Evaluation of a capacitance-based soil moisture sensor for real-time applications

A low resonant frequency, dielectric-based soil moisture sensor developed by Retrokool, Inc. (Berkeley, Cal.) was slightly modified and tested under static laboratory conditions using soil from three different series (Capay silty clay, Yolo loam, and Metz Variant fine sandy loam) of contrasting textural composition. The sensor response consisting of frequency and amplitude measurements was recorded over a range of volumetric moisture contents and salinity levels. The results indicated that the sensor was insensitive to changes in soil texture. The modification to the sensing circuit improved the moisture detection range for the sensor. However, the sensor response was influenced by changes in soil salinity. Empirical analyses showed that a normalized sensor output was highly correlated with the soil conductance. Under laboratory conditions, these estimated conductance values correlated well with soil moisture content (r² = 0.87). When this sensor was vehicle-mounted behind a tillage tool and tested under field conditions in a Yolo loam soil, estimated conductance values were well correlated with measured soil moisture content (r² = 0.78). The results suggest the sensor has good potential for routine applications in real-time measurement of soil moisture for precision agriculture applications.