Soil Phosphorus Storage Capacity and Fertilization in Southwest Florida Vegetable Production.

Using the Best Management Practice (BMP) program developed by the University of Florida, recommendations for phosphorus (P) fertilizer management are based on soil test P (STP; i.e., Mehlich-1) levels of the production site at the beginning of the season. Most vegetables in southwest Florida are grown on soils belonging to Alfisols or Spodosols with argillic (clay) or spodic (organic hardpan) sub-surface layers. Phosphorus fertilization to these soils may cause accumulation of P in soils and overestimation of plant-available P for BMP recommendations. Moreover, sandy soils in some areas are low in organic matter and Fe/Al-oxides to retain P in soils, and others may be saturated with P due to long-term P fertilization even with high soil pH and extractable Ca contents, possibly causing environmental impacts in the watershed of the region. Soil P storage capacity (SPSC) provides a direct estimate of the amount of P a soil can hold before exceeding a threshold soil equilibrium concentration (i.e., before the soil becomes an environmental risk), which the STP value itself cannot express. More than 800 soil samples were collected from 5 different vegetable farms using seepage irrigation within the same watershed to calculate SPSC. Soil pH ranged from 5.5 to 8.6 with an average pH of 7.3. The SPSC index categorized more than 50% of the soil sampled as predicted to lose P from the soil (i.e., environmental risk). It also appeared that most soils had excessive lime resulting in both high pH and extractable Ca, thus crop response to P fertilization would be likely due to excess Ca reacting with P and reducing plant-available P. The SPSC is useful when considering P movement and crop response, and indicates the need for improved soil pH management to benefit from P fertilization in southwest Florida vegetable production.