Texture and degree of compactness effect on the pore size distribution in weathered tropical soils

Soil pores can be functionally classified into macro-, meso- and micropores. The volume of these pores is highly affected by constitutive and state components of soils, such as particle size distribution and degree of compactness. In this study, we aimed to examine the impact of soil texture and degree of compactness on total porosity, and distribution of macro-, meso- and micropores. Undisturbed soil cores were taken in eight differently textured soils cultivated with sugarcane and equilibrated at matric potentials of − 30 and − 100 hPa for calculation of macropores (effective pore diameter – EPD > 100 µm), mesopores (EPD 100–30 µm) and micropores (EPD < 30 µm). The volume of each pore category was modelled using additive spline functions of soil particle size and degree of compactness. Soil organic carbon and mean weight diameter of soil aggregate (MWD) were also measured to support the findings. Total porosity and macropores were dependent on the degree of compactness and silt plus clay content, whereas silt plus clay content significantly affected meso- and micropores. Total porosity and macroporosity decreased with increasing compactness, mainly for soils with silt plus clay content lower than ~500 g kg⁻¹; especially for the macropores, these decreases were substantially reduced for further increase in silt plus clay content (> 500 g kg⁻¹) due to the higher MWD verified for these soils. Mesopores were predominant in soils with lower silt plus clay content, while their volume was practically null in soils with silt plus clay content higher than 500 g kg⁻¹, for which only macro- and micropores were observed. In conclusion, the increased degree of compactness may reduce both total porosity and macroporosity in a process whose magnitude depends on soil texture, whereas meso- and micropores have major changes associated only with particle size distribution. However, macropores showed to be also sensitive to soil aggregation rather than properly changes in total soil volume (e.g., degree of compactness).