Soil and Nutrient Losses in Runoff with Selected Cropping Treatments on Tropical Soils

The soil and nutrient losses in runoff for selected cropping systems were studied using artificial rainfall techniques on three soils in Puerto Rico in 1967. This study was undertaken to determine the erosion control effectiveness of and nutrient removal in runoff from selected cropping systems. Soils were Humatas clay, Juncos silty clay, and Pandura sandy loam. Artificial rain was applied at 6.4 cm/hr fox 60 rain (Storm 1), followed 10 min later by 12.7 cm/hr for 60 min (Storm 2). Storms and 2 were combined and designated storm 3, representing less than 2- to more than 500-year frequency storms. Cropping systems tested were fallow, conventional tobacco (Nicotiana sp.), mulch-tilled tobacco, tobacco in grass strips, pangolagrass (Digitaria decumbens), and pangolagrass with all aboveground parts removed. Slopes ranged from 26 to 46%; plots were 10.7 m long. Test areas had been in grass for several years. Final infiltration rates were 3.7, 0.9, and 6.2 cm/hr for Humatas clay, Juncos silty clay, and Pandura sandy loam, respectively. All soils were highly aggregated and remained so throughout all storms. The low infiltration rates of Juncos silty clay are misleading. Actually, most of the rainfall infiltrated this soil but returned as interflow at the lower end of the plots. All soils were quite resistant to erosion due to their high degree of aggregation and in part to the grass grown in previous years. Tillage increased storage capacity in the soil and reduced runoff. All cropping treatments were effective in reducing erosion when compared with fallow, except on Humatas clay, where the fallow treatment permitted the least erosion. Average erosion on all treatments from Humatas, Juncos, and Pandura soils were: Storm 1 — 0.87, 0.51, and 0.06; storm 2 — 5.12, 1.20, and 6.63; and storm 3 — 6.00, 1.72, and 6.69 MT/ha, respectively. Nitrogen, potassium, calcium, magnesium, sulfur, and chlorine in runoff were measured from conventionally tilled tobacco during storm 1. Treated plots received by broadcasting 1,120 kg/ha (2.27 kg/plot) of 12-6-16 (N, P₂O₅, K₂O) fertilizer 1 kr before tests began. Concentrations were high at the initiation of runoff and decreased with time as the volume of runoff increased. The average concentration of N ranged from 0.01 to 2.02 ppm, which is well below accepted public health standards. Potassium ranged from 0.01 to 2.29 ppm. Concentrations were highest fox all nutrients in runoff from the fertilized Juncos silty clay plots, because almost all runoff from this soil was actually interflow—infiltrated water that moved through the tilled zone and along the tilled-untilled interface and then surfaced at the lower end of the plot. Although measured as runoff, no surface flow occurred except at the lower 0.3 to 0.6 m of the plots.