Soil moisture sensor evaluation

An experimental apparatus was designed to conduct a comparative laboratory test of 23 soil moisture sensors representing seven different general types. Sensors evaluated were: a Troxler neutron probe, a Troxler Sentry 200-AP capacitance probe, an Aqua-Tel Capacitance sensor, time domain reflectometry (TDR) with two different waveguide configurations, gypsum blocks. Watermark PVC moisture blocks, and Agwatronics heat dissipation blocks. Research efforts were directed toward determining the prediction error of the volumetric water content of the soil for each sensor, over moisture contents ranging from saturation to below two percent by volume.

The apparatus was designed to monitor the weight of water in two columns filled with soil. Soil moisture sensors were placed in the center of the column and the soil subjected to four drying cycles of saturation and subsequent drying. The first drying cycle was used for sensor calibration, and the three following cycles were used to determine the prediction error of each sensor. Soil types used were a loam and a sandy loam. Two soil types allowed differences in performance due to soil type to be observed.

Capacitance sensors proved to be the most accurate sensors included in the study. The neutron probe was not the most accurate sensor, but it was one of two sensors studied that predicted valid values for all data points during the study. Resistance sensors also performed well over their limited range of water content.