Tuesday, November 14, 2006 - 9:00 AM

Validating the Phosphorus Loss Assessment Tool for the Organic Soils of North Carolina.

Rory Maguire, Laura Dell'Olio, and Deanna Osmond. North Carolina State Univ, Dept Soil Science, PO Box 7619, Raleigh, NC 27695

Phosphorus runoff and leaching from agricultural fields have been identified as major environmental concerns for the health of aquatic ecosystems.  North Carolina has responded by implementing the Phosphorus Loss Assessment Tool (PLAT). Based on previous research, the current version of PLAT is programmed to predict greater soluble P losses from organic soils than from mineral soils with the same M3P values. However, recent research specific to North Carolina’s organic soils has indicated decreased soluble P release in the presence of high Al concentrations. Our objectives were to determine (i) the Al content of organic soils (Typic Medisaprists and Terric Medisaprists) of North Carolina’s Lower Coastal Plain, and (ii) how the Al in these soils affects P retention. We sampled four organic soil series and determined M3P, M3Al, M3Fe, water-soluble P (WSP), total dissolved P, total P, pH, particle size distribution, and percentage organic matter. Water-soluble P and M3P were also measured in a 21-d incubation study in which P was added at a rate equivalent to 150 kg P ha-1. Preliminary analyses showed that as the percentage of organic matter increased so did WSP, indicating that organic matter could block P sorption. No relationships between M3P and WSP were evident, and wide distributions among soil series for several parameters assessed were observed, indicating that the organic soils have highly variable properties. In the incubation, the percentage of applied P that was sorbed (could not be extracted with water) was greater in soils with lower organic matter and/or higher M3Al. This shows that soil Al increases P sorption, while organic matter can prevent some P sorption. The results from this research will be used to modify as necessary the appropriate algorithms within PLAT.