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Summary Design, calibration, working precautions and field use of a soil‐moisture meter are described. The probe carries a source 241 Am‐Be and a detector 10 BF 3 in an Al case, 3.4 cm diam and 17 cm long. Source and detector are as close to each other as possible, so giving a desired linear relationship between thermal neutron count and the volumetric water content around the probe. The portable sealer incorporates some new features of circuit design that lead to ease of operation and small size: it weighs 5 kg. Factors involved in calibration or the interpretation of a calibration curve are: resolving time of the sealer, the volume of soil explored by the neutrons, the effect of a moisture gradient, and the possible effect of elements other than hydrogen. Field trials were in two groups. In 1962 and 1963 frequent measurements were made in bare clay soil and under a nearby barley crop: from the profiles, the extraction of water from the bare soil was limited to the top 30 cm, but the barley roots took water out at least as deeply as the maximum depth of measurement then possible, 90 cm. In 1964 similar measurements under first‐year Timothy and Meadow Fescue, irrigated and non‐irrigated, showed that the total water abstracted by each was nearly the same, but the Timothy took most of its water from the top 40 cm of the profile, whereas the Fescue took more water from the lower depths. Combining the estimated deficits with known rainfall gives values of periodic evaporation in good agreement with aerodynamic estimates based on temperature, humidity, and wind profiles above the crop, and in fair agreement with potential evaporation‐rates calculated from routine weather records.
Long et al. (Wed,) studied this question.