Soil Moisture Sensing in Variable Conditions

Abstract

The study presents the design, calibration, and testing of capacitive soil moisture sensors in laboratory conditions using construction sand. Five different sensor prototypes with different frequencies and designs were tested under four conditions: base case (tap water), low salinity (4.0 dS/m), high salinity (15.7 dS/m), and fertilizer addition. Known water volumes were added to a fixed volume of soil to establish reference volumetric water content (VWC), against which sensor outputs were calibrated by second-order polynomial fits. Each sensor's performance was evaluated using root-mean-square error (RMSE) and coefficient of determination R², and compared to a commercial TEROS 12 sensor. The prototype sensors showed good accuracy, with RMSEs ranging from 0.92% to 6.33% VWC, and with R² values between 0.763 and 0.995. The TEROS 12 showed larger errors, with RMSEs between 6.20% and 11.93% and R² values from 0.157 to 0.773. While the results are limited to laboratory conditions, they show that well-calibrated, low-cost capacitive designs can rival or exceed commercial sensor accuracy. Future work should consider testing at higher operating frequencies and using soil types that are more relevant to the agricultural sector.