Dissolution of north carolina phosphate rock in soils of the appalachian region

Forage production in the Appalachian Region is limited by soil acidity and nutrient deficiencies including phosphorus. North Carolina phosphate rock (PR), a carbonate substituted fluorapatite, should be reactive in these soils and may be appropriate as a low cost P fertilizer. An experiment was conducted to evaluate the extent of PR dissolution, extractability of P after dissolution, and soil factors controlling dissolution of PR and subsequent levels of extractable P in 32 acidic soil horizons. PR was mixed with each soil at a rate of 500 mg P/kg and incubated at a moisture content corresponding to 33 kPa tension. In an initial set of 15 soil horizons, samples were taken at 0.1, 3, 7, 14, 30, and 60 days and analyzed for PR dissolution by the NaOH (ΔP) and BaCl2-TEA (ΔCa) methods and for extractable P by the Bray and Olsen methods. A second set of 17 soil horizons was incubated for 30 days and analyzed for extent of PR dissolution and extractable P. North Carolina PR exhibited substantial solubility (1.3 to 71.5% after 30 days of incubation) in a majority of the soil horizons. Soil factors controlling the H supplying power of the soil (pH, pH-buffer capacity) and the concentrations of P and Ca in soil solution (exchangeable Al, P-sorption index, P-retention, soil moisture, organic C, and exchangeable Ca) were significantly correlated with extent of PR dissolution. Multiple regression equations indicated that no single soil property was responsible for promoting the dissolution of PR over a wide range of soils. The levels of extractable P in the soil were significantly related (P < 0.01) to the extent of PR dissolution but were modified by the characteristics of the individual soils. Soils with the greatest extent of PR dissolution did not necessarily have the highest levels of extractable P. Many of the soil properties controlling dissolution of PR were also significantly correlated with extractable P. Organic C apparently played an important role in maintaining the availability of P after PR dissolution. Results from this study suggest that North Carolina PR should have sufficient solubility and subsequent availability after dissolution to serve as a direct application P source in many pasture areas in the Appalachian Region.