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A tensiometer is an instrument used for measuring the moisture content of soil. It indicates the physical forces of gravity and molecular bonding that must be overcome in order to pull water out of the soil. The water held in the soil is known as the matrix water potential. A tensiometer is used to measure how easily water can be extracted out of the soil and to gauge how hard a plant is going to have to work to absorb water from its surrounding soil.
Tensiometers are very helpful to farmers in predicting how much water their crops are going to need. Knowing their irrigation requirements, farmers are able to set up irrigation schedules. They are able to assess more accurately how much water is needed by their crops and at what intervals they would need watering. This knowledge is particularly useful during the dry seasons. Tensiometers are also easily managed and can be automated to control irrigation.
A tensiometer usually is comprised of a long, sealed tube made of clear glass or plastic which is partially filled with water. A porous ceramic covering that allows water through is located on one end of the tube. The other end has a vacuum gauge that measures the pressure within the tube. Tensiometers range from 6 inches (15.24 cm) in length to around 4 feet (about 1.22 meters), although there are mini-tensiometers that measure matrix water potential at a depth of .5 inches (1.27 cm).
The ceramic tip of a tensiometer is inserted to the depth of the soil that is being monitored. The tube that is partially filled with water has a vacuum and if there has been rainfall or the soil is otherwise moist, the ceramic tip absorbs moisture from the soil and the vacuum in the tube becomes weaker. Conversely, if the soil is dry, water is drawn out of the tube into the soil, and the vacuum in the tube increases. The gauge measures the vacuum and registers its reading and in this manner, a tensiometer continually records changes in soil water.
Tensiometers typically have a restricted area of dependability. Their readings portray only the soil moisture in the small volume of soil surrounding the area in which it is planted, thus it cannot be counted on to give readings of an entire site. In addition, tensiometers generally do not perform as well in very dry or desert conditions as some amount of water needs to be present in the soil for the instrument to operate effectively.