A temperature compensated soil specific calibration approach for frequency domain soil moisture sensors for in-situ agricultural applications

Dielectric based capacitive soil moisture sensors are widely used because of their affordability and ease of use. We propose a simple temperature compensated soil specific in-field calibration method for frequency-domain soil moisture sensors and is implemented using the in-house developed soil moisture sensors. The proposed approach produces two different look-up table based calibration models, one corresponding to 22° C and another corresponding to 32° C. The sensor output frequency is mapped to the soil moisture with the help of linear interpolation using both the models whenever the soil temperature is in the range of 22° C - 32° C. If the soil temperature is outside this range, the calibration model closer to the temperature is used for soil moisture measurements. With the proposed calibration approach, the maximum difference between the gravimetric soil moisture and the measured values is observed as 3 % in comparison with the conventional oven-dry laboratory calibration approach, a labour-intensive method. Field experiments were carried out for five consecutive days using a wireless sensor network consisting of 3 sensor nodes where each node reported its sensor data every 3 hours. The field measurements with the proposed calibration approach showed a maximum deviation of 3.17% in comparison with gravimetric measurements.