Abstract: A sensor for measuring moisture changes in soil and having an electrically insulating cylindrical body along which bare electrical conductors are placed. A transmitter produces constant alternating current which is supplied to the sensor. The voltage which this current creates in the soil is detected by the electrical conductors on the sensor and is fed to a receiver. The receiver consists of electronic circuitry for determining the necessity for allowing irrigation to occur, and if so, switches on a solenoid controlling the water supply. When the sensor indicates sufficient irrigation has occurred, the electronic controls switch the irrigation off. A tube sensor comprising rigid metal tubes of equal length and secured by end members such that no two tubes touch is also provided, for example to measure moisture content changes of any granular material.