Sorption of Phosphorus from Swine,Dairy, and Poultry Manures

In most phosphorus (P) sorption studies, P is added as an inorganic salt to a predefined background solution such as calcium chloride (CaCl₂) or potassium chloride (KCl); however, in many regions, the application of P to agricultural fields is in the form of animal manure. The purpose of this study, therefore, was to compare the sorption behavior of dissolved reactive P (DRP) in monopotassium phosphate (KH₂PO₄)–amended CaCl₂ and KCl solutions with sorption behavior of DRP in three different animal manure extracts. Phosphorus single‐point isotherms (PSI) were conducted on eight soils with the following solutions: KH₂PO₄‐amended 0.01 M CaCl₂ solution, KH₂PO₄‐amended 0.03 M KCl solution, water‐extracted dairy manure, water‐extracted poultry litter, and swine lagoon effluent. The PSI values for the dairy manure extract were significantly lower than the CaCl₂ solution for all eight soils and lower than the KCl solution for six soils. The PSI values were significantly higher, on the other hand, for poultry litter extract and swine effluent than the inorganic solutions in four and five of the soils, respectively. Our observations that the sorption of DRP in manure solutions differs significantly from that of KH₂PO₄‐amended CaCl₂ and KCl solutions indicates that manure application rates based on sorption data collected from inorganic P salt experiments may be inaccurate.