# Tensiometer (soil science)

A tensiometer in soil science is a measuring instrument used to determine the matric water potential (${\displaystyle \Psi _{m}}$) (soil moisture tension) in the vadose zone. This device typically consists of a glass or plastic tube with a porous ceramic cup, and is filled with water. The top of the tube has either a built-in vacuum gauge or a rubber cap used with a portable puncture tensiometer instrument, which uses a hypodermic needle to measure the pressure inside the tensiometer. The tensiometer is buried in the soil, and a hand pump is used to pull a partial vacuum. As water is pulled out of the soil by plants and evaporation, the vacuum inside the tube increases. As water is added to the soil, the vacuum inside the tube pulls moisture from the soil and decreases. The actual gauge reading will vary according to the type of soil, the moisture content, and due to hysteresis, according to the saturation history of the soil.