Dispersive soils are soils where sodium occupies a large proportion of the exchangeable cations (Ca, Mg, Na, K). These clay soils tend to be swelling and be dispersive with soil characteristics of low permeabiliy and porosity. These type of soils are usually dense and prone to erosion.
From the aspect of onsite wastewater treatment design (where our goal is to dispose or reuse treated wastewater via deep seepage and evapotranspiration) dispersive and swelling clay soils are problematic in terms of during or following a saturated state, these soils become slowly permeable or impermeable which may cause system failures, eg. effluent appears at the surface. These undesirable soil characteristics can be ameliorated by methods including addition of gypsum and/or lime.
Here are a couple of photos of dispersive soils. If you see such soils you can tell without any kind of testing that you have encountered dispersive soils.
