Survey of water-extractable phosphorus in livestock manures

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) (23 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.