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, vegetables are grown on more than 300,000 acres throughout the state in various soils. Most soils in southwest Florida belong to Alfisols or Spodosols with argillic (clay) or spodic (organic hardpan) sub-surface. The M1 P levels may not represent adequate available P thus provide accurate justifications for the recommendation in some soils. Therefore, soils were collected from vegetable production sites in southwest Florida and available P levels by five different soil testing methods (M1, Mehlich-3, Olsen, Bray-1, and ammonium bicarbonate-DTPA) were compared. Surface soil pH ranged from 6.4 and 8.6. Correlation coefficients between available P by M1 and those by other 4 methods ranged from 0.606 and 0.733 (all p < 0.001). Compared to M1 method, all other 4 methods extracted less amounts of available P (59%, 22%, 51%, and 25% with M3, Olsen, Bray-1, and AB-DTPA, respectively). Depending on which soil testing method best represents plant available P, current BMP P recommendations should be revised.