Differential Availability of Manure and Inorganic Sources of Phosphorus in Soil
1997
As little information is available on whether differences in the availability of manure and fertilizer P in soil exist, manure management recommendations are being based on fertilizer response. Thus, the availability of P (as Fe-oxide strip P) from poultry litter leachate or KH₂PO₄ was investigated by incubation for 7 d with 193 soils with a wide range 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 (Fₖ of 0.50) than litter leachate (F₁ of 0.29). Although Fₖ and Fₗ were linearly related (r² of 0.92), relative availabilities were inversely related to the ratio of soil clay/organic C content (r² of 0.68), an estimate of the reactive surface area involved in P sorption and availability. The differential availability in soil of the two sources of P is attributed to the complexation of P with Ca and organo-Ca, Fe, and Al, with the addition of Ca and 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 fertilizer recommendations may be modified by soil CaCO₃, extractable Fe, and clay/organic C to account for the differential availability of manure and inorganic P sources in soil.
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