Nitrogen and phosphorus yields in run-off from silty soils in the Mississippi Delta, U.S.A.

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Nutrients and sediment concentration in surface run-off from an 18.7-ha watershed, planted to continuous cotton ( Gossypium hirsutum L.), were measured from March 1973 through February 1979 to determine: (a) nutrient and sediment yields; (b) mode of nutrient transport; (c) nutrient concentration frequency distributions; (d) nitrogen (N) and phosphorus (P) enrichment in sediments. The watershed, consisting of silt loam (72%) and silty clay (28%) soils, had been land-formed to a mean slope of 0.2%. The 6-year average annual rainfall, run-off and sediment yields were 1335 mm and 20.8 t ha −1, respectively. Nitrogen fertilizer was applied at the rate of 171 kg ha −1 year −1. P fertilizer was not applied because cotton growing on these soils does not respond to P fertilization. The 6-year average soluble N (ammonium N plus nitrate N) and P yields were 7.7 and 1.6 kg ha −1 year −1, respectively, compared with sediment N and P yields of 34.6 and 19.6 kg ha −1 year −1. Thus, about 18 and 7% of the total (sum of solution and sediment) N and P yields were transported in solution; the remainder was transported by the sediments. Sediment and nutrient yields were greater than expected from these flatlands. Without P fertilizer additions, soluble P concentrations in 77% of the run-off exceeded the 0.1 mg l −1 criterion proposed by the U.S. Environmental Protection Agency. Improved run-off and erosion control practices are needed to significantly reduce sediment and chemical yields.