A low cost stand alone sensor with a dial readout of soil water potential.
Tensiometers are simple instruments designed to simulate the amount of "suction" that the surrounding soil has on water molecules. They consist of a ceramic cup at the end of a tube with a pressure sensing dial (or other recording device) on the end. Water flows back and forward between the ceramic cup as it attempts to equalise with the surrounding soil. This flow creates positive or negative pressure which can be read by the pressure sensor. Readings of soil suction are in hPa with a zero reading for saturated soils to about 1000 hPa for very dry soils. The ideal pressure at which to irrigate is dependant on crop and soil type.