Sodicity – a dirty word in Australia

Key text

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.

Related Nova topics

• Monitoring the white death – soil salinity
• Toxic algal blooms – a sign of rivers under stress

Box 1. Some soil physics and chemistry basics

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 (Na_c ) to the divalent ions on the clay surface (Ca_c +Mg_c ) 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.

Box 2. 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.

Box 3. Rehabilitating sodic soils

Australia

During development of the Riverina District of New South Wales for irrigation, farmers encountered immense difficulties with impermeable, sodic soils. The clay fraction dispersed at the soil surface and, when dried, formed hard crusts that prevented the emergence of pasture and crop plants.

Investigations revealed that the Riverina clay contained 23 per cent exchangeable sodium (ESP), for which the threshold concentration solution was calculated to require an electrical conductivity of 1 deciSiemens per metre (dS/m). The problem for irrigators was that the water they were using had a conductivity of only 0.1 dS/m.

Once this was known, the remedy was simple: gypsum was dissolved in the irrigation water to raise conductivity above the threshold concentration. One dose of this treated irrigation water was sufficient for the successful establishment of the pasture plants. The same principles and technology for the dissolution of gypsum is currently being used to irrigate sodium-affected soils for the production of sugar cane in the Burdekin Valley of northern Queensland.

Large areas of the cropping lands of southeastern Australia have either sodic topsoils or sodic subsoils or a combination of both. Many land managers are using about 5 tonnes per hectare of gypsum every 10 years or so to ameliorate the sodic conditions. Spectacular improvements have occurred in soil structure and subsequent crop and pasture growth.

As well as improving agricultural profitability, reducing erosion and improving water quality, the application of gypsum has widened land use options and crop types. It has also reduced damage to infrastructure such as roads and buildings.

USA

Similar ingenuity might have rehabilitated a degraded farming region in Arizona, USA. For many years, it had been successfully irrigated with Colorado River water, which contained an electrolyte concentration sufficient to sustain soil permeability (that is, it was above the threshold concentration). However, during a shortage of river water, groundwater was used for 3 years, which increased the ESP to 25. When river water was re-introduced, the soil ‘froze up’ (became impermeable). The reason? The threshold concentration for this amount of exchangeable sodium had increased above the electrolyte concentration in the Colorado River water. This experience pre-dated notions of the threshold concentration concept. If this knowledge had been available, it would have been appropriate to mix the groundwater with the river water to obtain the threshold concentration. Over time, the proportion of river water could have been increased as the excess exchangeable sodium was gradually washed out of the system, until the original condition of the soil in equilibrium with the river water was attained.

Sounds good in theory, and in fact it has been done in practice. Water from the Salton Sea was mixed with Colorado River water to reclaim a saline soil with an ESP of 37 per in the Coachelle Valley, California. By ensuring that the manufactured water had an electrolyte level always above the threshold concentration, soil permeability was maintained, thereby reducing the period required for reclamation. During the reclamation process, the sea water was, in stages, increasingly diluted with river water; this favoured the exchange of calcium ions onto the surface of particles in the clay fraction.

Activities

• University of Adelaide (Australia)
  • Determining soil texture

• Hawaii Space Grant College (University of Hawaii)
  • Describing soils
  • Porosity

• Soil Science Education Home Page (Goddard Space Flight Center, NASA)
  • Texture by feel

• Agricultural Research Service (United States Department of Agriculture)
  • Soil recipes: Activity 1 (identifying soil components and predicting how soil affects water filtration).

Activity 1. A field test for sodicity

Test the surface and the subsoil separately to best understand the problem.

• Take two clean buckets into the field and collect samples from both the surface and the subsoil, according to standard procedures (a 5 centimetre soil auger would be ideal).

• Collect samples randomly from a minimum of 5 locations over a uniform 1-2 hectare representative area of the paddock, placing all the surface samples in one bucket and the subsoil samples in the other.

• If it isn't clear where the subsoil begins, take a sample from the top 10 centimetres of the soil profile. Then take a second 10 centimetre sample from somewhere deeper in the profile, within the range of 20-60 centimetres below the surface.

• Spread the soil from each bucket into a thin layer on a clean plastic sheet. Place in a well-ventilated location to get it air-dry, which may take several days.

• Then if necessary break the air-dried soil down into pieces of 1centimetre diameter, and mix the soil in each bucket thoroughly.

Measure the problem using the turbidity test for sodicity

• From each surface and subsoil sample weigh 100 grams of soil into a clean 600 millilitre glass jar with lid.

• Measure out 500 millilitres rainwater or distilled water to give a 1:5 ratio of soil to water.

  Making up a ratio of 1:5 soil to water (100 grams of soil and 500 millilitres water).

• Gently pour this water down the side, without disturbing the soil at the bottom.

• Invert the jar slowly and gently once and then return to its original position (avoid any shaking). Then let stand for 4 hours, with no vibrations or bumping.

