REACTION IN SOIL OF PHOSPHORUS RELEASED FROM POULTRY LITTER

Poultry production generates large amounts of litter or manure, which can be a valuable source of P for crops. However, litter application r ates are usually based on data for mineral P fertilizer recommendation s. In order to determine if this is agronomically and environmentally sound, the availability, fractions, and sorptivity of P from poultry l itter or KH2PO4 were determined in six Oklahoma soils following incuba tion for up to 28 d. An average 50% more P from KH2PO4-treated (78 mg kg(-1)) than from leachate-treated soils (52 mg kg(-1)) was bioavailab le, as determined by extraction with Fe-oxide-impregnated paper strips , after 28-d incubations. Conversely, more NaHCO3-extractable P was fo und in leachate-treated (66 mg kg(-1)) than KH2PO4-treated soils (42 m g kg(-1)). Calculated from Langmuir isotherms, P sorption maxima avera ged 548 mg kg(-1) for leachate-treated and 304 mg kg(-1) for KH2PO4-tr eated soils, while binding energies averaged 0.527 and 0.456 L mg(-1), respectively. The higher P sorption maxima and binding energies of le achate-treated soils may result from the formation of Ca-P complexes, given the increased Ca content of these soils. The different reaction in soil of P added as poultry litter leachate to that added as KH2PO4, indicates manure application rates should be based on soil tests that are sensitive to P source-dependent sorption characteristics and/or m anure trials, and not just on mineral fertilizer trials.