Survey of Water‐Extractable Phosphorus in Livestock Manures

Abstract

Water‐extractable P (WEP) in manure is increasingly used as an environmental indicator as it is correlated with P in runoff from soils recently amended with manure. Little information exists on WEP variability across livestock manures. A survey of 140 livestock manures was conducted to assess trends in WEP (dry weight equivalent) related to livestock types and manure storage. Manure WEP ranged widely (0.2–16.8 g kg−1), with swine (Sus scrofa domestica L.) having the highest average concentrations (9.2 g kg−1), followed by turkey (Melleagris gallopavo) (6.3 g kg−1), layer chickens (Gallus gallus domesticus L.) (4.9 g kg−1), dairy cattle (Bos taurus) (4.0 g kg−1), broiler chickens (Gallus gallus domesticus L.) (3.2 g kg−1), and beef cattle (Bos taurus) (2.3 g kg−1). Manure WEP also differed by general storage system; dry manures contained significantly lower WEP concentrations (3.9 g kg−1) than manure from liquid storage systems (5.4 g kg−1). Within liquid storages, no significant differences in WEP were observed between covered and uncovered storages or between bottom‐loaded and top‐loaded storages. Dry‐matter (DM) content of manure was weakly correlated to WEP across all manures (r = −0.44), but strongly correlated with WEP in liquid swine manure (r = −0.87) and dairy manure (r = −0.72), suggesting dissolution of phosphate compounds as manure solids are diluted in storage. Varying positive correlations were observed between WEP in manure and water‐extractable Ca, Mg, and Fe, or total P, depending on livestock category. Results of this study show that livestock manure can be categorized by WEP, a key step toward differential weighting of agricultural P sources in P site assessment indices.