Recent studies suggest that phosphorus (P) saturation of the soil's sorption complex may be a better predictor of potential P release from soil and consequent loss to the environment than agronomic soil tests. Traditional laboratory procedures to determine P sorption capacity (PSC) are time-consuming and expensive to conduct, and therefore not readily adaptable to routine use. More recent work has focused on development of predictors of sorption capacity and P saturation using Mehlich-3 or oxalate extractable Fe and Al in the soil. Application of poultry manure or liming materials to naturally acidic soils increase calcium levels and soil pH, thereby altering the soil's capacity to sorp P. To determine the effects of poultry and dairy manure application on the P sorption capacity of naturally acidic soils, batch P sorption isotherms were conducted on 17 New York State soils; 7 dairy manure amended soils, 7 poultry layer manure amended soils and 3 soils with no recent manure history. The soils were analyzed for Mehlich-3 extractable Fe, Al and Ca to compare sorption estimation equations with maximum sorption capacities determined by the batch equilibrium studies. Poultry manure applications increased Mehlich-3 extractable Ca levels from 3.8 – 10.7 g Ca kg^-1 and total Ca levels from 6.4 – 51.2 mg Ca g^-1. Batch studies showed an increase in maximum P sorption capacity of the poultry manure amended soils. Because all fields consisted of a similar soil type, Fe and Al content did not change significantly between soils, suggesting that the increase in maximum PSC in the poultry manure amended soils was due to Ca addition to the soils. For accurate predictions of PSC based on Mehlich-3 data, Ca must be included.