Agassi, M., Soil Erosion Research Center, Israel
Morin, J., Soil Erosion Research Center, Israel
Shainberg, I., Institute of Soils and Water, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel

The arable soils in the arid and semi-arid regions of Israel contain 2-6% exchangeable sodium in the upper layer. Samples of five soils, typical of the region, were exposed to simulated rainfall. Various amounts of phosphogypsum (0, 5 and 10 ton/ha) were applied directly on the surface of some soil samples and mixed into the upper parts of others. Six consecutive simulated rainfalls were applied to some samples, at intervals ranging from 3 to 5 days and with total amounts of 210 mm/ Infiltration rates of the soil samples were measured, and the amounts of runoff from fields were calculated by combining the infiltration rate information with data from typical rainstorms in the region. The infiltration rates (IR) of the five soils dropped sharply to final values of 1.5 to 2.5 mm/h. Phosphogypsum prevented sharp drops in the IR and maintained final IR at 8-12 mm/h. Spread over the soil, phosphogypsum was more beneficial than when mixed with the soil, and the optimal amount of phosphogypsum was found to be 5 ton/ha. The beneficial effect of phosphogypsum was maintained in the six consecutive simulated rainstorms. The soil surface was believed to be particularly sensitive to a low level of exchangeable sodium due to both the mechanical impact of the raindrops and the low concentrations of electrolytes in the rain and in the soil solution at the surface. Phosphogypsum, which dissolves readily, contributes Ca-electrolytes to the soil solution and prevents chemical dispersion of the clay. Calculating the effect of phosphogypsum on the amount of runoff in the region, we concluded that application of 5 ton/ha reduced the runoff to 0-43% of the control. © 1982.