Field Calibration of a Capacitance Water Content Probe in Fine Sand Soils

Abstract

Fine sand soils important to Florida agriculture have volumetric soil water content values (θv) of <0.10 cm3 cm−3 after drainage due to gravity has ceased. Small changes in θv in the range of 0.02 to 0.08 cm3 cm−3 can greatly affect plant available water and, therefore, good calibration 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 (θv). Many fine sand soils in Florida have plant available θv values of ≤0.08 cm3 cm−3 The manufacturer's calibration curve has very few data points <0.10 cm3 cm−3 θv and no data in the 0.02 to 0.04 cm3 cm−3 θv range. Because of the lack of data in this range, a calibration curve from 0.02 to 0.08 cm3 cm−3 θv was developed for Candler fine sand (hyperthermic, uncoated Typic Quartzipsamments), Apopka 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 exponential calibration curve was developed This equation provides substantially different estimates of water content in the 0.02 to 0.08 range than values obtained from the manufacturer's calibration. This improved calibration extends the useful range of the EnviroSCAN to include an important group of soils with very low water holding capacity.

Density-dependent calibration of multisensor capacitance probes in coarse soil

“…Calibrations for specific soils have also been reported by others, including Mead et al (1995) for sandy loam, Paltineanu and Starr (1997) for silt loam, and Morgan et al (1999) for fine sand; however, the validity of manufacturer's calibration has not been demonstrated for coarse-textured soils. Gardner et al (1998) and Gupta and Jangid (2011) found that for a given u v a greater bulk density (r b ) results in a larger relative dielectric permittivity (K a ) (commonly referred to as dielectric constant) than a lower density.…”

“…Multiple capacitance sensors can be placed on a single probe, with the number and spacing of sensors set by the user. Sensor reading-time intervals are also flexible and can be set by the user to vary from 1 min to 6.9 d. The accuracy of these sensors can be affected by: uncontrolled humidity inside the access tubes, air gaps around the tubes caused by poor field installation, positional changes in orientation within the tube, and alteration of soil properties within the sensor's zone of influence (Morgan et al 1999;Starr and Paltineanu 2002;Evett et al 2006). If care is taken in the installation of the access tubes, these potential problems can be limited; however, a soil-specific calibration is still required.…”

Calibration equations for two capacitance water content probes

“…The calibration showed significant differences even within the two medium soil textures, which only differ in bulk density. Morgan et al (1999) calibrated the EnviroScan probe to three sandy soils in Florida, USA with very satisfactory coefficients of determination (R 2 =0.83). They also compared their equation with the manufacturer default equation and concluded that the latter underestimates the soil water content in these soil types.…”

“…Specifically for the EnviroScan and the Diviner 2000, the manufacturer default equations derive from an average of three soil materials (sands, loams and clay loams) and provide relevant measurements of soil water content. The measurements resulting from these equations are inaccurate and often overestimate the soil water content (Leib et al, 2003;Starr and Paltineanu, 1998) or, in the case of light soil, underestimate it (Morgan et al, 1999).…”

“…Geesing et al (2004) calibrated the Diviner 2000 probe for two soil types (silt-loamy Cambisol and loamy Cambisol) and presented an equation with a relatively high coefficient of determination (R 2 =0.78) compared with the large number of samples (n=282). In the same paper, they compared their equation to the equations of Morgan et al (1999) and Paltineanu and Starr (1997) the manufacturer default equation. Groves and Rose (2004) proposed a laboratory calibration method of the Diviner 2000 probe.…”