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For nutrient management purposes, it is important to understand the impact of manure application on soil phosphorus (P) sorption characteristics and what it means with regard to potential environmental problems. The objectives of this study were (i) to determine whether previous manure applications impact P sorption capacity and strength; (ii) to characterize the relationship between water soluble-P (WSP) and degree of P saturation (DPS); and (iii) to characterize the relationship between WSP and P sorption capacity and strength.

Phosphorus sorption isotherms were constructed for seven pairs of soils with and without a history of manure application. Within a soil series, P sorption isotherms were compared to determine the effects of manure application.

Manure application increased sorption capacity significantly in the Nicollet soil series. Phosphorus sorption capacity was unchanged by manure application in the Waukegan soil series. Manure application reduced P sorption capacity in the Port Byron, Sanburn, Verndale, Ves, and Barnes soils. Phosphorus sorption strength decreased in five of the seven soil series after manure application. The degree of P saturation and soil test P were strongly linearly correlated to WSP. Water soluble-P was less than 1 mg L−1 when DPS was less than 21.7%.

When manure application increased DPS to more than 26%, sorption strength was reduced; P was bound less tightly to the soil on low strength sorption sites, such that P was more readily available. Sorption strength was used to determine the soil test P level above which there is increased risk to water quality because of reduced sorption strength and subsequent increases in WSP; this occurred when Bray-P was greater than 60 mg kg−1. Phosphorus sorption strength seems to be the sorption property that is most sensitive to applications of P.

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