Soil physical quality: Part III: Unsaturated hydraulic conductivity and general conclusions about S-theory

The index of soil physical quality, S, which was proposed in Part I and which was applied to the problems of tillage in Part II is applied in Part III to the unsaturated hydraulic conductivity of soil. S is equal to the slope of the soil water retention curve at its inflection point. This curve must be plotted as the logarithm (to base e) of the water potential against the gravimetric water content (kg kg−1). It is suggested that S is a measure of the micro-structural porosity of the soil. It is shown through an approximate theory, through simulations and through experimental results that the value of S at the inflection point is related to the unsaturated hydraulic conductivity of soil at the inflection point. It is proposed that the inflection point can be used as a “matching point” in studies of unsaturated hydraulic conductivity. Pedo-transfer functions are used to explore the predicted effects of soil texture class and bulk density on the values of the unsaturated hydraulic conductivity at the inflection point. The conclusions from this series of three papers are summarized and as a result, it is recommended that S be used as an index of soil physical quality that enables different soils and the effects of different management treatments and conditions to be compared directly. The use of S for prediction of a range of soil physical properties is summarized and is called S-theory.