Phosphorus Dynamics in Amended Soils during a Growing Season: I. Availability and Plant Uptake.

Recent studies indicate that soil extractable P and P in runoff are greatest immediately after fertilizer and manure applications. Our objective was to evaluate changes in Mehlich 3– and water– extractable P in soils amended with cattle (Bos taurus) manure and monoammonium phosphate (MAP) throughout two growing seasons. Cattle manure and MAP were applied at a rate of 184 and 300 kg P ha^-1, respectively, on a Pullman clay loam (Fine, mixed, superactive, thermic Torrertic Paleustoll). Unfertilized checks were included for P extractability comparisons. Grain sorghum (Sorghum bicolor (L.) Moench) was planted after fertilizer applications and received supplemental irrigation. Soil samples (0-150 mm) were collected prior to fertilizer applications and periodically throughout the growing season. Changes in Mehlich 3– and water– extractable P with time exhibited a rapid initial increase after fertilizer applications followed by seasonal fluctuations and a decline 15 weeks after planting.