Extractable Soil Phosphorus Related to Phosphorus Fractions and Sugarcane Yield On Florida Histosols.

Optimizing P fertilizer recommendations with proper soil testing will reduce over-application and in south Florida is vital for reducing P discharge into the Everglades. Five field experiments were conducted on different organic soils in the Everglades Agricultural Area with the objectives of 1) quantifying the forms of soil P and 2) relating extractable P using water, acetic acid, Bray 2, and Mehlich 3 extractants to these P fractions. Results of two additional P rate experiments were added to the dataset to further evaluate the relationships between extractable P and relative sucrose yield of sugarcane (Saccharum spp.). The percentages of five P fractions generally increased with increasing recalcitrance in the order of labile P, Fe-Al bound P, humic-fulvic bound P, Ca-Mg bound P, and residual P with the exceptions of two acid soils in which the highest percentage of total P was in the humic-fulvic fraction. The water extractant measured only labile P with a large increase in extractable P at pH<6. Acetic acid-extractable P included some of the more recalcitrant forms of soil P, residual P and possibly Ca-Mg bound P, and was assumed to extract the most P not considered plant available. Bray 2 primarily extracted labile P and Fe-Al bound P with possible contribution from humic-fulvic bound P and residual P. Mehlich 3 was the only extractant tested which included labile and non-labile (primarily Fe-Al bound) P while excluding residual P, thus was deemed to best indicate the plant-available P pool. Mehlich 3 also related best to relative sucrose yield and a new soil test calibration is proposed with a maximum rate of 36 kg P ha^-1 with < 10 g P m^-3 in pre-plant soil samples and no P recommended with > 30 g P m^-3.