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 field 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.