tensiometers

Tensiometers are devices used to measure soil matric potential, an indicator of how tightly soil water is held and thus its availability to plants. They are commonly employed in agricultural irrigation management, horticulture, and soil science to monitor relative soil moisture and guide watering decisions.

The basic tensiometer consists of a sealed, water-filled chamber connected to a porous ceramic cup that is buried at the measurement depth in the soil. A manometer, pressure gauge, or electronic sensor is mounted on the top of the chamber to read the pressure of the water column. The ceramic cup allows water to exchange between the soil and the tensiometer while preventing soil particles from entering the tube.

Principle of operation: When the soil around the ceramic cup contains water under tension (negative matric potential), water is drawn from the tensiometer into the soil until equilibrium is reached. The water column inside the tensiometer develops a negative pressure that reflects the soil’s matric potential. The gauge or sensor records this negative pressure, which is typically reported in kilopascals (kPa) or bars. The tensiometer thus provides a direct, near-real-time measure of soil water potential in a specific depth.

Installation and maintenance: Tensiometers must be primed with water, often degassed to remove air, and installed at the desired depth with the ceramic cup fully in contact with the soil. They require periodic refilling and venting to remove air bubbles; accuracy diminishes if air enters the system. The effective measurement range is limited, commonly up to about −0.8 MPa, beyond which the water column risks cavitation or air ingress.

Limitations and applications: Tensiometers perform best in well-structured, moist to moderately dry soils and are less reliable in very dry or sandy soils. They are widely used for irrigation scheduling, crop-water budgeting, and research to relate soil moisture to plant water use.Alternative methods include time-domain reflectometry and gypsum blocks.