Sodicity – a dirty word in Australia
Box 2 | Diffuse double layers
Clay is sometimes described as 'colloidal', a term that comes from a Greek word for glue. Clay is sticky: you only have to do a little pottery to realise that. But what causes clay to lose its stickiness, which is what happens in sodic soils? The answer can be found in the theory of diffuse double layers.
Diffuse double layers
The diffuse double layer occurs at the interface between the clay surface and the soil solution. It is made up of the permanent negative charge of the clay and the cations or counter-ions in the soil solution that balance the negative charge. The counter-ions are influenced by two equal but opposing forces – the electrical force attracting the positive ion to the negative surface, and the diffusive or thermal forces (responsible for Brownian motion) which tend to move the cations away from the surface. The balance of these two forces gives rise to a distribution of cations in water adjacent to the clay surface. This distribution, described as a diffuse electrical double layer or simply diffuse double layer, is made up of the negative clay surface and the spread-out (diffuse) distribution of the counter-ions.
The thickness of the diffuse double layer can change
The diffuse double layer, in effect, occupies the space between the clay surface and the soil solution and has a thickness less than one-millionth of a centimetre (10-6 cm). The thickness of the diffuse double layers decreases with an increase in the electrolyte concentration – in this case, the double layer is said to be compressed. The diffuse double layer is thinner when calcium ions (with a double positive charge) balance the negative charge rather than ions such as sodium that have a single positive charge.
Furthermore, the clay particles in a soil are assembled to give a compound particle called a clay domain, consisting of many particles in parallel alignment. In such assemblages the particles overlap, and very strong forces cause these particles to be stable. As the amount of exchangeable sodium in a soil increases, these particles become increasingly unstable, leading to a disruption of the soil structure and a blocking of the large water-conducting pores of a soil.
Sodic soils disperse
When two clay particles with a high concentration of sodium counter-ions sit close to one another, their double layers overlap or interact. As a consequence, the total concentration of the ions at the plane mid-way between the two particles is greater than that in the soil solution in which the particles are immersed. This creates a difference in osmotic pressure which will draw water between the particles, causing them to move further apart – this is the swelling associated with sodic soils. In the presence of free water (eg, excess rainfall or low electrolyte irrigation water) at a soil surface, a sodic soil may move a stage further in disruption so that the particles become dispersed in this water. Dispersion can be decreased to a considerable extent in the presence of high concentrations of electrolytes.
The situation with respect to clay particles with relatively higher concentrations of calcium counter-ions is different for two reasons. First, since the doubly charged calcium ions are more strongly attracted to the clay surface, the thickness of the double layer is less and the tendency to swell is correspondingly less. Secondly, and much more importantly, the particular organisation of the clay particles where they overlap restricts the swelling, due to diffuse double layers.
Treating sodicity
There are two strategies which might be used to prevent the disruption of sodic surface soils and both strategies involve the use of gypsum. Gypsum has a relatively large solubility, producing calcium ions when dissolved.
The first strategy involves the application of large quantities of powdered gypsum, so that the exchangeable sodium in the top 5 centimetres of soil is largely replaced. This may require additions of gypsum of the magnitude of 5 tonnes per hectare.
The second strategy involves what can be called the electrolyte effect, in which the addition of calcium to an irrigation water suppresses the development of double layers on the soil particles. As a result, the adverse physical effects of exchangeable sodium are avoided. This strategy involves very much smaller amount of gypsum and is indicated where the addition of large quantities of gypsum is uneconomical.
Posted June 1999.