• Check the suspension above the sediment at the bottom of the jar and score the amount of cloudiness using the photographs for comparison. Make up another soil suspension and repeat the process if unsure.

  Estimating turbidity (soil sodicity) in a 1:5 soil/water suspension. 1. Clear or almost clear – not sodic 2. Partly cloudy – medium sodicity 3. Very cloudy – high sodicity

• A white plastic spoon or spatula, that reflects light, when placed in the centre of the suspension can help identify the level of turbidity.

  Estimating turbidity using spatula visibility. 1. Plastic spatula visible – not sodic 2. Plastic spatula partly visible – medium sodicity 3. Plastic spatula not visible – high sodicity

• When land managers determine remediation procedures for sodicity, they should take into account other degradation issues such as acidity and salinity – a multi-pronged approach may be needed.

Further reading

Useful sites

This site includes general information about soils. Click on 'What are soils good for?' and then 'Issues' to find out more about erosion, salinity, acidity, pollution and fertility. The salinity issue includes information about sodicity.
http://www.adelaide.edu.au/agcareers/Content/content-framesets/TeacherResources.html

Managing sodic soils (Colorado State University Cooperative Extention, USA)

Describes sodic soils and how to manage them.
http://www.ext.colostate.edu/pubs/crops/00504.html

Salinity and sodicity in North Dakota (North Dakota State University, USA)

Describes the related problems of salinity and sodicity. Some explanations use technical language and the areas discussed are in the USA but this site contains good information.
http://www.ext.nodak.edu/extpubs/plantsci/soilfert/eb57-1.htm

The Australian Soil Classification: How to use this classification (CSIRO Division of Land and Water)

Explains the features used to classify soils. Sodic soils are dealt with in detail in the sodosol order. The detailed descriptions use technical language.
http://www.clw.csiro.au/aclep/asc_re_on_line/soiusing.htm

The salinity audit of the Murray-Darling Basin (Murray-Darling Basin Commission)

Both the full report and a summary of the salinity audit are available as PDF files. Also of interest is Water and land salinity, which gives a good coverage of these problems in the Murray-Darling Basin.
http://www.mdbc.gov.au/naturalresources/salt_audit/salinity.htm

Glossary

electrical conductivity. When a voltage is applied across a substance, an electric current will only flow if the substance conducts electricity. When salts dissolve in water, ions are formed and the solution (the electrolyte) will conduct electricity. As a general rule, the higher the concentration of ions in solution (ie, the higher the salt concentration) the better the solution conducts electricity; in other words, its electrical conductivity increases. Electrical conductivity is often expressed in units such as deciSeimens per metre (dS/m). Rain water, for example, has a conductivity of 0.02-0.05 dS/m, while sea water has a conductivity of 50-60 dS/m.

electrolyte. A substance that produces ions (particles with an electric charge) when dissolved in water. The resulting solution (which can also be referred to as an electrolyte) conducts electricity.

exchangeable sodium percentage (ESP). Calculated by dividing the concentration of sodium ions clay particles by the concentration of divalent ions on clay particles, then multiplying by 100. A high ESP is an indication of a sodic soil.

ion, anion, cation, and divalent ion. An ion is an electrically charged atom or group of atoms. The charge is the result of the loss (positive ion) or gain (negative ion) of one or more electrons.

The gain of one or more electrons produces an ion with a negative charge (anion). The loss of one or more electrons produces an ion with a positive charge (cation). Ions that have gained or lost two electrons are called divalent ions.

sodium adsorption ratio (SAR). The ratio of the concentration of the sodium ion concentration in the soil solution to the square root of the total divalent ion concentration. A high SAR indicates a sodic soil.

soil salinity. This characteristic of soils relates to their salt content. These salts usually involve sodium chloride, but other salts occur in some soils. Soil salinity can be measured by determining the electrical conductivity of a solution, obtained by saturating a soil sample with water (a soil 'saturation extract'). A soil is said to be slightly saline when the electrical conductivity of a soil 'saturation extract' is 4-8 deciSeimens per metre (dS/metre), moderately saline at 8-16 dS/metre, and strongly saline at more than 16 dS/m.

soil structure. Refers to the arrangement of soil particles and the pore spaces between them.

The mineral or inorganic part of soil consists of particles of different sizes. Between the soil particles are spaces, called pores. Their number and size strongly influence soil structure. Pores are important in allowing air and water to penetrate the soil. Small pores enable the soil to hold greater amounts of water; larger ones allow for faster drainage. Good soil needs many pores, varying in size with some holding air and some water. Loss of pores changes soil structure for the worse as far as plant growth is concerned.

turbidity. A measure of the amount of suspended solids (usually fine clay or silt particles) in water. Increased turbidity reduces the penetration of light through water, reducing the growth of aquatic plants. For more information see Water quality: Turbidity (NSW Environmental Protection Authority).