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Introduction and description

The Equitensiometer measures soil matric potential - that is the negative pressure (or suction) required to extract water from between the matrix of soil particles. It is an important indication of plant water stress. The value of soil matric potential measured depends mainly on the quantity of water present and the make up of the soil, though it is also affected by temperature and salinity.

The Equitensiometer consists of a precision soil moisture sensor (the ThetaProbe) whose measuring rods are embedded in a porous material (the equilibrium body). This material has a known, stable relationship between water content and matric potential. When the probe is inserted into the soil, the matric potential within the equilibrium body rapidly equilibrates to that of the surrounding soils. The water content of the matric material is measured directly by the ThetaProbe, and this can be converted into the matric potential of the surrounding soil using the calibration curve supplied with each Equitensiometer.


The Equitensiometer should be thoroughly wetted before use, and installed at a horizontal or slanting angle. Vertical installation may slow the response time, and also lead to incorrect readings because rainfall running down the side of the probe housing may wet the soil around the probe excessively. This is particularly important if the probe is being installed below the soil surface using a probe extension tube. Any gaps between the Equitensiometer and soil should be filled with quartz powder suspension. Small changes to the soil structure surrounding the probe will not affect readings.

It is important to protect the Equitensiometer from strong temperature fluctuations and in particular to avoid exposing the electronics and equilibrium body to different temperatures, e.g. by installing it with the case in strong sunlight. Avoid flexing the PVC cable at low temperatures. It is advisable to protect the cable before burying it in conditions where it may be attacked by soil insects etc.