A sodic soil is one in which there is a high ESP - that is, there is a relatively large number of sodium ions on the clay surface. Following rain in which the electrolyte concentration is relatively small, the combination of sodicity and raindrop action causes the clay particles to disperse instead of remaining in their original arrangement. The disruption of the soil structure, together with clay dispersion, greatly reduces the soil permeability since the larger pores are blocked. The pores are the passageways along which water, plant roots and soil microorganisms move. When the excess water finally evaporates, say in summer, the soil sets hard and crusty.
Plants find it hard to penetrate.
Fewer seedlings emerge and plant growth is adversely affected. Fewer, smaller plants mean fewer roots to bind the soil making it more vulnerable to water and wind erosion. Therefore, a doubling in the concentration of sodium ions is likely to lead to an increase in the ESP and, consequently, an increase in sodicity. The calcium ions replace the sodium ions on the clay particles, stickiness increases and sodicity decreases. About 6 per cent of Australia s land surface is saline as well as sodic.