infiltration studies using a rainfall simulator to form a crust at the soil surface yielded the same cumulative rain and final infiltration rate for both the Na-Mg and Na-Ca systems. The results suggest that the “specific” effect of exchangeable Mg is evident only in measurements that are sensitive to small differences in clay dispersion, such as HC measurements. In a system where the impact energy of the raindrops, and hence the physical dispersion of the soil aggregates, predominates the chemical effects of MG vis-à-vis Ca on the permeability of the soil are obscured.

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