Water-extractable phosphorus in animal manure and manure compost: Quantities, characteristics, and temporal changes

Water-extractable P (WEP) in manure and manure compost is widely used as an indicator of P release to runoff from manure and compost that are land applied. A survey of 600 manures and composts was conducted to assess trends in WEP (dry weight equivalent) related to manure and compost types from sources in the Mid-Atlantic region. Manure and compost WEP ranged from 0.2 to 20.8 g kg^-1. Mean WEP was highest in turkey and swine manures (manure: 4.1-5.6 g kg^-1; no composts tested), followed by layer and broiler chicken manures (manure: 3.0-3.5 g kg^-1; compost: 4.6-5.1 g kg^-1), cattle manure (dairy and beef manure: 2.1-2.8 g kg^-1; compost: 1.1-2.7 g kg^-1), and horse manure (manure: 2.7 g kg^-1; compost: 1.9 g kg^-1). Across all manures and composts, WEP was negatively correlated with manure dry matter content (r = 0.42, p < 0.001). Moreover, WEP was strongly correlated (r = 0.66, p < 0.001) to degree of P saturation expressed as a molar ratio of total P to total metals (Ca, Mg, Fe, Al, and Mn). Although WEP levels of beef, broiler chicken, and turkey manures from this survey are similar to those from a decade ago, WEP is now significantly lower for dairy (30%, p < 0.001), swine (46%, p < 0.001), and layer chickens (39%, p < 0.05). Lower WEP resulted from decreasing total P and/or increasing P sorption capacity, combined with increasing dry matter content. Results highlight the potential to use degree of P saturation to predict WEP and suggest an opportunity to reduce WEP by managing manure handling, storage, and chemistry.