Phosphorus forms and extractability from three sources in a recently exposed calcareous subsoil

Irrigation-induced erosion and land leveling have decreased crop yields on approximate to 800 000 ha of south-central Idaho silt loam soils because of topsoil removal. Phosphorus availability is a known production problem aft er topsoil removal. This study evaluated the effect of three P sources on s oil P solubility by three standard methods for calcareous soils. A long-ter m study was initiated on a Portneuf silt loam (Coarse-silty, mired, superac tive, mesic Durinodic Xeric Haplocalcid) by removing the surface 0.3 m of t opsoil from strips between undisturbed topsoil strips. Phosphorus treatment s applied across all strips were conventional fertilizer (applied according to soil test), dairy manure, and cheese whey. All treatments increased the freshly exposed subsoil bicarbonate extractable ortho P concentrations up to or greater than the topsoil concentrations, which were more than adequat e for economical crop production. The high-whey and manure treatments incre ased the subsoil saturation paste and 0.01 M CaCl2 extractable ortho-P conc entrations up to or greater than the untreated topsoil ortho P concentratio ns. The initial topsoil ortho-P solubility was along the beta-triralcium ph osphate (beta-TCP) isotherm and the initial subsoil was well below the beta -TCP isotherm, The ortho-P solubility of the subsoil monocalcium phosphate (MCP) treatment remained just below the beta-TCP isotherm. The cottage chee se whey treatment increased subsoil P solubility up to the beta-TCP isother m and the manure treated subsoil ortho-P solubilities were between the beta -TCP and octacalcium phosphate (OCP) isotherms, Most subsoil ortho-P concen trations by all three extraction methods decreased from spring to fall and then increased over winter in the subsequent spring samples, Soil solution ortho-P concentrations decreased with time in the subsoil treatments except immediately following treatment applications. The topsoil ortho-P extract concentrations by all three methods varied among samplings but remained abo ut the same during the study period.