Soil physical quality: Part II. Friability, tillage, tilth and hard-setting

The index of soil physical quality, S, which was introduced in Part I is applied to problems of agricultural soil mechanics, especially soil tillage and hard-setting. S is equal to the slope of the water retention curve at its inflection point. The retention curve must be plotted as the logarithm (to base e) of the water potential against the gravimetric water content (kg kg⁻¹). The use of S is illustrated with examples of soils with different friabilities, tillage at different water contents and the aggregate size distribution resulting from tillage. It is shown that friability, and hence the ease of working of the soil, is linearly and positively correlated with S. It is also shown in a short theoretical study that S can be used in a simple equation for estimation of the hard-setting behaviour of soil on drying. In combination with pedo-transfer functions, this enables the hard-setting behaviours of soils of different textures to be predicted and shows how hard-setting may be expected to increase with soil compaction. However, the predictions of hard-setting should be considered as speculative until they have been tested experimentally.