Use of neutron and gamma radiation meters to estimate bulk density and correct for bias in sampling for water content in a swelling clay soil

Two radiation methods were used to estimate the bulk density of a three-dimensionally swelling grey clay soil used for furrow irrigation at Narrabri, N.S.W. Firstly, gamma ray scattering was calibrated with measurements of wet bulk density derived from undisturbed soil cores. Secondly, a high correlation between neutron counts and gravimetric water content in this soil provided a basis for predicting bulk density corrected for bias in sampling of shrinkage cracks by using a published theoretical model of three-dimensional soil shrinkage. The gamma ray backscattering technique was an inefficient means of estimating wet or dry bulk density in this swelling soil, and is not recommended. However, the neutron moisture meter accurately predicted gravimetric water content and hence bulk density corrected for three-dimensional shrinkage. For soils which undergo three-dimensional normal shrinkage over the range of water content of interest, accurate predictions of bulk density and hence volumetric water content, both corrected for three-dimensional shrinkage, can be made once the derived constants of minimum bulk density, minimum air-filled porosity of fully swollen soil and soil particle density are obtained. The method eliminates the need for using a gamma density meter in conjunction with the neutron moisture meter, and should therefore streamline and improve the accuracy of measurements of soil water in highly swelling clays. 5 tabs