The tensiometer indicates the effort that the roots must make to obtain the humidity necessary for the crop from the ground. It does not measure the humidity percentage in soil, but operates as a true artificial root. The dry soil extracts liquid from the tensiometer producing a vacuum in the instrument that is reflected in the gauge.

The dryer the soil becomes, the higher is the value registered in the dial of the gauge. When the soil gets moist as a consequence of the rain or the watering, the tensiometer absorbs the humidity from the soil, which reduces the tension and the gauge indicates a lower value decreasing to 0.

This indicates that the soil has reached its maximal capacity to retain humidity again, which is called “field capacity”.