Characterization of Macropore Morphology in a Sandy Loam Soil Using X-ray Computer Assisted Tomography and Geostatistical Analysis

ABSTRACT

Our understanding of the factors affecting the leaching of chemicals such as pesticides and fertilizers into soils is limited, due in part to a lack of adequate non-destructive measurements of soil pore size distribution. The recent application of X-Ray Computer Assisted Tomography scanning to non-medical fields such as soil physics has allowed the development of a new and non-destructive technique with considerable potential for quantification of soil and water processes. The present study was undertaken to develop an innovative protocol for the characterization and visualization of soil macropores. It focuses primarily on the characterization of macropore morphology in large undisturbed soil columns using a 4th generation X-ray CAT scanner. The objective of the study is to assess, in quantitative terms, variations in the morphology of macropores as a function of depth. In order to evaluate these variations, the number of macropores, soil macroporosity, macropore size, hydraulic radius, and circularity have been computed and are presented in the paper. Aspects of geostatistics, which have been used to describe the spatial distribution and spatial continuity of soil macroporosity are also presented. Pores larger or equal to 1.0 mm in equivalent diameter were readily detected. It was found that the average macroporosity of the four soil columns fluctuated between 2.1 and 3.8%. Macroporosity was found to decrease significantly with depth. Similarly, the number of macropores decreased by 0.7% to 2.5% over the entire depth of the soil columns. It was also found that the macropore-size distributions exhibited a peak for all soil columns and at all depths for macropores having an equivalent cylindrical diameter ranging from 2–4 mm. The number of macropores in this category decreased with depth. From the hydraulic radius distribution, it was inferred that about 20% of macropore throats have a diameter of 1.6 mm. Moreover, it was noted that the relative percentage of circular macropores increased with depth. Semi-variograms showed spatial dependency for most soil sections up to 22 pixels (17 mm).

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