Capacitance-Based Soil Water Sensors for Irrigated Agriculture: Examples from Drip Irrigation in a Humid Climate.

Capacitance sensors respond to the dielectric properties of the soil surrounding them. Soil water content, electrical conductivity, and temperature all affect sensor response. We have used such sensors, in the form of multi-sensor probes, for a number of seasons in mulched drip-irrigated watermelon in Delaware. The probes are available with single frequency sensors to measure volumetric soil water content, and more recently with dual-frequency sensors that can also respond to electrical conductivity and so give an indication of salinity or nutrient content. By logging the probes, the dynamics of soil water content resulting from processes such as root water extraction and irrigation/rainfall can be clearly seen. The dynamics of soil water salinity is more complex, but can also be seen albeit more qualitatively. Laboratory studies have been conducted to better quantify the salinity response. We have used the probes under various irrigation management treatments, and conclude that they can be a useful tool for optimizing irrigation amount and timing, particularly during seasons with significant rainfall. There is good potential to develop improved irrigation management strategies from our data that are based on ET₀ and the pattern of rainfall events.