DIVISION S-2—SOIL CHEMISTRY Phosphorus-Sorption Characteristics of Calcareous Soils and Limestone from the Southern Everglades and Adjacent Farmlands

in calcareous soils of the southern Everglades and of adjacent areas is needed urgently. The understanding of P sorption and desorption by soils is imporThe relationships between P-sorption capacity, P tant for safeguarding water quality and for fertilizer management. availability and soil components in calcareous soils have Little is known about the P-sorption characteristics of the calcareous been studied in other states and countries. Ryan et al. soils and limestone bedrocks in southern Florida. In this study, 19 samples of calcareous soil and subsurface limestone bedrock were (1985) reported that P sorption in soils is related more collected from the southern Everglades wetland, pineland, and nearby to their Fe oxide than CaCO3 content. Solis and Torrent farmland. At very low P concentrations, P sorption in these soils fit (1989) found P-sorption capacity of soil to be highly the linear isotherm. The equilibrium P concentration at zero-net P correlated with Fe oxide and clay content, and that sorption (EPCo) of soils correlated positively with P saturation. PhosCaCO3 plays a less important role in P sorption. Howphorus-sorption data of soils at medium P concentrations fit the ever, Sharpley and Smith(1985) observed that concenFreundlich and Langmuir equations up to a point at high P concentratrations of labile P in calcareous soils after fertilizer tions where the slope of the isotherm changed abruptly. In bedrock applications are negatively correlated to CaCO3 content. samples the sorption-isotherm-inflection point, which is caused by P Afif et al. (1993) also reported that the ratio of Olsen-P precipitation, occurred at much lower solution P concentrations than to applied P is negatively correlated to Fe oxides content in the case of soils (4–18 vs. 400–600 mg mL 1 ). Also bedrock samples at low P application rates and to CaCO3 content at had significantly lower Freundlich values, Kf, than soils. The sorption of P in soils occurred at relatively low solution P concentrations (as high P application rates. Most previous studies involved indicated by Freundlich value, Kf ), and appears to be caused by strong agricultural soils with active CaCO3 content of 300 g affinity of the noncarbonate clay, while the P sorption at relatively high kg 1 (Holford and Mattingly, 1974, 1975a). Thus the solution concentrations (as indicted by Langmuir maximum sorption, P chemistry in nonagricultural soils or agricultural soils Smax ) appears to be caused by the affinities of both the noncarbonate with CaCO3 contents as high as 928 g kg 1 has not clay and carbonate clay. Phosphorus-sorption values (Psorption ) estibeen elucidated clearly. Calcareous soils often contain mated from the one-point isotherm were comparable with the Smax limestone fragments or lie directly upon limestone bedvalues calculated from the Langmuir isotherm. Phosphorus saturarock. Holford and Mattingly (1975b) measured the tion and the P-retention capacities (Smax or Psorption ) were correlated P-sorption capacity of ground limestone. Since the strongly to the percentage of P desorption. P-sorption capacity of carbonate depends strongly on its surface area (Amer et al., 1985), more research is needed to study the P-sorption characteristics of unground intact limestones. T chemistry of P in Florida’s acid and organic The ability of soil to release sorbed P to the environsoils has been well documented (Flaig and Reddy, ment is dependent on both P-sorption capacity and 1995; Reddy et al., 1995; Harris et al., 1996; Zhou et al., amount of P sorbed. When the amount of sorbed P 1997; Li et al., 1999). The relative P sorption, a P-sorpincreases after P application to soils, soil-P desorption tion index of the relative P-sorption capacity of sandy tends to increase, and this in turn, leads to increases in soils, has been developed using a one-point isotherm P loss through runoff or leaching. The concept of the for Florida’s acid soils (Harris et al., 1996). However, degree of P saturation (DPS) has been introduced rethe P-sorption capacities and desorption potentials of cently as an environmental index of soil P available to calcareous soils in south Florida have not been reported. be released through runoff and leaching to surface and This is a serious gap in knowledge because P is considsubsurface waters. This concept has been applied sucered to be a major factor responsible for ecological cessfully to different soils in the Netherlands and to degradation of Everglades. Calcareous soils under both some soils in the USA (Breeuwsma and Silva, 1992; sawgrass (Cladium jamaicense Crantz) wetland and Sharpley, 1995). Phosphorus saturation is defined as: pineland occupy most of the area of the southern Everglades. Clearly an indepth understanding of P chemistry DPS (%) Extractable P ( g/g) P-sorption Maximum ( g/g) 100 [1] Tropical Research & Education Center, IFAS, University of Florida, 18905 SW 280th Street, Homestead, FL 33031. Florida Agricultural Abbreviations: EPCo, equilibrium phosphorus concentration at zeroExperiment Station Journal Series No. R-07432. Received 20 Mar. net P sorption; DPS, degree of phosphorus saturation; PSI, phospho2000. *Corresponding author ([email protected]). rus-saturation index; *,**,***, Significant at the 0.05, 0.01, and 0.001 probability levels, respectively. Published in Soil Sci. Soc. Am. J. 65:1404–1412 (2001).