CALCIUM SULFATE CRYSTALLIZATION ALONG CITRUS ROOT CHANNELS IN A FLORIDA SOIL EXHIBITING ACID SULFATE PROPERTIES

Abstract

We observed euhedral crystals in Manatee soil (coarse-loamy, siliceous, hyperthermic Typic Argiaquolls) in a citrus grove in St. Lucie County, Florida. The material was identified as gypsum (CaSo4·2H2O) using x-ray diffraction and infrared spectra. Photomicrography and scanning electron microscopy revealed that gypsum accumulated both in old root channels and within citrus root tissue of the Btg horizon.

The subsurface horizons had elevated sulfate levels, a low initial pH, a drop (0.5 unit) in pH upon air-drying. Electrical conductivity paralleled the concentration of water-soluble sulfate. High levels of calcium and sulfate occurred for horizons above the water table. This accumulation is attributed to groundwater bearing these ions and subsequently discharging them to the overlying soil. Dead citrus roots appear to act as wicks to aid water transfer from lower to higher horizons. The roots and their empty channels provide spaces in which the gypsum can precipitate if the concentrations of calcium and sulfate in the evaporating groundwater exceed the solubility product of gypsum.