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<br />VIII. 7 <br /> <br />00122S <br /> <br />Tensiometers indicate soil-water content only from saturation (zero <br />tension) to a tension of less than one atmosphere, whereas, tension at the <br />wilting point of plants commonly is considered to be about 15 atmospheres. <br />Within their range, however, tensiometers are reliable if they are serviced <br />properly. Permanently placed tensiometers permit repeated measurements at <br />the identical point in the soil, an advantage where continuous records are <br />required. <br /> <br />Electrical resistance of porous blocks <br /> <br />When placed in goad contact with soil, blocks of porous material such <br />as gypsum, fiberglass, or nylon acquire a moisture cOntent which is in <br />equilibrium with that of the soil. Measurement of the electrical resist- <br />ance between two electrodes embedded in the block indicates this acquired <br />moisture content, which then can be related to the tension or free energy <br />of the soil water. The device is similar to the tensiometer in that it <br />must be calibrated if a measure of the volumetric soil-water content is <br />desired. <br /> <br />Errors may result from the influence of the wetting-and-drying <br />hysteresis. Also, accumulation of salts in the soil water may change <br />the relationship of soil-water tension to electrical resistance of the <br />block. This could introduce substantial error, especially with units <br />made solely of fiberglass or nylon. Despite these limitations, electrical- <br />resistance blocks have been popular because they "read out" from the same <br />point in the soil each time, and because they can be observed very rapidly <br />by connecting the lead wires to a resistance meter or portable Wheatstone <br />bridge. Reliability of the data obtained from these blocks will depend <br />upon completeness of their contact with the soil, and on adequacy of <br />original and check calibrations. <br /> <br />Other methods <br /> <br />Other procedures for measuring water content of soil involve porous <br />absorption blocks that are weighed -after being in contact with' the soil, <br />also thermal or electrical conductivity of soil as an index of its water <br />content. These have been successful in some situations but not all. <br /> <br />