Dissolution characteristics of central Florida phosphate rock in an acidic sandy soil

Florida has more than 14 million ha of acidic soils and most of these acidic soils are very sandy. Transport of phosphorus from these soils to surface water is a great concern in Florida. One of the promising approaches is to develop slow release P fertilizers to replace the regular water soluble fertilizers. Dolomite phosphate rock (DPR) is an ideal material for making this type of fertilizer for acidic sandy soils, which are wide-spread in Florida. The effects of soil moisture, particle size, and source of DPR on the extent and rate of DPR dissolution in a representative acidic sandy soil were examined in an incubation study. DPR dissolution estimated by the NaOH extraction method was strongly dependent on the mineralogy and chemistry of the DPR source materials. Decreasing particle size increased the percentage dissolution of the DPR in soil. Soil moisture affected the initial dissolution rate of the DPR and the time to reach equilibrium. Soil extractable P (Olsen-P) generally increased with DPR dissolution, but the ratios of Olsen-P to dissolved DPR-P (NaOH-P) were significantly different between the two DPR sources. Dissolution of DPR in the acidic sandy soil was well described by the Langmuir and Elovich kinetic equations, with the Langmuir equation being better. Constant A derived from the Elovich equation and potential maximum dissolution (PMDPR) from the Langmuir equation were closely correlated with the increases in pH and exchange (Ca+Mg) after DPR application. The two parameters appeared useful in evaluating DPR dissolution and timing of DPR application in acidic sandy soils.