A classification of soil aggregates based on their coherence in water

Soil aggregates have been divided into seven classes by observing the coherence of the clay fraction after reacting aggregates with water. The reactions used were: immersion of dry aggregates in water, immersion of wet remoulded aggregates in water, and suspension of aggregates in water. One further class was distinguished by the presence of carbonate. Illite and montmorillonite clays were modified so as to exhibit the physical properties of some of the aggregate classes. The results with the clays were then used to explain the characteristic properties of aggregates derived from a wide variety of soils in the various classes. For example, class 2 aggregates show partial dispersion when placed, dry, in water. The minimum percentage of exchangeable sodium present in aggregates of this class was equal to that required for dispersion of the dry Na/Ca-clays immersed in water. Examples are given of where the detection of dispersion from aggregates can be useful in the field. As only simple tests are used, the proposed method of classification can be easily carried out under field conditions.