Dispersive soils are more common in older landscapes where leaching and illuviation processes have had more time to work. A source of sodium is also required. Possible sources can include weathering from soil parent materials or wind-blown salt deposition. Sodium ions are highly mobile in the soil solution and so they accumulate in the lower parts of the landscape.
The dispersive portion of a soil profile is generally confined to the subsoil, where soil-forming processes concentrate clay minerals and sodium. This means that dispersive soils may not be identified until they are disturbed in a way that exposes the subsoil to running water.
When observed in situ, dispersive soil textures may feel soapy, and in many cases the physical structure of subsoil layers will be prismatic or columnar. A simplified version of the Emerson soil dispersion test can be completed in the field on a 20-minute to two-hour timescale.
Laboratory tests used to diagnose a soil as dispersive focus on the cation exchange capacity of a soil sample and its cation breakdown. Soil cations are dominated by Ca, Mg, K, and Na, as well as H in acidic soils. The exchangeable sodium percentage ( "ESP", (sodium / (total cations)) * 100 ) is a key indicator derived from these measurements. Where ESP exceeds 5%, dispersive behaviour becomes possible, and is highly likely where ESP exceeds 15%.