EVALUATION OF A CAPACITANCE-BASED SOIL MOISTURE SENSOR FOR REAL-TIME APPLICATIONS

Abstract

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