Frequency, electrical conductivity and temperature analysis of a low-cost capacitance soil moisture sensor

This study evaluated the family of ECH2O sensors (EC-5 and ECH2O-TE) for measurement of soil moisture content (theta), bulk electrical conductivity (ECb) and temperature for a range of soils, across a range of measurement frequencies between 5 and 150 MHz. Measurement frequency is one of the primary factors affecting the sensitivity of capacitance sensor measurements to soil variables such as soil texture, electrical conductivity, and temperature. Measurements in both soil and solution demonstrated that the ECH2O EC and TE measurements were accurate. Using a measurement frequency of 70 MHz, a single calibration curve was determined for a range of mineral soils, independent of soil salinity, suggesting there might be no need for a soil specific calibration. When combining all data for each soil type, the R-2 values remained high (R-2 = 0.98) with little probe to probe variability. After laboratory calibration, the error for theta was about 2%, independent of soil ECb, up to a soil solution EC of about 12 dS/m. Our results showed that a single calibration curve could be used for all tested mineral soils, independent of soil salinity. The bulk soil ECb - water content data were excellently described by a polynomial expression. Measurements of temperature sensitivity to soil water content and ECb were sufficiently small. For example, for a temperature change of 10 degrees C, measurements of theta and ECb were affected by about 0.02 cm(3) cm(-3) and 0.02 dS/m, respectively. Limited sensor calibration requirements are important, when large networks of soil moisture sensors are being deployed. It is concluded that an accurate, cost-effective soil moisture sensor is available that operates at a measurement frequency of 70 MHz, with a low sensitivity to confounding soil environmental factors. (C) 2008 Elsevier B.V. All rights reserved.