Guideline Index

Chapter 7: Managing Limiting Soil Factors

7.2 Slaking and dispersion

Slaking is related to soil structure and particularly to structural stability, which is the soil’s ability to retain aggregates and pore spaces under various environmental conditions. Slaking is the result of lack of organic matter. Dispersion is usually a problem of soil chemistry (namely, high levels of exchangeable sodium), although it can occur in non-sodic soils due to excessive mechanical disturbance of the soil, with associated loss of organic matter. Slaking and dispersion can occur together. When this happens, both problems will have to be managed.

7.2.1 Slaking

Slaking is the breakdown of aggregates into smaller aggregates or single particles. It occurs when a dry clay soil becomes wet. The clay swells and the air within the pore spaces in the aggregates is compressed. This builds up pressure, resulting in the ‘explosion’ of the aggregate – See Figure 7.1, or the animation on Victorian Resources Online .


Figure 7.1   Diagrammatic illustration of slaking and dispersion
Figure 7.1 Diagrammatic illustration of slaking and dispersion

Slaking is severe in some soils with low organic matter and can occur within minutes of the soil becoming wet. When a slaked soil dries, crusting (hardsetting) of the soil can occur. This limits water infiltration and seedling emergence. The hardsetting can be limited to the top few millimetres of soil or can extend through the entire soil profile.

7.2.2 Dispersion

Dispersion is the separation of the clay particles from the aggregates when the soil is wet – See Figure 7.1. Clay particles carry a negative electrical charge and tend to repel each other. Calcium, magnesium, sodium and potassium all carry positive charges and are attracted to the clay particles, forming a ‘bridge’, or bond, between the negatively charged clay particles.

Calcium (Ca2+) ions, followed by magnesium (Mg2+) ions, are the strongest ‘bridge formers’ because they have two positive charges. Potassium (K+) and sodium (Na+) ions only have one positive charge, and their bonding of the clay particles is much weaker. If calcium is forming the bridge, the clay particles will hold together when they are wet. However, if sodium is forming the bridge, the bonding is much weaker and the clay particles tend to separate and repel one another when they are wet (in other words, they tend to disperse).

Cloudy or muddy water in puddles is an indication that a soil may be dispersive. A continual stream of cloudy water running out of a mole drain outlet is also indicative of a dispersive clay-type soil. Mole drainage and open drains in dispersive soils may lead to severe soil erosion.

When dispersion occurs, the dispersed clay particles fill up the pores between soil particles and aggregates; and when the soil dries out, the dispersed clay blocks up soil pores. This restricts seedling emergence, water and air movement, and root penetration. Dispersed soils are generally hardsetting and may form a surface crust – See Figure 7.2. Dispersion with no slaking results in a ‘concrete-like’ lump being formed.

Figure 7.2   Soil crusting due to dispersive sodic subsoil layer being exposed after flooding. (Photograph by David Hall).
Figure 7.2 Soil crusting due to dispersive sodic subsoil layer being exposed after flooding. (Photograph by David Hall).

Soils prone to dispersion are very susceptible to tunnel and gully erosion if incorrectly managed.

Dispersion is an indicator of sodic soils. Sodic soils are those that contain such a high level of sodium cations that it affects soil structure. However, ploughing or other mechanical treatment of some non-sodic soils can also result in dispersion.

7.2.3 Simple tests for determining slaking and dispersion

Place several large aggregates (5 to 10 mm in diameter) into a shallow dish containing distilled or rain water (for irrigated pastures use the irrigation water).

If the soil is a slaking soil, the aggregates will fall (or “slump”) apart within a few minutes to a few hours. Cloudy water will not appear around the slumped aggregates.

If the soil is dispersive, a cloud of clay-sized particles, or muddy water, will form around the aggregates – See Figure 7.3, or the animation on Victorian Resources Online. Dispersion can take several hours to occur, and a visual assessment should be made after about 2 hours and again after about 24 hours. See Chapter to work out the Clay Dispersion Index.


Figure 7.3   Increasing levels of soil dispersion, from left to right, when dispersion is determined using deionised water. Source:
Figure 7.3 Increasing levels of soil dispersion, from left to right, when dispersion is determined using deionised water.

Another test can be done to determine whether a soil is dispersive and will respond to gypsum. Place a small handful of soil into each of two clear glass jars half filled with distilled or rain water. Add a small handful of gypsum to one of the containers only (label it with a marker). Shake the two jars and leave them for 24 hours. If the soil is dispersive and responsive to gypsum, the soil will settle out in the jar with gypsum and will remain cloudy in the jar without gypsum. See Section 7.2.5 for further information on gypsum and dispersive soils.

7.2.4 Management of slaking soils

Slaking, which is related to soil structure and particularly to soil stability, can be managed by increasing the level of organic matter in the soil.

Organic matter reduces slaking by reducing the rate of aggregate wetting and by more strongly binding the soil particles together.

The best ways to increase the organic matter level in the soil are to:

  • Grow highly productive pastures, especially perennial ryegrass and white clover and, where possible, deep-rooted legumes, such as lucerne.
  • Use minimum tillage or no-tillage techniques for crop and pasture establishment.

See Chapter 5 on Soil Biology for further information on increasing organic matter levels.

Organic matter levels and stable soil aggregates can be easily destroyed by: excessive cultivation; cultivation when the soil is too dry or too wet; or stock trampling (pugging), particularly when the soils are wet. Cultivation increases the rate at which organic matter is broken down by soil organism activity (mineralisation of soil organic matter). Cultivation machinery compacts the soil, as does stock trampling. In fine-textured soils, cultivating when the soil is too wet breaks down aggregates, and cultivating when the soil is too dry creates large clods that are not easily penetrated by roots or seedlings.

