Characterisation of the soil pore system in relation to its hydraulic functions in two South African aeolian soil groups

Abstract

The characterisation of soil pores has a prime importance in soil hydrology particularly for soil water management in both irrigated and dryland agriculture. Hence, this study describes the soil pore system in the Clovelly and Hutton soil forms and its roles in soil hydrology by quantifying the different functional pore classes. The soil pore size–pore volume (PS-PV) response curve, which shows the relationship between cumulative pore volume to the corresponding pore sizes, was estimated from the soil water retention curve (SWRC) by applying the capillary theory. The SWRC was measured in the laboratory using a hanging water column and pressure plate apparatus. Saturated hydraulic conductivity was determined in situ with a constant head permeameter, whereas unsaturated hydraulic conductivity (KL) and diffusivity (Df) were estimated from the SWRC by the van Genuchten conductivity model. The classification of soil pore sizes given in the literature was modified based on the concept of field water capacity. Therefore, pore sizes greater than the drained upper limit (DUL) were classified as transmission pores. Similarly, the pore size between the DUL and the lower limit (LL) were classified as storage pores, and pore sizes smaller than the LL were residuals and bonding pores collectively. Soil water retention increased non-linearly as pore sizes decreased and the opposite was true for KL and Df. Generally, macropores were important for water movement, whereas micropores were responsible for water retention. The findings are important for soil water management of cultivated aeolian soils in semi-arid and arid regions of sub-Saharan Africa. The proposed method to classify soil pores on the basis of their function also gives a consistent and realistic value compared with the fixed boundary classes, although the method requires further development.