Soil Moisture Sensors

All sensors are transmitting the data via radio frequency to a home computer. The following sensors are being used to determine triggers for irrigation and soil moisture uniformity:

• Water content reflectometer
• Water content reflectometry (WCR) is a method which uses time-domain reflectometry to obtain soil water content. The WCR consists of two stainless steel rods connected to a printed circuit board which is encapsulated in epoxy. The two rods on the sensor measure dielectric permittivity of the soil. It also propagates an electromagnetic pulse along two rods which act as a wave guide (Campbell Scientific 2006). The WCR works at frequencies between 15 and 45 MHz.It can be installed at the desired depth.

• Electronic tensiometer
• A tensiometer measures soil suction and consists of a sealed, water-filled, vacuum chamber with a porous ceramic cup connected to a display on the top. As the soil dries, water is pulled out of the chamber and seeps into the surrounding soil to reach an equilibrium, thus creating a greater vacuum and sipalys higher tensions. Tensiometers can be installed at different depths.

• Capacitance probe
• Capacitance probes consist of two electrodes separated by a dielectric (a material that does not readily conduct electricity). Capacitance probes are usually cylindrical in shape, and have multisensory capabilities. When they are inserted into the soil, the soil becomes part of the dielectric. An oscillator applies frequency (50 to 150 MHz) to the electrodes which in turn provides a resonant frequency. The greater the soil moisture content, the smaller the resonance frequency (Prichard et al 2004). The capacitance sensors have the ability to oscillate over 100 MHz inside the plastic tube, in free air. The output of the sensor is the frequency response of the soil’s capacitiance due to the soil moisture level (Fares and Polyakov 2006). Capacitance probes have multi-sensors which allow readings at several soil depths.

• Hand-held portable TDR
• The time-domain reflectrometer (TDR) equipment allows for spot measurements, taking an average soil moisture reading of 0-20cm. It also has a port for connecting an external GPS system that can be used to pin-point previously sampled locations, which is helpful for re-sampling.This piece of equipment shows the data points displayed on the equipment, and they are logged into the sensors' memory and downloaded for viewing