Phosphorus speciation, mineralization, and uptake by corn in a soil amended with liquid swine manure

The soil application of liquid swine (Sus scrofu domesticus) manure as a source of P for crop production provides a means for the utilization of waste material. Manure rates have usually been based upon manure N concentrations, but this has sometimes resulted in excess P loadings due to the high N requirement of some crops, as well as the lower availability of N in manures relative to that of P. Applying higher than recommended P rates may result in greater runoff losses, which could lead to eutrophication of surface waters. As a result, future regulations may require that manure applications be based on P rather than N content, which would increase the amount of land needed for application. A two-year field study was conducted to examine the variability in nutrient content of soil-applied liquid swine manure, to determine the effects of liquid swine manure application on the distribution of soil P fractions, and to assess the availability of manure P to com (Zea mays L.). Liquid swine manure was surface-applied to plots in an attempt to provide 15, 30, and 60 kg P ha-1. Control plots received 60 kg P ha-1 as triple superphosphate and 170 kg N ha-1 as ammonium nitrate. Manure plots also received 170 kg N ha-1 by assuming 40 percent availability of manure N and using ammonium nitrate to supplement the deficit. Plots were planted with \u27DeKalb 689\u27 and \u27Pioneer 3343\u27 varieties of com. Prior to and after manure application and when com was at four leaf, ten leaf, and silking growth stages, depth incremented soil samples were collected for P analysis. Plant samples were also collected at each growth stage. Soil samples were analyzed to determine total and organic P, and a sequential dissolution procedure was used to fractionate the inorganic P into operationally defined aluminum (Al-P), calcium (Ca-P), iron (Fe-P), and reductant-soluble (Red-P) phosphate chemical pools. Available P was estimated using Olsen and Mehlich 3 extractants. Plant samples were analyzed for P content, and yields were calculated from grain harvested from the plot center rows. Nutrient concentrations in liquid swine manure were highly variable, and resulted in lower than expected P application rates. In 1992, manure P rates were 17, 32, and 49 kg ha-1 while 1993 rates were 5, 11, and 22 kg ha-1. Effects of P rate on soil P concentrations were observed only in samples taken from the surface 10 cm. Of the inorganic P fractions measured, Al-P was most responsive to P application. Mehlich 3 and Olsen extractable P were also consistently affected by manure and fertilizer application. Manure P was generally less effective than triple superphosphate in maintaining or increasing soil P levels. Com yields were not affected by manure or TSP addition in either year. Ear leaf P content increased with P rated in 1992 but was unaffected in 1993. Correlation of P uptake with soil P fractions suggested that Al-P and Mehlich 3 P were the most accurate predictors of P availability, but correlation coefficients were low and not consistent between years. In order to assess P fertility status in this manure-amended soil, methodologies are needed which are more sensitive to changes in the pool of available P