Highly weathered soils in the southeastern United States may suffer from dispersion-related degradation of physical properties with additions of Na, either from fertilizer or wastewater sources. A clayey Southern Coastal Plain soil used in this study showed severely reduced infiltration and much higher soil loss rates, when NaNO3 was surface-applied to the soil at fertilizer rates in a rainfall simulator experiment conducted in small runoff pans. Gypsum applications, often used to reclaim naturally sodic soils, resulted in significant increases in infiltration and decreases in soil loss, when applied both alone and with NaNO3. Gypsum added to a previously Na-impacted soil improved infiltration and soil loss levels to roughly the level of untreated soil. Clay dispersion was observed in the untreated soil in the form of primary clay in the sediment; the amount of this sediment clay was greatly increased by NaNO3 treatment and was virtually eliminated by gypsum treatment, applied either alone or in combination with NaNO3. We suggest this dispersion accounts for the observed differences in runoff and soil loss between NaNO3, gypsum, and untreated soils. The use of gypsum may be advised on similar soils if Na is added and may be a useful amendment in the absence of applied Na as well.
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