Soils with high levels of P can contribute to excess P in runoff and subsequently pollute the surface water. Excess P in the soil can be removed from the system by harvesting crops. The objectives of this study were to evaluate corn (Zea mays L.) P removal effects on soil P reduction, and to evaluate various corn hybrids and soybean [Glycine max (L.) Merr.] varieties for differences in grain P concentration and P removal. Soil with varying P levels as a result of annual or biennial beef cattle (Bos taurus) feedlot manure or compost application was cropped to corn for 4 yr without any P addition. In other studies under various water and N regimes, corn hybrids and soybean varieties were evaluated for grain P concentration and P removal. Four years of corn production without P addition lowered surface soil (0-15 cm) extractable P level (Bray and Kurtz no. 1) from 265 mg kg-1 to 171 mg kg-1 in the biennial N-based compost treatment. Based on a decay equation, it would have required 10 yr of corn P removal to lower the soil P level to the original 69 mg kg-1 that existed before treatment application. The rate of decrease in extractable soil P was greater when soil P was higher and reduced with decreasing soil P level. Most of the P in the plants was absorbed from the 0- to 15-cm soil depth since no significant reduction in soil P level was observed from 1996 to 1999 in the 15- to 30-cm soil depth. Across 2 yr, there was as much as 54% difference among corn hybrids for grain P removal. The differences in P concentrations among corn hybrids indicated that hybrids could be selected for low P uptake when lower P level in ethanol production by-product or in animal ration and subsequently in manure is desired. Soybean grain P concentration was nearly twice that for corn but grain P removal was less for soybean than for corn. Crop P removal can significantly reduce soil P level with time.
