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Effect of Sodium Polymetaphosphate on Soil Crust Formation and Runoff/Rain Relations¹

ABSTRACT

The effect of sodium polymetaphosphate (NaPMP) on crust formation and on the amount of runoff was studied with two soils, Typic Rhodoxeralfs (sandy loam) and Xeric Torripsamments (loamy sand), using a rainfall simulator. Increasing the rate of NaPMP application in the range 100 kg ha^–1, increased the rate of crust formation and decreased the final infiltration rate (IR). The increased rate of crust formation increased the fraction of runoff of the total rainfall and decreased the threshold rain needed to form runoff. The dispersant, NaPMP, weakened the stability of the aggregates and increased the amount of clay that migrated with the infiltrating water and accumulated in the "washed in" layer. Hence, the effect of NaPMP was more pronounced in a loamy sand, where the low amount of clay limits the formation of a crust, compared with the sandy loam, which contains enough clay to form a crust. In consecutive short storms of 18 mm each with complete drying between storms, the rate of crust formation was slow and the amount of runoff was low in the control treatment, compared with the high efficiency of NaPMP for increasing runoff. The low amounts of NaPMP required to significantly increase runoff gives the potential for commercial use in runoff farming.

¹ Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, no. 1435-E, 1985 series.

² Soil Scientists, Institute of Soils and Water, ARO, The Volcani Center, Bet Dagan, Israel.

Received for publication November 4, 1985.