Availability of residual phosphorus in manured soils

In many areas with confined animal operations, continual manure application has increased soil P above amounts sufficient for optimum crop yields. In these areas, it is of economic and environmental importance to determine how long high-P soils will remain above crop sufficiency and identify soils where P contents would decrease most rapidly under similar management conditions. Thus, the surface 5 cm of 23 high-P soils (85-419 mg kg(-1) Mehlich-3 P) in Oklahoma and Texas, which had received beef feedlot, poultry, or swine manure (90-1880 kg P ha(-1) yr(-1) for up to 35 yr) were successively extracted with Fe-oxide-impregnated paper strips to investigate residual soil P availability. A decrease in strip P with successive extractions followed the equation: Strip P = a(extraction number)(-b) (r(2) of 0.88-0.98). The rate of P release to strips (exponent b) decreased more rapidly as soil P sorption saturation increased (R(2) of 0.79). Phosphorus saturation also accounted for 85% of the variation in the total amount of P released to strips from manured soils in 15 successive extractions (51-572 mg kg(-1)). Fractionation of soil P before and after strip extraction showed bicarbonate inorganic P contributed most of the P released to strips (46%). The above equation also described soil P release in several published held studies (r(2) of 0.77-0.98). Thus, successive strip extraction of soil has the potential to describe soil factors controlling the availability of residual P and identify soils where high P contents may be less buffered and, thus, decrease more rapidly than others under similar management conditions.