THE SELECTIVE EROSION OF PLANT NUTRIENTS IN RUNOFF

The enrichment of several P forms (Bray I, labile, inorganic, and organic), N, C, and K in runoff sediment was investigated for six soils of varying physical and chemical composition, using simulated rainfall (60 and 120 mm h** minus **1). Differing enrichment ratios (ER) for C, N, and organic P (2. 00, 1. 61, and 1. 47 avg for the six soils) indicate that erosion may reduce the C/N/organic P ratio of the remaining surface soil. Average ERs, for Bray I (2. 45) and labile P (2. 89) were significantly greater than for the other P forms (1. 48). This was attributed to less aggregation of sediment compared to source soil for the major proportion (70%) of the runoff events studied. Phosphorus desorption-sorption characteristics, buffer capacity (1. 49), sorption index (1. 56), equilibrium P concentration (1. 80), and exchangeable K (2. 46) were also enriched in runoff sediment compared to source soil. The logarithm of ER for each P form, N, C, and K was related to the logarithm of soil loss, which ranged from 10 to 800 kg ha** minus **1. Statistically significant differences between regression equations for each nutrient indicate that more than one equation is needed to estimate different nutrient ERs.