Distinguishing Indigenous Phosphorus Effects On Soil and Water Quality in the Inner and Outer Bluegrass of Kentucky.

The addition of organic phosphorus (P) is necessary for sustainable productivity of cropping systems. The soils of Inner and Outer Bluegrass regions of Kentucky formed from Ordovician phosphatic limestones resulting in naturally high inorganic P levels. Because soils have a finite P sorption capacity the addition of organic P amendments in agricultural systems increases concerns about P enrichment of water resources. Characterization of 32 soils in the Inner and Outer Bluegrass regions showed similarities between the regions in terms of particle size, pH, CEC, and base saturation, but considerable differences in mineralogy and orthophosphate levels. The differences are likely resulting from significantly less interbedding with shales in the Inner Bluegrass region imparting different inherent mineral characteristics and higher indigenous extractable and soluble P. Mineralogical characterizations with XRD and TG analyses of the clay fraction suggested HIV, mica, kaolinite, and quartz as dominant minerals in both regions. However, unique to the Inner Bluegrass was the presence of fluorapatite and Mg-phosphate mineral phases. Soil solutions extracted by high speed centrifugation from in situ soil samples showed surface horizon concentrations of water soluble inorganic P to be significantly higher for Inner Bluegrass soils, likely resulting from an increase in weatherable P mineral phases in surface horizons. A 30 day aerobic incubation of soils amended with cattle manure, swine manure, and biosolid, was also used to assess the impact of organic P amendments on soil solution and mineralogy changes via the Visual MINTEQ equilibrium speciation program. Understanding how indigenous P mineral solubility is affected by organic P amendments will improve estimations of potential P release into solution and help develop proper management strategies to alleviate adverse impacts on water quality resources in the area.