Source: Department of Ag and Food, WA

What is a Dispersive Soil?

A dispersive soil is structurally unstable (breaks down in water). In dispersive soils the soil aggregates – small clods – collapse when the soil gets wet because the individual clay particles disperse (Come apart) into solution. This collapse of structure causes the soil to slump, lose porosity and become denser thus restricting root growth of annual crops and pastures.

What causes Soil Dispersion?

Soils often disperse when they are sodic (high sodium), which means they contain enough sodium to interfere with the how the soil holds together (structural stability). Clay particles have a negative charge on their surface; this charge is balanced by positively charged nutrient (cations), such as Ca2+, Mg2+, K+ and Na+, distributed around the surface of the clay. Cation exchange capacity (CEC) is a measure of the total number of exchange sites in a given mass of a soil. When the ratio of sodium to other ions at these exchange sites is high, clay particles are less tightly bound to each other and the soil aggregates easily disperse when the soil becomes wet.

The impact of Soil Dispersion

When a dispersive soil wets, the structure of the soil collapses and becomes ‘soupy’. Under the impact of rain the soil slumps, soil pores collapse or are filled by dispersed clay particles. This results in sealing of the soil like a dam bank and causes a reduction in soil pore space. The consequence of this depends on whether the soil topsoil, subsoil or both are sodic.

Sodic topsoils are prone to surface sealing, and hard setting (surface crusting) when they dry. This causes ponding and low soil oxygen levels leading to poor crop emergence and root growth. It can cause root death if water ponds for a long time. When wet these soils are boggy and prone to water erosion and compaction. Because of this behaviour, strongly dispersive soils can rapidly go from being too hard when dry to being too wet for seeding after rainfall.
Dispersive subsoils, like the topsoils, have low porosity with dense (massive/ poor) structure and high soil strength when dry. Movement of air into these subsoils is poor resulting in low oxygen availability
Water infiltration is slow resulting in waterlogging or perched water tables. They are usually moderately to strongly alkaline, and often contain toxic concentrations of boron and salt which restricts root growth.