AGGREGATE SLAKING AND DISPERSION CLASS, BULK PROPERTIES OF SOIL

Diffuse double-layer swelling of soils with aggregates in dispersion classes 1 and 2 can cause piping failures in earthen structures. For saturated aggregates in classes 3 or 4 from clay soils, the matric suction at which shrinkage starts depends on the size of the clay particles. As a result, cracks form sooner in highly swelling soils and aggregates around the cracks do not slake. This makes the soils easy to irrigate. Non-cracking clay subsoils may be difficult to drain because porosity created by ripping is lost by slaking. Very few Na-ions are needed on exchange sites to move class 3a aggregates into class 2. The number required is increased in neutral to acid soils because of cationic forms of aluminium on exchange sites. Aggregates from the tops of columns of a natrustalf were found to be in class 3, the bases in class 2. It is deduced that it is unlikely that the tops move seasonally. The results axe also used to illustrate the difficulty of defining natric horizons by exchangeable cation composition. The main challenge to improving soil structure and crop yield is where surface soils with aggregates in class 3a overlie clay subsoils with aggregates in classes 1 or 2.