Distribution of Phosphorus Forms in Virgin and Cultivated Soils and Potential Erosion Losses

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Abstract
The relative amounts and distribution of P forms in virgin soil profiles are compared with those of similar soils that had been cultivated and fertilized for at least 15 years. Eight agriculturally important soil series, representative of different U.S. cropping areas, are included. Cultivation and fertilizer P application resulted in an increase in the total and inorganic P content for five soils and a decrease in organic P content for seven of the eight soils. Overall the change in total, inorganic, and organic P contents of cultivated surface horizons was +25, +118, and −43%, respectively. Fertilizer P application and mineralization of organic P during cultivation increased the available P (Bray‐I) content of all surface horizons. The content of all P forms in the subsoils was relatively unaffected by cultivation. Little change in total N/organic P ratio, exchangeable K, and cation exchange capacity was observed with cultivation and profile depth. The change in content of P forms with cultivation and fertilizer P application was closely correlated with P content of the virgin soils and amount of fertilizer P added during cultivation. The effect of sheet erosion of the surface 15 cm of virgin and cultivated soil on the content of P, N, and C forms, was estimated by comparing the nutrient contents of the 0‐ to 75‐ (uneroded) and 15‐ to 90‐cm (eroded) depths. Sheet erosion of the surface 15 cm of soil would reduce the inorganic P, organic P, available P, mineralizable N, and organic C contents of the virgin soil profile by an average of 15, 18, 25, 46, and 31%, respectively, while increasing P sorption index by 51%. The percent decrease in these P, N, and C forms (16, 2, 9, 35, and 22%) in the cultivated soil profile following sheet erosion was smaller than for the virgin soil. In general, simulated sheet erosion affected the P status of the soil profiles less than that of N.