Performance evaluation of the AquaTag, a prototype near-field RF-based soil moisture sensor

Soil sensor activated irrigation scheduling is used in agriculture to optimally dose water and improve water use efficiency. Soil moisture sensors give point-related information and due to heterogeneities in soil hydraulic properties, use of a limited number of sensors may lead to errors in the average soil moisture content obtained for a field. As prices for individual IoT-based soil moisture sensors are still high, there is a need for low-cost soil moisture sensors. The AquaTag prototype is a passive, impedance-based sensor tag and a hand-held readout device for measuring soil water content. It operates contactless using a near-field, narrow-band RF-technology at 27 MHz. The tag has a lower accuracy than regular FD or TDR sensors, but it has the potential to be produced at very low-cost. Growers with small-scale, low-tech soil-grown crop production systems could use it to save water at acceptable cost. The aim of this work was to evaluate performance of prototypes, especially for the effects of production variability, reader-sensor positioning, temperature and soil type calibration. Sensor repeatability was ±0.62% and the overall accuracy tested with well-saturated glass beads was ±10.4%, taking effects of functional calibration, reader positioning and temperature into account. Production variability was improved by functional testing, selection and optimizing the reader signal analysis. Measurements for dry and wet sandy soils are possible at sensor-reader distances up to 10 cm. The reader angular position influences readings only marginally, if measurements are taken with care. Soil temperature affects sensor readings considerably, but the effect of the tag temperature is only marginally. A single calibration curve for two loamy-clay sandy soils was obtained. The AquaTag seems suitable to determine soil moisture content, but in order to reach an accuracy of below ±10%, sensors must be tuned individually, for temperature a compensation is required and in-situ calibration for well-saturated soils is advised. Sensors with a longer shaft are required to use the tag for measurements at common crop rooting depths.