Soil phosphorus forms after successive pig slurry application in a native pasture

The swine industry generates large amounts of pig slurry (PS), which is reused as agricultural fertilizer. However, if applied at excessive rates, pig slurry becomes a severe environmental pollutant. The objective of this study was to evaluate forms of soil phosphorus accumulation after successive swine manure applications. The experiment was installed in 1995, in an area of native pasture. Rates of 0, 20 and 40 m(3) ha(-1) of PS were applied periodically (at 45 to 60 days intervals) during five years. In 1999, after 28 PS applications totaling 0, 560 and 1.120 m(3) ha(-1) of PS, soil samples were collected from the layers 0-2.5, 2.5-5.0, 5-10, 10-15, and 15-20 cm. Soil phosphorus in the samples was fractionated using Hedley's fractionation scheme (Hedley et al., 1982). The maximum phosphorus adsorption capacity (MPAC) was determined in soil samples without PS application using the Langmuir model. Results showed that phosphorus added as PS is accumulated essentially in inorganic forms, mainly those extractable by anion exchange resin, NaHCO3 0.5 mol L-1, NaOH 0.1 mol L-1 and HCl 1.0 mol L-1.