Drainage and soil structure - a review

Abstract

Definitions of soil structure and drainage are proposed that would allow a more useful interaction between the two areas of interest that appear to have developed independently. Drainage phenomena in three structural systems are quantitatively described. Firstly a simple uniform profile is described; then wormholes and stones are added to the upper layer of this soil; finally the upper layer is given a 23% stable macropore space and its depth is proportionately increased by biotic activity. Numerical solutions of these geometries (supported by experimental work) show that if the structure enhancements are confined to the upper horizons, the drainage behaviour of the matrix is unaffected and only the early stages of the drainage hydrograph are affected. More complex structures are considered qualitatively. The influence of entrapped and encapsulated air within the soil pore space is outlined. It is concluded that a major problem in correlating drainage phenomena with soil structure is that the structural description of wet soils is rarely attempted. In particular, the descriptions of structures, naturally occurring and those enhanced by cultivation, which change with time and with wetting and drying, are presently only described by soil surveyors in qualitative terms.