The interaction of rainfall with the soil surface has a major effect on agricultural productivity and environmental quality in the Southeastern U. S. High-intensity rainfall during the summer months often causes surface crusting, which inhibits seedling emergence, decreases water infiltration into the soil, and causes accelerated soil erosion. Dispersion of soil clays and associated aggregate breakdown have been implicated in the process of soil crusting. The application of gypsum, particularly high-purity phosphogypsum, has the potential to flocculate soil clays into micro-aggregates, and thereby delay or decrease crust formation, provided that ionic strength of the soil solution is the primary factor responsible for de-flocculation.
The studies reported here show that gypsum does increase water intake rate and reduce soil loss, and that the mechanism is primarily an ionic strength effect. A new approach to simulating rainfall in the laboratory and field settings has been developed and tested, allowing runoff and erosion measurements to be more easily collected. The results from three soils tested under a single-nozzle simulator show significantly higher infiltration rates of soils recieving 5 mt/ha gypsum, with a 50% reduction in soil loss. The sediment produced from gypsum-amended plots was flocculated into silt-sized particles, which are less transportable and less likely to enter surface waters. In another experiment using a Greenville soil, Na- and CaSO4 amendments also produced the expected results of respectively increasing runoff and erosion (for Na) and singnificantly decreasing both factors for Ca-treated soils.
Laboratory studies on selected topsoils have demonstrated the importance of soil pH, soil solution ionic strength, and sodium levels in determining dispersive behavior. Even low levels of sodium, coupled with the low ionic strength of southeastern topsoils, leads to high dispersibility, which allows the fine fraction of the soil to clog water transmission pores and reduce water intake rates. Water conductivity decreases by a factor of two at sodium levels of 2-5% and ionic strength < 2 mM, compared to Ca-saturated systems at 5 mM, at pH 5-6. In a study of 9 Georgia topsoils, even a 10% saturation of gypsum in the solution phase caused significant flocculation of clays, although soils differed in their sensitivity to ionic strength changes. The flocculating effect of gypsum on phosphatic clays was evaluated in a separate study, and found to have little effect in the range of 10-20% solids suspensions of the clays, largely because the clays were already flocculated in these concentrated suspensions.
Field studies on the Appling soil have shown the utility of surface-applied gypsum over the growing season in increasing water infiltration and decreasing soil loss. The longevity of this effect appears to extend over at least a crop growing season under average field conditions. Regular yearly applications, perhaps at a rate of 1-2 mt/ha, seem to be necessary for a continued effect, although longer-term changes in soil properties may also be occurring. In one study cotton emergence was marginally increased by gypsum application. The economics of gypsum use under this scenario are uncertain: yield increases demonstrated in other studies may marginally cover the costs of application for some crops on responsive soils, but not on others.
William P. Miller, University of Georgia. December 1989.