Developing Optimum Ratios of Soil Extractants as Indicators of the Phosphorus Available to Plants in Florida Calcareous Soils.

Phosphorus (P) is a plant essential nutrient, but can also be a pollutant. Therefore, best management practices (BMPs) are being developed by the University of Florida and adapted by Florida farmers to efficiently manage the fertilizer applied to their crops and minimize environmental pollution. The C-139 Basin in southwest Hendry County, a tributary to the Everglades, contains sandy soils with pH > 7.5 and calcium content exceeding 2000 ppm. In this soil, applied P can precipitate with calcium and become unavailable to plants. Most soil extracting solutions are acidic and dissolve precipitated P causing an overestimation of the P available to plants. The purpose of this research was to determine the optimum ratios of soil to extraction solutions that will provide a reliable analysis of bioavailable P in calcareous soils. Soil P contents using ten soil to extractant ratios were determined and compared with plant available P using sequential analysis. For Mehlich 3, Bray, Olsen, and ABDTPA, the optimum solution to soil ratios were 40:1, 40:1, 50:1, and 30:1, respectively. With the exception of Mehlich 1, the selected optimum ratios (for Mehlich 3, Bray, Olsen, and AB-DTPA) were superior to the standard ratios. These findings are of critical importance to Florida farmers growing crops on calcareous soil because this allow them to apply the correct amounts of P if needed and reduce excess P application.