DIFFERENTIAL AVAILABILITY OF MANURE AND INORGANIC SOURCES OF PHOSPHORUS IN SOIL

As little information is available on whether differences in the avail ability of manure and fertilizer P in soil exist, manure management re commendations are being based on fertilizer response, Thus, the availa bility of P (as Fe-oxide strip P) from poultry litter leachate or KH2P O4 was investigated by incubation for 7 d with 193 soils with a wide r ange of properties associated with P sorption, Phosphorus availability , calculated as the slope of the linear relationship (F) between strip P and P added (0-108 mg inorganic P kg(-1)), was consistently greater for soils treated with KH2PO4 (F-k of 0.50) than litter leachate (F-l of 0.29), Although F-k and F-l were linearly related (r(2) of 0.92), relative availabilities were inversely related to the ratio of soil cl ay/organic C content (r(2) of 0.68), an estimate of the reactive surfa ce area involved in P sorption and availability, The differential avai lability in soil of the two sources of P is attributed to the complexa tion of P with Ca and organo-Ca, Fe, and Al, with the addition of Ca a nd soluble organic compounds in poultry litter leachate, Thus, manure management recommendations should be based on yield trials for manure and its availability in soil rather than on fertilizer P, Current fert ilizer recommendations may be modified by soil CaCO3, extractable Fe, and clay/organic C to account for the differential availability of man ure and inorganic P sources in soil.