101 Questions answered
Sodic vs Dispersive Soils: High Sodium Content vs Structural Collapse
The terms are often used interchangeably but are not the same thing.
A sodic soil has a higher proportion of sodium; an exchangeable sodium per cent (ESP) of 6% or more in the topsoil and 15% or more in the subsoil is considered sodic.
When dispersive soils are exposed to water, the aggregates swell and collapse, then dry into a hardened mass. The cause is often high ESP but organic matter, salinity, and clay mineralogy also affect dispersion.
Sodic soils are often dispersive, but not all dispersive soils are sodic. There is no definitive ESP at which soil disperses. For example, when 306 soil samples from southern NSW and northern Victoria were analysed, 17% of the samples with an ESP > 6 did not disperse (Vance et al. 2002).
