Seasonal Dynamics of Phosphorus in Sulfur-Amended Organic Soils of South Florida.

The Everglades Agricultural Area (EAA) in south Florida was drained in the early 1900s for agricultural production. The dominant series is Histosols with organic matter content up to 80-90%.  Long-term tillage resulted in incorporation of CaCO₃ into the soil profile from underlying bedrock, increasing the pH to approximately 7.0-7.5.  In some cases, elemental S is used to reduce soil pH for the purpose of increasing micronutrient and P availability, which may also increase P mobility and potential runoff from fields.  The objectives were to quantify the seasonal dynamics of P distribution in various fractions as affected by S amendment.  Elemental sulfur was introduced in granular form (90% S) at 0, 112, 224, and 448 kg S ha^-1.  Phosphorus sequential fractionation was employed to determine sizes of different P fractions during the sugarcane growing season.  Pools of P fluctuated seasonally and their response to sulfur amendment varied.  Organic P was the major form in the soil while the Ca-bound P (Ca-P) dominated the inorganic fraction.  Labile P and Fe-Al bound P (Fe-Al-P) had the least contribution to total P and declined during the growing season, corresponding to P uptake patterns by sugarcane.  The size of Ca-P fraction was relatively stable but also subject to decrease during the growing season.  Humic-fulvic acid P and residual P fluctuated seasonally.  Total P stocks declined over time as a result of continuous reduction in inorganic fractions by sugarcane uptake.  Application of sulfur at rates up to 400 lb S acre^-1 had limited effects on altering soil pH.  Yet, a small reduction in soil pH did promote P accumulation in both labile P and Fe-Al-P fractions within 3 months of application.  None of the Ca-P, humic-fulvic acid P, and residual P responded to any rate of S addition.