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Effect of Exchangeable Sodium and Phosphogypsum on Crust Structure—Scanning Electron Microscope Observations¹

ABSTRACT

The effect of soil sodicity and phosphogypsum treatment on the structure of the crust of two soils (sandy loam and clay soil) was studied using a scanning electron microscope (SEM). The crust of the sandy loam with ESP 1.0, and the two soils at all levels of sodicity when treated with phosphogypsum, consisted of a compacted skin seal attributed to the mechanical impact of raindrops. Little clay movement or development of a "washed in" layer was observed under these conditions. Conversely, when the ESP > 1.6 (in the two soils) the crust consisted of naked sand and silt grains over a dense "washed in" layer. The compacted layer of disintegrated aggregates is not stable when soil and water conditions are favorable for clay dispersion (sodic soils and low electrolyte soil solutions). In soils with no exchangeable sodium, aggregate breakdown and surface compaction by raindrops are the dominant mechanisms for crust formation. In soils with ESP values above 1.5, chemical dispersion of the soil clays (enhanced by the sodicity of the soil, the low electrolyte concentration in the rain, and the stirring effect of the raindrops) and the downward movement of the clays into the "washed in" layer clogging the pores in this layer were the dominant mechanisms in crust formation. Soils with a "washed in" layer had a very low infiltration rate.

¹ Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel. No. 754-E 1983 series.

² Soil Scientists, ARO.

³ Physicist, ARO.

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