Soil Phosphorus Persistence after Manure Phosphorus Loading.

A timeframe for phosphorus (P) drawdown from soils historically treated with manure is necessary due to political and environmental drivers in areas where agricultural nutrient contributions to surface water are regulated. Our objective was to observe changes in Mehlich-3 (M3P), Ammonium oxalate (OxP), and water extractable (WEP) soil P concentrations and soil P saturation after cessation of manure applications. We calculated P saturation using ammonium oxalate (DPS_ox) as well as Mehlich-3 (M3PSR). The study was conducted on three sites in Maryland with the following soil classifications: a Mattapex silt loam and a Nassawango silt loam, which were treated with broiler litter, and a Donlonton fine sandy loam, which was treated with dairy manure. Treatments at these sites were four manure-P treatment rates and a no P control. The manure treatments were applied annually from 1994 to 1997. We examined the soils in 2001, 2004, 2006, and 2012. The effects of the manure treatments were clearly visible in all of the analyses across all years of the study. The M3P concentrations significantly declined for all treatments from 2001 to 2012, with only a few exceptions; however, little to no significant decrease was observed in M3P, DPS_ox, M3PSR, or WEP from 2006 to 2012 at all sites. Some convergence occurred between adjacent treatments. The highest manure treatment resulted in M3P concentrations of greater than 200 mg kg^-1 at all sites in 2012. Slow decline in M3P and the P saturation (DPS_ox and M3PSR alike) poses challenges for animal producers who have high-P fields from historic manure application, but still need to manage animal waste under P application restrictions. Data suggest there is sufficient available P in solution to sustain crop growth in scenarios were M3P is in the range that P application may be limited by a Phosphorus Site Index.