# The Application of Computed Tomography to Study the Soil Porosity of Mountain Red Earth
## 3 Results and Discussion  
### Parameter Quantization  
The number and size of soil pores are critical indicators in describing the structure of soil porosity. The count and distribution significantly influence the soil's permeability to air and water, as well as its water retention capacity. Figure 6 shows the changes in the pore count at different depths. The number of pores ranges from 77 to 216. As depth increases, the count first rises from 137 at the top to a maximum of 216 and then falls to 139 at the bottom. The increase in the number of pores from the surface upwards may be due to the predominance of larger pores at the surface; though fewer in number, these pores have a larger diameter. The variation in pore area with depth is illustrated in Figure 6. As seen in Figure 7, the total percentage of pore area (or total pore area) ranges from 2.02% (1084 mm\({}^{2}\)) to 52.25% (24,702 mm\({}^{2}\)). With increasing depth, the percentage of area decreases gradually from a maximum of 52.25% at the top to a minimum of just 2.02% at the bottom, showing a clear linear relationship with depth. Figure 6: Variation of pore number with depth. The observed changes in pore number and area are mainly due to significant biological activity in surface soils, which includes plant root growth and soil organism movement, forming additional pores. These pores are more concentrated and larger, enhancing water and gas transport efficiency and supporting plant root growth. Surface soil has larger pores despite being fewer in number. As depth increases, biological activity decreases, and physical/chemical factors become more prominent, resulting in smaller, less numerous, and more scattered pores. This arrangement affects water infiltration, causing slower movement through deeper layers of Mountainous Red Earth. In addition to the number and total area of pores, the scale of individual pores also significantly affects soil water and gas movement. Currently, there is not a clear, unified standard for classifying the scale of soil pores. Some researchers, after conducting CT scanning experiments on undisturbed soil columns containing various large pores and filled soil columns with known large pore diameters, generally agreed that pores with equivalent diameters larger than 0.5 mm should all be classified as large pores. In soil science, pores are classified into three types based on equivalent pore diameter: inactive pores (\(\leq\)0.002 mm, suction > 1500 mb), capillary pores (0.002 mm-0.02 mm, suction between 1500 mb and 150 mb), and aeration pores (\(\geq\)0.02 mm, suction < 150 mb) [45]. Due to the resolution limits of the CT equipment in this study, the smallest discernible pores were 0.64 mm \(\times\) 0.64 mm in 2D images. Given the lack of standardized pore classification, pores were categorized by pixel size into micro-pores (1 pixel), small pores (2-20 pixels), medium pores (21-50 pixels), and large pores (>50 pixels). Analysis of pore distribution from slices enabled calculations of pore area percentages and quantities by pore size (Figures 8-10), with seven sample depths taken at equidistant intervals from 24.5 mm to 192.5 mm. Within the depth of 1 mm to 165.5 mm, the distribution trends of the four types of pores are rather stable. Small pores generally account for the largest proportion, followed by medium and large pores, while micro-pores cover the smallest area. However, overall, the area proportion of large pores is higher than that of both micro-pores and medium pores. This is indicative of the broad distribution of large pores in the surface soil layer, largely due to active biological activity. Within the depth range of 65.5 mm to 200 mm, the proportional distribution of the four types of pores displays a slight shift compared to the patterns observed previously. This is consistent with the pore distribution derived from the CT value extraction (Figure 8). As depth increases, the amount of biological matter decreases along with the distribution of root systems.