Sodicity – a dirty word in Australia
Key text
This topic is sponsored by CRC for Soil and Land Management and the Australian Government's National Innovation Awareness Strategy.
Soil sodicity hasn’t hit the headlines yet, but in terms of area and impact it far outweighs salinity as a problem in Australia.
Soil sodicity is an obscure cousin of that infamous phenomenon, soil salinity. The two are often confused, probably because both involve sodium, a metal widespread in Australian soils.
Sodicity may be the more obscure problem, but it is a more widespread form of land degradation. It affects nearly a third of all soils in Australia (including a third of all agricultural soils), causing poor water infiltration, surface crusting, erosion and waterlogging.
It costs agriculture as much as $2 billion each year in lost production. And its impacts extend to water catchments, infrastructure facilities and the environment. Run-off from sodic soils carries clay particles into waterways and reservoirs causing water turbidity, or cloudiness. The effects of turbidity, and its removal, are very costly for industrial and domestic water users. Turbidity also causes environmental problems in rivers and wetlands. In addition, run-off from sodic soils is more likely to carry higher levels of nitrogen and phosphate into waterways and reservoirs. These are the nutrients that contribute to algal blooms, another significant environmental problem.
The difference between saline and sodic soils
In saline soils, sodium has a partner in crime, chlorine, with which it forms a salt. The presence of salt in the soil reduces the availability of water to plants and at high enough concentrations can kill them.
In sodic soils, much of the chlorine has been washed away, leaving behind sodium ions (sodium atoms with a positive charge) attached to tiny clay particles in the soil. As a result, these clay particles lose their tendency to stick together when wet – leading to unstable soils which may erode or become impermeable to both water and roots (Box 1: Some soil physics and chemistry basics).
Sodic soils are waterlogged
Sodicity can occur in the top 30 centimetres or so of the soil, or it may occur lower down, but it is in the top 5 centimetres where the real difficulties are encountered.
If sodicity occurs below the root zones of plants, its effect on crop productivity may be less apparent but it can still cause significant problems. For example, in a high rainfall area on sloping land, subsurface water will flow over the sodic layer and be lost in lateral drainage. On flatter land, the sodic layer may not permit water to drain, leading to waterlogging at the surface.
Sodic soils erode easily
Sodic topsoils in arid and semi-arid regions are subject to dust storms, which create major environmental and human problems.
Sodic soils on sloping land are also subject to water erosion, which means that important fertile topsoil is lost from agricultural land. When water flows in channels or rivulets, soil is washed away along these lines forming furrows called rills. In some cases, even larger channels of soil removal, called gullies, develop.
In other situations where only the subsoil is sodic on sloping land, subsurface water flowing over this sodic layer will create tunnels, leaving cavities that eventually collapse to form gullies.
Salinity can suppress sodicity
Sodic soils that are also saline contain high concentrations of both sodium and sodium chloride. Strangely enough, such soils will usually not exhibit symptoms of sodicity because the sodium and chloride ions formed by the dissolved sodium chloride (an electrolyte) in the soil solution prevents the clay particles from dispersing (Box 2: Diffuse double layers). The amount of electrolyte required to prevent decline in soil structure is called the threshold concentration. It can be calculated quite easily and has been used in the reclamation of soils which have become sodic (Box 3: Rehabilitating sodic soils). The adverse symptoms of sodicity will start to appear if the concentration of electrolyte falls below the threshold concentration.
Treating the sodicity problem
Sodicity can often be treated. Most commonly, calcium-containing substances like gypsum are applied to the affected soil. Other substances are also effective, including the direct application of sulfur, aluminium and iron sulfates or iron pyrite, all of which form gypsum in soils containing calcium carbonate. Even acidic cottage cheese whey has been used with some success, but gypsum is the cheapest and most effective treatment readily available for treating large areas.
Such additives may not always solve the problem in the long term. For example, very large quantities of gypsum may be needed if the additions are to have anything more than a short-term effect. And sub-soil sodicity may not be affected by the addition of gypsum at the surface, unless the soils are also deep-ripped to aid penetration.
The wider view
Soil sodicity is only one of many potentially overwhelming land management problems faced by farmers: others include salinity, erosion, compaction, acidity, pests and diseases, and waterlogging. Many are interrelated, and there is no easy cure for any one of them.
In some cases, the way ahead is becoming clearer. For example, researchers are advocating, and some farmers are adopting, a whole-farm approach to the management of sodic and other problem soils. This is based on an understanding of hydrology and the distribution of soils down the slopes and combines a number of practices in the most effective way.
Inevitably, greater understanding will lead to better practice. By combining science and common sense, agriculturalists increase the chances of finding effective and sustainable ways to manage Australia's fragile soil.
Boxes
1. Some soil physics and chemistry basics
2. Diffuse double layers
3. Rehabilitating sodic soils
Related Nova topics
Monitoring the white death – soil salinity
Toxic algal blooms – a sign of rivers under stress
Posted June 1999.