Accurately calibrating a neutron probe for the field measurement of soil water content is not a simple task. The most straightforword calibration technique is done in the field by determining the volumetric soil water content of soil cores taken around the access tube and relating it to the instrument reading. Although the sampling procedure is time-consuming and sometimes arduous, it is probably the most accurate method currently in use. Plastic cylinders of different outside diameters have been found to be valuable intermediate standards for transferring the field calibration from one neutron probe to another when the detector type, source strength, and geometry are similar. This new technique will greatly facilitate the calibration of any number of neutron probes in many different soils.
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