Towards water-saving irrigation methodology: Field test of soil moisture profiling using flat thin mm-sized soil moisture sensors (MSMSs)

Abstract

This study examined flat thin mm-sized soil moisture sensors (MSMS) fabricated using thermal press technology on thin compact disc (CD) to solve the long-standing problems of soil moisture profiling. The 10-month field tests conducted at a farm site compared three groups of MSMS with commercial capacitance-type soil moisture sensors (SMS) in terms of accuracy, sensitivity to environmental variations (e.g. water shock, temperatures, dry/wet seasons) and long-term stability. MSMS sensors were mounted on the shallow, middle and deep locations of a hollow plastic rod (length: 1.1 m) and installed along the soil depth to profile the soil moisture variation. The resistance readings of MSMS sensors along soil depth were recorded in a real-time mode. Due to soil settlement over time after installation, the MSMS sensors in the shallow soil suffered from unstable readings, while the MSMS sensors in the middle and deep soil exhibited high stability and had the best correlation with water content values of commercial sensors (R2 value: 0.6264). The contact between MSMS surface and soil particles appeared to be a critical factor determining the stability of MSMS readings. In addition, MSMS sensors showed a prompt response to the sharp change of soil moisture in the water shock tests. The soil moisture profiles collected from MSMS sensors captured the spatiotemporal variation of soil moisture, which enabled the simultaneous profiling at multiple locations. This field study demonstrated the great potential of mass deployment of low-cost but accurate MSMS sensors to achieve high resolution profiling for water-saving irrigation.