Extractable phosphorus following soil amendment with manure from swine fed low-phytate corn

Manure application rate recommendations have been based almost exclusively on nitrogen (N) management considerations, which can result in over-application of phosphorus (P) and its accumulation in soil. Lowphytate (LP) corn (Zea mays L.) was introduced into swine (Sus scrofa domesticus) feed to reduce the amount of P in manure. Unlike ruminants, swine cannot utilize phytate, the major form of P in normal hybrid corn. The objectives of this study were to determine extractable P following swine manure application from different manure sources and compare the relative P availability of LP-manure to standard (S) manure. A laboratory incubation study was conducted using a Waukegan silt loam, a Verndale sandy loam soil, and five levels of total P (Pt) from three P sources (KH₂PO₄, S-manure, LP-manure). Extractable P from manures of swine fed low-phytate corn and standard corn diets were compared with a soluble P (KH₂PO₄) source. Slope ratio procedure was used to determine a phosphorus availability index (PAI). The effect of the P application rate on extractable Bray-1 P was linear (r²>0.97). Small but significant P mineralization was observed for the Verndale sandy loam soil. Mineralization of organic P was detected only at the highest LP-manure application rate for the Waukegan silt loam soil. Relative to KH ₂PO₄, the LP-manure and S-manure had PAI of 0.55 and 0.61, respectively. Swine manure P availability was about 60% that of the KH ₂PO₄ source for both manure sources. The total P concentration for the LP-manure was 42% lower than that for the S-manure. The important factor in the difference between manure sources is the 42% lower total P content for the LP-manure, not a difference in availability. Further evaluation of available P from LP-manure is needed under field conditions.