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Tensiometers are used where the soil moisture content will be maintained at about 50-70% of the Available Water (AW). In sandy soils, it can measure much greater than 50% of the total AW (i.e., because of the relatively large pores in these soils which release water at a lower tension). In clayey soils, it will read about 80 centibars (cb) of tension when the soil contains about 75% of the AW. The useful limit of most tensiometers is at about 80 cb of maximum tension. Thus, they can be used where irrigation water management is aimed at maintaining low-tension conditions which are the most optimal for plant-growth. The tensiometer covers most of the range of readily-available soil moisture. They can not indicate the osmotic tension (i.e., the effect of soluble salts) of soil water, unless equipped with an auxiliary salt sensor. Thus, total soil moisture tension will be greater than that measured by the tensiometer (e.g., a soil solution containing an Electrical Conductivity (EC) of 1.5 mmhos/cm at irrigation, will contribute an additional 54 cb of tension). For furrow or flood irrigation, the tensiometer should be placed at 2/3 of the way down the run. If the length of the field or row is especially long, a station at each end of the field is preferred. The use of two tensiometers per site is recommended, with the shallow tensiometer placed at about 1/4 of the actual depth of the root zone. The larger tensiometer is placed in the lower root zone. The shallow tensiometer indicates the status of the soil moisture in the very active root zone area (i.e., it is used to schedule irrigations), while the larger one will detect any under or over-irrigations (e.g., it can be helpful in determining to which depth the soil moisture has actually penetrated and how quickly). The tensiometer continuously registers the readily AW for any soil type; however, the measurement is only for the depth at which the porous ceramic tip (which is about 2.5" in length) of the tensiometer is placed, and cannot measure moisture conditions above or below this point. It is used to monitor changes in soil moisture tension, i.e., water that is adsorbed on the soil particles and held between them by surface tension (matric potential). The tensiometer is in effect a dummy root equipped with a gauge that continuously registers the amount of soil moisture tension required by the plant to extract water (e.g., the higher the reading in cb of tension, the more energy is being used by the plant to maintain the transpiration process); i.e., the drier the soil, the harder the plant roots have to work to uptake water. A reading of 40 cb of tension indicates that the roots are extracting the same amount of moisture whether they are in a sandy or clayey soil. The tensiometer readings are related to a given quantity of soil water that a soil can retain at a given tension, which is unique for each soil type. A zero reading indicates saturated conditions. Readings of 0-10 cb (i.e., saturated to wet soil) often occurs for a day or two following a irrigation. Continued readings in this range indicate over-irrigation (e.g., water-logged soils) or a high water table. Readings of around 10 cb correspond to Field Capacity (FC) (i.e., the maximum amount of water that a soil can hold, with any additional amount draining below the root zone) for coarse-textured soils, while a reading of 30 cb are FC for finer-textured soils.
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Note: Technical Questions
may possibly be quickly answered by contacting USDA, Natural Resources Conservation
Service Soil Conservationists: Holistic Irrigation Technology (HIT) Rudy Garcia, Soil Conservationist (Water Quality), rgarcia@nm.nrcs.usda.gov or call: (505) 522-8775, ext. 116 Remote Sensing & GIS Technology Dave Christenson, Soil Conservationist (Remote Sensing), dchriste@nm.nrcs.usda.gov or call: (505) 522-8775, ext. 115 Send mail to LaUnion@zianet.com with questions or comments about this web site.
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