Soil Water Sensors for Agriculture – Theory and Issues

Soil water sensors have been used for irrigation and water management in agriculture for many years, but with limited success in many cases. Nonetheless, the use of soil water sensors is increasing as water scarcity increases and, conversely, as problems associated with over irrigation increase. Common problems with soil water sensing include sensor failure, problems with wiring, lack of or failure of data telemetry, inaccurate data, lack of timely data, excessive labor requirements and interference from dynamic soil temperature and bulk electrical conductivity changes. There are many sensors available, but only four main technologies: neutron thermalization, resistance blocks, capacitance sensing (frequency domain sensing), and travel time sensing (time domain reflectometry and time domain transmission modes). Understanding the theory of these offers insight into what a user can expect from each technology in terms of accuracy, stability and representativeness of the readings. The presentation will cover the types of sensors available, the operational theory of each sensor type, and explanations, with examples, of how the physical theory of operation dictates the limits of sensor calibration and performance, and of sensor representativeness in given soils.