Available Phosphorus by Five Different Soil Testing Methods and Fractionation in Southwest Florida Vegetable Production.

Under Best Management Practice (BMP) program developed by the University of Florida, recommendations for phosphorus (P) fertilizer management are based on Mehlich-1 (M1) soil test P levels of the production site at the beginning of the season. However, most vegetables in southwest Florida are grown on soils belonging to Alfisols or Spodosols with argillic (clay) or spodic (organic hardpan) sub-surface. The M1 P may not represent adequate available P thus provide accurate justifications for the recommendation in alkaline and calcareous soils. Therefore, soils (0-6” and 6-12”) were collected from vegetable production sites in southwest Florida for 2 years and available P by five different soil testing methods (M1, Mehlich-3, Olsen, Bray-1, and ammonium bicarbonate-DTPA) were analyzed. Sequential fractionation analyses of the top soil were also performed. Correlation coefficients between available P by the five test P methods and the labile P (the sum of water- and bicarbonate-extractable P) ranged between 0.379 and 0.726 (all p < 0.001) with the Mehlich-3 P being the best for the whole sample set for the first year. However, with data limited to < 300 mg P kg^–1 for the available P, Bray-1 P best correlated with the labile P (0.701, p < 0.001) for the same dataset. Depending on which soil testing method best represents plant available P, current BMP P recommendations for southwest Florida vegetable production should be revised.