Index of structure: evaluation of a modified method of determining aggregate stability

By determining total and natural clay by the pipette method, it is possible to obtain a more reproducible measure of aggregate stability in soils. This index has been called the “index of structure” and is reported on a scale from 0 to 100. High values represent water stable materials. Where total clay contents are below 25%, multiple determinations are needed to produce reliable results. The index of structure can be used to differentiate between parent materials since it does not vary appreciably in the drainage catenas examined so far. It gives a good measure of the relative water stability of aggregates, especially when the time on the mixer is altered during the natural clay determination. The index is significantly related to porosity but there are appreciable differences between major soil groups. Humid region soils appear to give a more consistent relationship than those of seasonally dry areas.