Poorly structured heavy soils are difficult to work and can have multiple constraints to root growth and crop yields. With a low infiltration rate, the soil is prone to run-off, waterlogging and trafficability issues. The soil can be too dense for crop roots to grow through, limiting their access to water, oxygen and nutrients.
These soils can also suffer from transient salinity, a form of salinity influenced by seasonal rainfall and crop evapotranspiration rather than by groundwater. This type of salinity tends to be more prevalent in regions and years with less than 200 millimetres of rainfall.
Figure 2: Soils with transient salinity can have a powdery appearance. Photo: Quenten Knight
Two mechanisms that contribute to its transient nature are salt movement in the soil profile and changes in the ratio of salt and water. As crops transpire, they draw water and salts to the surface. As the soil dries the salt concentration in the root zone increases. When it rains the salts are diluted or leached and salinity drops. In drier years there is less water in the soil and therefore a higher ratio of salt to water.
When transient salinity is present it causes the same problems as permanent salinity – reduced root growth, the salt decreasing water availability for crop growth, and nutrient issues such as exacerbated boron toxicity.
Salinity is typically caused by a variety of salt components – sodium, potassium, magnesium, calcium, chloride, sulphate, carbonate, bicarbonate and nitrate. Further challenges arise when there is too much of one salt, sodium chloride, and transient salinity occurs alongside soil dispersion.
Soil is described as dispersive when clay particles separate upon wetting, causing soil aggregates to break down. The soil loses its structure, the dispersed clay clogs soil pores and, as the soil dries, it sets hard and can form a crust at the surface. This makes it difficult for germinating seeds to emerge upwards and for roots to grow downwards. With limited soil pores, high soil density limits water infiltration and oxygen availability to roots.
Soils often disperse when there is too much sodium attached to soil particles. When in water, the sodium acts like two positive ends of a magnet and pushes the soil particles apart. This is why soil dispersion is often called sodicity, although the two are not synonymous.
Laboratory tests for cation exchange capacity measure the amount and percentage of exchangeable sodium, calcium, magnesium and potassium. When the exchangeable sodium percent (ESP) is greater than 6.0, a soil is considered sodic.
Although sodium is the most common cause of dispersion, it is not the only cause. Dispersion is complex and affected by clay mineralogy, organic matter, electrolyte concentration and exchangeable cation composition. There is no definitive ESP at which soil disperses, and as salinity increases, soil dispersion decreases.
Higher salt concentration in the soil solution promotes flocculation, which can stop the soil particles from separating. For example, a non-saline soil may disperse at ESP 4.0 while a more saline soil might not disperse when the ESP is 10.
Transient salinity and dispersion
In theory, transient salinity can suppress dispersion. However, whether or not transient salinity levels can get high enough to do so is unclear to researchers because landscapes behave differently and there is no universal index or tipping point at which salinity from sodium chloride begins to suppress dispersion. But there are guides (outlined in Step 3 below).
Knowing if a dense soil is dispersive and/or saline, and if salinity periodically suppresses dispersion, is critical to choosing the best management option. Gypsum is the most common treatment for dispersive soil but might not work in all situations. There are a series of tests to help diagnose the problem(s) and to check if gypsum will be beneficial.
Diagnosing the issue
Step 1: Dispersion test
The best way to know if a soil is dispersive is to do a dispersion or ‘jam jar’ test. Gently place small, dry soil aggregates in distilled or rainwater and see if the water turns cloudy. In highly dispersive soil, the water will turn cloudy within minutes. Wait a full 24 hours before making your assessment, as dispersion can take a while to show up in low to moderately dispersive soil. More detailed instructions are outlined in the Dealing with dispersive soils fact sheet.
If the aggregates disperse, the next step is to see whether the soil will respond to gypsum (Step 2). If the aggregates did not disperse but you suspect transient salinity is suppressing dispersion, consider the electrochemical stability index (ESI) in Step 3.
Step 2: Dispersion test + gypsum
To test whether a dispersive soil will respond to gypsum:
- Half-fill two glass jars with distilled or rainwater.
- Place three scoops of soil into each jar.
- Add one scoop of gypsum to one jar and label it.
- Shake both jars and leave for 24 hours.
If the soil is dispersive and responsive to gypsum, the soil will settle out in the jar containing gypsum. The jar without the gypsum will remain cloudy.
Step 3: Transient salinity
There are a few ways to gauge whether transient salinity is influencing dispersion in your landscape.
Calculations
The electrochemical stability index (ESI) was developed to describe the relationship between salinity and dispersion. ESI is calculated as: ESI = EC1:5/ESP.
Soils with an ESI less than 0.05 are potentially dispersive. Using the ESI, for an ESP of 6, a salinity level (EC1:5) of 0.3 dS/m is the tipping point at which dispersion may be an issue. Salinity levels below this suggest dispersion will become more of a problem. Note that this criterion was developed on cotton soils in the eastern states and models that only use ESP and EC will not explain all dispersive behaviour; use it as a guide only.
Another calculation in development is net dispersive charge (Rengasamy et al. 2016). It describes the interaction between the dispersive charge of exchangeable cations (calcium, magnesium, potassium, sodium) and the flocculating charge of these soluble cations in the soil. If transient salinity causes the flocculation charge to be equal to dispersive charge, dispersion will be suppressed. Calculating net dispersive charge needs laboratory analysis for both exchangeable and soluble cations.
Field indicators
In-paddock observations are usually a better guide than laboratory tests. If the regular signs of soil dispersion – cloudy puddles, very sticky soil and surface crusts are only sometimes present after rain – there might be transient salinity. Salinity can make some soils look puffy and dusty, such as in Figure 2. Areas of transient salinity usually show up on biomass or yield maps due to poor crop growth. As dispersive soils are often alkaline, calcium carbonate nodules may also be present.
More dispersion tests
Another option is to do dispersion tests throughout the year. Collect aggregates from the same location and soil layer. You might see variability in dispersion influenced by the rise and fall in salinity.
If transient salinity is suppressing dispersion, there could still be value in applying gypsum. Gypsum can improve soil structure in dispersive sodic soils and help leach salts below the root zone.
References
Rengasamy, P. et al. 2016. Exchangeable cations and clay dispersion: Net dispersive charge, a new concept for dispersive soil. European Journal of Soil Science Vol 67, pages 659-665.
This article was produced as part of the GRDC ‘Maintain the longevity of soils constraints investments and increase grower adoption through extension – western region’ investment (PLT1909-001SAX). This project is extending practical findings to grain growers from the five-year Soil Constraints – West suite of projects, conducted by the Department of Primary Industries and Regional Development, with GRDC investment.