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Noncrystalline Phosphates in Florida Phosphatic Soils

Soil Science Dep., Univ. of Florida, Gainesville, FL 32611

* Corresponding author.

ABSTRACT

Six Florida soils formed over phosphoritic deposits were analyzed by horizon to verify the presence of noncrystalline P and to determine the P forms associated with Ca, Fe, and Al. Each soil contained only one phosphate mineral, as detectable by x-ray diffraction (XRD): carbonate-fluorapatite, wavellite, or crandallite. Some horizons contained no detectable crystalline P, despite the presence of appreciable total P. Quantities of fluorapatite and wavellite, determined by XRD and thermal analysis, respectively, were insufficient to account for total P. Higher proportions of noncrystalline P near the surface were substantiated by a decrease in ratios of oxalate-extractable P to total P with depth. Oxalate-extractable P was higher for most samples than for standard minerals. Oxalate was more effective than dithionite-citrate-bicarbonate (DCB) in extracting P associated with Al, but DCB extracted more P associated with Fe. Endothermic differential scanning calorimetry peaks (90–100°C) were reduced in size or eliminated by oxalate or DCB extractions, suggesting that the peaks originated from the dehydration of noncrystalline Al and Fe with which some P could be associated. The fraction of total P extracted by soil-testing reagents (Mehlich I and Olsen) was generally higher for horizons containing appreciable noncrystalline P associated with Al and Fe. Crandallite was resistant to dissolution by all extractants.

A contribution of the Florida Agric. Exp. Stn., Journal Series no. R-00804.

Received for publication June 18, 1990.

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