SOIL EXTRACTABLE PHOSPHORUS CHANGES WITH TIME AFTER APPLICATION OF FERTILIZER: II. MANURE FROM SWINE FED MODIFIED DIETS 1.

Little is known about how diet modification and manure amendments affect extractable P in soils, which can be related to runoff P. The objectives of this study were to determine if there were any effects on water extractable soil, Mehlich 3, Morgan, or Olsen P with time due to fertilization with manure from dietary modification or manure amendment treatments. Soils containing high and low soil test P (STP) were fertilized with swine manure from two diets (normal and phytase) and two manure treatment regimes (control and aluminum chloride). Soils were incubated and analyzed at 0, 2, 4, 8, 12, and 16 weeks after fertilization. After 16 weeks, water extractable soil, Mehlich 3, and Morgan P decreased by 35, 14, and 66%, respectively, in high STP soil and 75, 27, and 74%, respectively, in low STP soil. Final Olsen extractable P levels were similar to initial values for high STP soil and were reduced by 59% in the low STP soil. Phytase increased water extractable P by 13 and 6 mg [middle dot] L^-1, in high and low STP soils, respectively, whereas AlCl3 decreased water extractable P by 8 and 6 mg [middle dot] L^-1 in high and low STP soils, respectively. When phytase and AlCl3 were used together, resulting P levels were not statistically different than unfertilized soil at 16 weeks. These results indicate that use of dietary modification and manure amendments can prolong the capacity of soils to receive manure applications, particularly if threshold STP levels are used as part of a manure management program.