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A Field-Calibrated Portable Neutron Rate Meter for Measuring Soil Moisture in Citrus Orchards¹

ABSTRACT

Selected features of investigations previously described in the literature have been incorporated with new ideas in the construction of a dependable portable neutron-scattering device for measuring soil moisture. The equipment and method of calibration have given very satisfactory results.

The detector tube consists of a proportional counter tube filled with an enriched BF₃ gas and a fast neutron source attached in an annular container at the mid-plane of the counter tube. Thermal neutron densities in the soil are measured by inserting the detector tube into lined access holes in the field. When removed from the access hole, the detector tube is drawn into a shield composed of lead, paraffin, and cadmium. The paraffin acts as a shield for the fast neutrons and also serves as a standardization material for adjusting the meter.

A rate meter was chosen to indicate the slow neutron flux returning to the detector tube. The counter tube voltage was at first supplied by a vibrator type power supply, which was later replaced by a transistorized power pack.

The total weight of the equipment and shield was too much for one operator to carry conveniently; the entire unit was therefore mounted on a modified golf cart.

A definite curvilinear relationship was shown between the percent volume of water in the soil and the meter reading. The regression equation of soil moisture on meter reading was = 0.2462 + 0.4337X + 0.0101X². The correlation between Y and was R = 0.9833. The standard error of estimate from the mean Y (18.34) was 1.42. This study further confirmed other investigations, in that a single calibration curve served for many soil types.

¹ Paper No. 969, University of California Citrus Exp. Sta., Riverside, Calif.

² Assistant Irrigation Engineer and Assistant Horticulturist, respectively. The authors acknowledge the contributions of G. D. Morehouse and B. J. Wagoner, Motorola Research Laboratory, Riverside, Calif., and of S. J. Richards, Dept. of Irrigation and Soils, Citrus Exp. Sta., toward the construction and development of the equipment used in these studies.

Received for publication December 15, 1956. Accepted for publication June 5, 1957.