Phosphorus-sorption characteristics of calcareous soils and limestone from the southern Everglades and adjacent farmlands

The understanding of P sorption and desorption by soils is important for safeguarding water quality and for fertilizer management. Little is known about the P-sorption characteristics of the calcareous soils and limestone bedrocks in southern Florida. In this study, 19 samples of calcareous soil and subsurface limestone bedrock were collected from the southern Everglades wetland, pineland, and nearby farmland. At very low P concentrations. P sorption in these soils fit the linear isotherm. The equilibrium P concentration at zero-net P sorption (EPCo) of soils correlated positively with P saturation. Phosphorus-sorption data of soils at medium P concentrations fit the Freundlich and Langmuir equations up to a point at high P concentrations where the slope of the isotherm changed abruptly. In bedrock samples the sorption-isotherm-inflection point, which is caused by P precipitation, occurred at much lower solution P concentrations than in the case of soils (4-18 vs. 400-600 mg mL (1)). Also bedrock samples had significantly lower Freundlich values, K-f, than soils. The sorption of Pin soils occurred at relatively low solution P concentrations (as indicated by Freundlich value, K-f), and appears to be caused by strong affinity of the noncarbonate clay, while the P sorption at relatively high solution concentrations (as indicted by Langmuir maximum sorption, S-max) appears to be caused by the affinities of both the noncarbonate clay and carbonate clay. Phosphorus-sorption values (P-sorption) estimated from the one-point isotherm were comparable with the S-max values calculated from the Langmuir isotherm. Phosphorus saturation and the P-retention capacities (S-max or P-sorption) were correlated strongly to the percentage of P desorption.