SINGLE AND DUAL PROBE NUCLEAR INSTRUMENTS FOR DETERMINING WATER CONTENTS AND BULK DENSITIES OF A CLAY LOAM SOIL

The objective of this work was to characterize the shrinkage and swelling of a clay loam soil containing about 14% (by weight) smectite, and to evaluate the usefulness of commercially available single and dual nuclear probes for soil water and density measurements in such soils. The probes were evaluated both in a packed bin and in the field. Factory-supplied calibration coefficients for density measurements with the single probe appeared to be applicable to the soils tested; this was not the case for the dual probe which required calibration. For soil water determinations both instruments gave quite good results, although the single probe values were primarily influenced by near-surface contents. Such dependence on near-surface conditions could result in erroneous measurements of soil profiles with a steep gradient in water content. The potential for such errors is much less with the dual probe where both the source and detector of neutrons were located much closer to the region where γ-ray attenuation was being recorded. Despite this advantage, both instruments were found to be about equally accurate for determining soil wet densities, which may be a more suitable indicator of soil compactness than the more usual dry bulk density in shrink-swell soils. © 1990.