Sodicity – a dirty word in Australia
Box 1 | Some soil physics and chemistry basics
To understand why soils become unstable when clay particles don’t stick together, we need to dig into some soil physics and chemistry.
Soil particles
Soil is composed of particles which vary in size from sand to clay. Descriptions such as ‘sand’, ‘sandy loam’, ‘clay loam’ and ‘clay’ reflect the relative proportion of the different-sized particles. The clay particles in a soil are described as the ‘active fraction’ because of their small size and consequently large total surface area. They are flat, or plate-like, which means that while they usually smaller than 2 micrometres (0.002 millimetre), their surface area can vary from 10,000 to 100,000 square metres per kilogram of clay. Each clay particle has an overall negative charge, which is usually balanced by positive ions (often called ‘counter-ions’) such as calcium (Ca2+ ), magnesium (Mg2+ ) and sodium (Na+ ).
Cation equilibrium in a wet soil
A soil is porous, which means that there are holes, or pores, between particles. When a soil is wet, the pores are filled with a solution containing varying amounts of calcium, magnesium, sodium and potassium (K+ ) ions as well as the anions chloride (Cl- ), sulphate (SO4 2- ), bicarbonate (HCO3 - ) and small quantities of various other cations and anions. These cations are in equilibrium with the counter-ions balancing the negative charge on the clay. This equilibrium is governed by the Gapon equation:
The Gapon equation is very useful for soil scientists. Put into words, it says that the ratio of sodium ions balancing the charge of the clay surface (Nac ) to the divalent ions on the clay surface (Cac +Mgc ) is proportional to the ratio of the sodium ion concentration ([Na]) in the soil solution to the square root of the total divalent ion concentration ([Ca]+[Mg]). The right-hand side of the equation is called the sodium adsorption ratio (SAR). The left-hand side of the equation (multiplied by 100 to give a percentage) is called the exchangeable sodium percentage (ESP).
Clay particles disperse in sodic soils
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. High levels of sodium make the clay particles less ‘sticky’, so they don’t adhere, or hold together, so well (Box 2). 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 they become blocked, incoming water has nowhere to go: the net result is a waterlogged soil surface over the whole paddock. 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.
Salinity can suppress the expression of sodicity
The Gapon equation shows how salinity (an increased concentration of sodium chloride) influences the amount of sodium ions on the clay surface. If the concentration of sodium ions in the soil solution doubles, then to maintain the same ESP the concentration of calcium (and magnesium) ions in solution must increase fourfold. 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. On the other hand, an increase in the concentration of calcium ions in solution – if gypsum is added, for example – will lead to a decrease in ESP (Box 2). 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. In these soils the electrolytic effect of the sodium chloride will mask the harmful symptoms of sodicity. In soils that are both saline and sodic, correcting the salinity problem will cause sodicity to appear.
Additional information
Click here for more detailed information on sodic soils, written by Professor J.P. Quirk, AO, DSc, HonDAgrSc, FAA, FTSE, a leading expert on the physical chemistry of soils.
Posted June 1999.