A tensiometer in

soil science Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, soil classification, classification and Soil survey, mapping; Soil physics, physical, Soil chemistry, chemical, Soil biology, biologica ...

is a

measuring instrument Instrumentation is a collective term for measuring instruments, used for indicating, measuring, and recording physical quantities. It is also a field of study about the art and science about making measurement instruments, involving the related ...

used to determine the matric water potential (

\Psi_m

) ( soil moisture tension) in the

vadose zone The vadose zone (from the Latin word for "shallow"), also termed the unsaturated zone, is the part of Earth between the land surface and the top of the phreatic zone, the position at which the groundwater (the water in the soil's pores) is at ...

. This device typically consists of a

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plastic Plastics are a wide range of synthetic polymers, synthetic or Semisynthesis, semisynthetic materials composed primarily of Polymer, polymers. Their defining characteristic, Plasticity (physics), plasticity, allows them to be Injection moulding ...

tube with a porous

ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

cup and is filled with

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

. 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 A hypodermic needle (from Greek Language, Greek ὑπο- (''hypo-'' = under), and δέρμα (''derma'' = skin)) is a very thin, hollow tube with one sharp tip. As one of the most important intravenous inventions in the field of drug admini ...

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. When the soil is wetted flow can also occur in the reverse direction: as water is added to the soil, the vacuum inside the tube pulls moisture from the soil and decreases. When the water pressure in the tensiometer is determined to be in equilibrium with the water pressure in the soil, the tensiometer gauge reading represents the matric potential of the soil. Such tensiometers are used in irrigation scheduling to help farmers and other irrigation managers to determine when to water. In conjunction with a water retention curve, tensiometers can be used to determine how much to water. With practice, a tensiometer can be a useful tool for these purposes. Soil tensiometers can also be used in the scientific study of soils and plants.

References

* Rawls, W.J., Ahuja, L.R., Brakensiek, D.L., and Shirmohammadi, A. 1993. ''Infiltration and soil water movement'', in Maidment, D.R., Ed., Handbook of hydrology, New York, NY, USA, McGraw-Hill, p. 5.1–5.51.

External links

The Experimental Hydrology Wiki Soil matric potential - tensiometer (T4)

The Experimental Hydrology Wiki Soil matric potential - tensiometer (T5)
Irrigation Soil physics Earth observation in-situ sensors {{soil-sci-stub