Addition of 25% FGD gypsum by weight to poultry litter effected maximum reduction in H₂O-extractable P and a similar increase in NaHCO₃-extractable P when litter was incubated alone or mixed with soil. This shift of P from one fraction to the other did not affect plant P availability as measured by the Mehlich-1 P test. Thus, addition of FGD gypsum to litter may reduce the risk of contaminating surface runoff with P without apparently affecting P availability. Additional work should be conducted to evaluate the effect of FGD gypsum addition to poultry litter on actual plant P uptake to confirm our findings. Increasing incubation time also resulted in a reduction in H₂O-extractable P in litter with or without FGD gypsum addition, but while in litter alone this reduction was accompanied by an increase in the NaHCO₃ fraction, in litter mixed with soil it was followed by an increase in the NaOH fraction. In contrast to results from the laboratory incubation, stacking litter for 31 days did not affect H₂O-extractable P, but it increased P in the NaHCO₃ fraction by 25%. These results suggest that stacking poultry litter may increase the amount of labile P.

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