Nitrogen and Phosphorus Losses in Runoff from No-Till Soybeans
ABSTRACT NO-TILL soybeans and corn were grown on highly erodible loessial soils in north Mississippi with reduced soil and plant nutrient losses. The average an-nual soil loss for the two-year period 1972-1973 from no-till soybeans, planted directly through previous crop residues, was only 0.3 metric tons per hectare (t/ha) as compared with 29 t/ha from conventional till. Total (solution plus sediment) nitrogen (N) and phosphorus (P) losses in 1973 from no-till soybeans were only 4.7 and 2.8 kg/ha, respectively, as compared with 46.4 and 17.6 kg/ha from conventional till. No-till effectively reduced soil and associated plant nutrient losses, but increased solution P concentrations and losses in runoff. Solution P concentrations and losses, which were related to crop management practices, increased in the following order: conventional till soy-beans (continuous) < no-till corn-soybeans (rotation) < no-till soybeans (continuous) < no-till soybeans-wheat (double-cropped). The increase in solution P concentra-tions and losses from no-till is attributed to (a) insuffi-cient sediment to sorb P from solution; (b) the additional P fertilizer applied to the double-cropping system; (c) the limited sorption of fertilizer P by the soil caused by decreased fertilizer incorporation; (d) the release of P from crop residues; and (e) possibly a greater phosphate supplying capacity of sediments in runoff from no-till. The mean solution P concentration in runoff from no-till soybeans was 0.40 mg/L, as compared with 0.02 mg/L from conventional till soybeans receiving the same amount of phosphate fertilizer but with more incorpora-tion into the soil at planting. The low erosion rates from no-till practices are signifi-cant because of the potential reduction in soil loss from the increasing acreages of soybeans being planted on marginal land. Farmers can continue to plant soybeans on sloping land and minimize time and labor input without serious erosion and total plant nutrient losses. Thus, where applicable, no-till is a best management practice for reducing potential nonpoint pollution by sediment and total plant nutrients in surface runoff from agriculture.