7.2.5 Management of dispersive soils

In the short term, gypsum will reduce dispersion on sodic soils.

Lime can be used to reduce dispersion (to a lesser extent) on acidic sodic soils, but it is much less soluble than gypsum. In both cases, the sodium cations attached to clay particles are replaced with the stronger-bonding calcium cations. Some magnesium cations will also be replaced by calcium cations. The clay particles then bond together, or aggregate, by flocculation. However, aggregates formed solely by flocculation generally are not very stable.

In the longer term, dispersion management involves increasing the organic matter level in the soil, which will help to form stable aggregates that hold together.

See Chapter 5 on Soil Biology for more information on increasing organic matter levels.

The sodium cations that are exchanged for calcium cations on the clay particles don’t disappear. They enter the soil solution, where they can reattach to clay particles when the opportunity arises. Adequate drainage; resulting in removal of sodium-rich soil water from the root zone, will give longer-term responses to gypsum applications. What is gypsum?

Gypsum is the common name for hydrated calcium sulphate (CaSO4 • 2H2O).

There are two basic sources of gypsum: mined gypsum (natural deposits) or by-product gypsum.

Natural deposits of gypsum occur in many parts of inland Australia and vary widely in purity. The effectiveness of these sources of gypsum depends largely on the purity of the deposit and how finely the gypsum is ground. Classification of gypsum products

Gypsum products are classified under state legislation in Australia into three grades, based on their sulphur and calcium content:

  • Grade 1 gypsum must contain a minimum of 15% sulphur and 19% calcium.
  • Grade 2 gypsum must contain a minimum of 12.5% sulphur and 15.5% calcium.
  • Grade 3 gypsum must contain a minimum of 10% sulphur and 12.5% calcium.

The regulations also require that the label on any gypsum product must specify the fineness by stating the percentage of gypsum capable of passing a 2-mm sieve.

For more information on gypsum refer to:,-a-,%20Plant%20Tests/~/media/Gypsum%20Fact%20Sheet.ashx Uses of gypsum

The main use of gypsum is as a source of calcium to improve soil structure in dispersive soils.

Some farmers apply gypsum as a source of sulphur.

In dairying areas throughout Australia, some farmers apply gypsum on clay soils in the belief that it will help reduce waterlogging by improving soil structure. However, many clay loam soils, despite having waterlogging problems, will not improve in soil structure (and consequently drainage), after an application of gypsum. This is because these soils already contain adequate amounts of calcium ions, clay dispersion is not a problem, and adding further calcium in the form of gypsum is usually a complete waste of time and money. Soil testing is recommended to determine if calcium is required.

In the Northern Victoria irrigation areas on the red-brown earths the soils generally have low calcium levels and in some cases are also sodic (See Victorian Resources Online ). In northern Victoria, particular attention should be given to laser-leveled paddocks with exposed subsoils that will have low organic matter. In laser-leveled paddocks, gypsum should be incorporated into the plant root zone before sowing. It is usually applied before the final grading.

However, in northern Victoria, good pasture cover is the priority on recently lasered permanent pastures. Gypsum topdressing is considered if pasture cover is poor (usually related to poor structure or sodic soils) or on paddocks that have been badly pugged, exposing the subsurface soils. The aim of the topdressing is only to stabilise the surface soil. This will result in the soil retaining its crumb structure and improving water penetration in the summer by not sealing over.

In the Upper Torrens region of South Australia, calcium in the form of Nutrilime™ is being applied to correct low Ca:Mg ratios. The soils here were originally high in magnesium and had a low Ca:Mg ratio. Many years of irrigation using water with very high magnesium levels has aggravated the problem and this is resulting in the collapse of the soil structure.

Reclaimed soils in the lower Murray region of South Australia are experiencing structural problems, waterlogging and poor pasture growth due to high sodium levels. These soils are also benefiting from high application rates of gypsum. Determining the need for gypsum

Requirements for gypsum can be predicted from soil tests – See Chapters 8 & Chapter 9 . However, predictions based on cation measurements alone may be inaccurate. Therefore, a clay dispersion test should also be done (see Section 7.2.3 and Chapter 9.2.2 .4). Rates and application notes

Gypsum can be incorporated into the soil before sowing or before the final laser grading. Alternatively, it can be spread over the soil surface (topdressed).

Gypsum does not dissolve readily, although it dissolves more readily than lime (see Table 7.1), so it should be applied well before sowing to allow time for it to react with soil. Application rates of about 2.5 to 5 tonnes per hectare are needed on many soils, and the effect lasts from 1 to 20 years depending on the soil type and rate used.

Table 7.1   Solubility of lime and gypsum
Table 7.1 Solubility of lime and gypsum


When only the soil surface needs to be treated with gypsum it is topdressed and lower application rates are required. Rates of 1.2 to 2.5 t/ha are commonly used, and repeat applications may be required the following year.

Note that in a topdressing situation on irrigated, southern Australian dairy pastures:

  • Wait until a long rotation in the autumn.
  • Graze the paddocks out completely.
  • Topdress the gypsum before natural rainfall.

Cattle may be reluctant to graze the pastures until the gypsum fines have been washed off the leaves.

Gypsum and lime are both a type of salt, so application to an already saline soil may temporarily increase the total soluble salt levels.

Unless drainage is adequate, the sodium (displaced by the calcium) may not be flushed or leached beyond the plant root zone. Improved drainage may be necessary so that the excess sodium can be leached out of the soil. If drainage is not adequate, it is possible that the soils may quickly revert to their waterlogged and dispersive nature.

When buying gypsum by the tonne, it is important to know the water content of the product as well as its particle sizes when comparing products, as high freight costs may be incurred.

Pasture growth responses to topdressed gypsum on existing pasture are uncommon unless sulphur is deficient.