Phosphorus Sorption in Organic Soils in South Florida

The Everglades Agricultural Area (EAA), downstream of Lake Okeechobee in South Florida, was initially drained in the early 20th century for agriculture and flood protection. Drainage water from the EAA, enriched with P, has impacted the downstream Everglades ecosystem. The organic soils in the EAA have become shallower as a result of continuing oxidation. Mixing of the organic soils with the underlying limestone bedrock may have resulted in increased CaCO3. Our objectives were to determine the P sorption capacity of selected Histosols in the EAA with various soil depths and its relationships with selected physicochemical properties of the soils. Sorption experiments were performed on field-moist soils representing three series. These soils are taxonomically similar and differ in 0 horizon depth to bedrock. Both Langmuir and Freundlich sorption models fit the data well. Langmuir P sorption maxima (S-max) on a mass basis ranged from 733 to 6767 mg kg(-1) and are comparable to mineral soils on a volume basis. In general, P sorption variables were not different among the soils investigated except for the Freundlich sorption coefficient K-P which was lowest in the Pahokee soil, indicating lower P sorption capacity. Langmuir S-max was positively correlated with pH and oxalate-extractable Fe and Al, but not with CaCO3 content. The S-max was inversely correlated with organic matter and water-extractable P. Phosphorus sorption in these organic soils is affected by an increased mineral content, mainly poorly crystalline Fe and Al oxides, but we found no evidence that shallower soils had higher sorption capacities due to increased CaCO3 as hypothesized.