Flat thin mm-sized soil moisture sensor (MSMS) fabricated by gold compact discs etching for real-time in situ profiling

In situ direct soil moisture monitoring suffers from crucial problems including bulky sensor size, disturbance of soil texture, low accuracy and costly sensors. This study aimed at tackling this challenge by developing a novel flat thin mm-sized soil moisture sensor (MSMS) using gold compact discs etching. MSMSs were examined in six soil samples (sand/silt/clay mixtures), with soil moisture in each varying from zero to saturated. The capacitance and resistance readings of MSMSs were real-time recorded, and compared with soil moisture derived from conventional measurement. Both capacitance and resistance were found to correlate linearly with soil moisture, and the regression slopes were linearly correlated with soil porosity. This unique relationship facilitates accurate calculation of soil moisture based on MSMS readings for any given soil texture, which eliminates the need for tedious calibration of MSMSs in field applications. Four-week stability tests of MSMS showed the resistance readings did not drift throughout the test period, and the sensor surface observation indicated anti-scratching capability, demonstrating high stability for long-term continuous in situ monitoring of soil moisture.