DIFFERENTIAL PHOSPHORUS RETENTION IN SOIL PROFILES UNDER NO-TILL CROP PRODUCTION

Field trials in Ohio have shown that the Bray-1 P soil-test level for optimum corn (Zea mays L.) production is greater for no-till than for conventionally (moldboard plow) tilled corn. Studies using Hoytville silty clay loam (fine, illitic, mesic Mollic Ochraqualf) and Canfield silt loam (fine-loamy, mixed, mesic Aquic Fraguidalf) taken from long-term no-tillage plots indicated decreased retention of P against the Bray-1 extractant in surface soils, compared with samples from deeper within the soil profile. In an effort to further examine the P-retention characteristics of no-till soil profiles, experiments were prepared using P-sorption isotherms, before and after a series of sequential anion resion extractions. Prior to sequential extractions, the isotherms showed decreased sorption of P at the soil surface (0-2 cm), compared with deeper soil layers. Resin extraction removed more P from surface layers than deeper depths, and the quantity of P removed with each extraction declined much more quickly in the surface layers than in lower depths. After resin extraction there was a trend for greater retention of P against the Bray-1 extractant in the upper soil layers. Sorption isotherms created after resin extraction show increased P sorption at all depths and closer agreement between the shape of the isotherm curves. It appears that accumulated P on the soil surface saturates P fixation sites, resulting in differential retention of P with depth.