Evaluation of soil structure by using computer-assisted tomography

Abstract

Computer assisted tomography applied to gamma ray attenuation (CAT scanning) has been evaluated as a method for directly measuring the spatial distribution of soil macroporosity and for monitoring changes which occur during wetting and drying processes, in a non-destructive manner. Total porosity calculated from the bulk density of soil and glass tube samples varying in bulk density from 0.759 to 1.602 g cm-3 correlated well with total porosity calculated from CAT scan data. The system was able to detect changes in bulk density for a loam soil of the order of 16 mg cm-3, and objects of 2.0 by 2.0 mm dimensions could be pictorially resolved in the scanning plane. Statistically, the system resolves pixels differing by at least 6.14% in attenuation coefficient value from surrounding pixels. Average macroporosity and the spatial and frequency distribution of macroporosity for soil samples were determined by assigning the value of zero macroporosity to pixels having gamma attenuation coefficients corresponding to the bulk density of a soil aggregate, 100% macroporosity to pixels with zero attenuation coefficients and proportional values to pixels with intermediate coefficients. The CAT scanning of soil samples before and after a wetting and drying cycle illustrated the greater reduction in macroporosity for soil wetted under flooding compared with that wetted under capillary action. The CAT scanning clearly has considerable potential for studying the spatial distribution of soil macrostructure and for monitoring the changes in macroporosity which occur during wetting and drying processes in soil columns.