Quantifying Long-Term Availability of Phosphorus in Swine Manure.

Recent research advances indicate that legacy phosphorus (P) in soils can be usable by crops in the following years, while its availability varies depending on P source and the accompanying components. Long-term availability of manure/fertilizer P must be quantified on a source-specific base to develop economically and environmentally sound BMPs. We conducted an 8-year study to evaluate the effects of repeated application of chemical fertilizer and three forms of swine manure (liquid, LM; solid, SM; and liquid manure compost, MC) on crop yield, P uptake, and changes in soil test P (i.e. Olsen P) under corn-soybean rotation in a clay loam soil. Phosphorus from each source was added to the corn phase at 100 kg P ha^-1 yr^-1. Manure P source coefficient (PSC) was calculated in equivalency to the chemical fertilizer. Corn yield over the 8-year period followed the order of LM>SM≥MC, while soybean yield did not differ amongst the three forms of swine manure. Similar patterns were observed for total P uptake and grain P removal for both corn and soybean. Level of soil test P increased with year of application, but the incremental varied with the source of P added. When taking crop P removal and the increases of STP into account, each unit increase of STP required 12.2, 9.8, 16.1 and 10.7 kg ha^-1 net P addition for CF, LM, SM, and MC, respectively. The calculated manure PSC values were 1.10, 1.07 and 1.02 for LM, SM, and MC, respectively, indicating that long-term P availability of swine manures was about identical to that of chemical fertilizer, with some potential for mobilizing indigenous P in soil, regardless of the form. Long-term cumulative P agronomy efficiency, defined as grain yield produced with each kg net P input in a given year-period since its 1st application, decreased linearly with increases in net P input over time, regardless of crop species and P source. P-based manure application must be optimized on a manure-form specific base. Long-term P management must take the effect of legacy P into consideration.