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

A soil physical parameter, S, is defined. It 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). The value of S is indicative of the extent to which the soil porosity is concentrated into a narrow range of pore sizes. In most soils, larger values of S are consistent with the presence of a better-defined microstructure. Much previous work has shown that this microstructure is responsible for most of the soil physical properties that are necessary for the proper functioning of soil in agriculture and the environment. The use of S is illustrated with examples of soils with different texture, density (or degree of compaction), and organic matter (OM) content. The effects of S on root growth in soil are investigated, and S is shown to be a better indicator of soil rootability than bulk density. It is suggested that S can 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.

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