EFFECT OF LOW LEVELS OF EXCHANGEABLE SODIUM AND APPLIED PHOSPHOGYPSUM ON THE INFILTRATION RATE OF VARIOUS SOILS 1

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Abstract
The effect of exchangeable sodium percentage (ESP) on the infiltration rate (IR) and crust formation of four soils, varying in their texture, mineralogy, and CaCO 3 content was studied, using distilled water (EC ∼.05 mmho/cm) in a rain simulator. The IR of the four soils was very sensitive to low levels of ESP (5), whereas the hydraulic conductivity (HC) of these soils, as determined by Felhendler et al. (1974), was less sensitive. The presence of lime in the soils was effective in preventing HC decreases due to low levels of exchangeable Na. Conversely, lime was not effective in maintaining the IR of the calcareous soils. Phosphogypsum prevented the sharp drop in IR of the soils at all levels of ESP. The mechanical impact of the raindrops, the low concentration of electrolytes in the rain and in the surface soil solution, and the relative freedom for particle movement at the soil surface accounts for the higher susceptibility of the soil surface to exchangeable Na. The high rate of dissolution of phosogypsum, leading to the presence of electrolytes in the surface soil solution, accounts for the effectiveness of phosphogypsum in maintaining the high IR. The effect of low levels of ESP and the soil solution concentration on the IR of soils suggests that crust formation in soils is due to two mechanisms: (1) a physical dispersion of soil aggregates caused by the mechanical impact of the raindrops; and (2) a chemical dispersion caused by the soil ESP and the low electrolyte concentration in the applied water.