Field calibration of a capacitance water content probe in fine sand soils

Fine sand soils important to Florida agriculture have volumetric soil water content values (theta(v)) of <0.10 cm(3) cm(-3) after drainage due to grav ity has ceased. Small changes in theta(v) in the range of 0.02 to 0.08 cm(3 ) cm(-3) can greatly affect plant available water and, therefore, good cali bration of soil water content sensors is necessary. The EnviroSCAN (Sentek Pty, Ltd., South Australia) is a multiple sensor capacitance probe capable of continuous measurement of soil water content by volume (theta(v)). Many fine sand soils in Florida have plant available theta(v) values of less tha n or equal to 0.08 cm(3) cm(-3). The manufacturer's calibration curve has v ery few data points <0.10 cm(3) cm(-3) theta(v) and no data in the 0.02 to 0.04 cm(3) cm(-3) theta(v) range. Because of the lack of data in this range , a calibration curve from 0.02 to 0.08 cm(3) cm(-3) theta(v) was developed for Candler fine sand (hyperthermic, uncoated Typic Quartzipsamments), Apo pka fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudults), and Immokalee fine sand (sandy, siliceous, hyperthermic Arenic Alaquods) in two locations in Florida. Since calibration curves for the three soils did not differ significantly, data from the three soils were combined. An exponent ial calibration curve was developed (RMSE = 0.0085, R-2 = 0.83). This equat ion provides substantially different estimates of water content in the 0.02 to 0.08 range than values obtained from the manufacturer's calibration. Th is improved calibration extends the useful range of the EnviroSCAN to inclu de an important group of soils with very low water holding capacity.