Dispersive (sodic) soils in Western Australia

Page last updated: Monday, 25 July 2022 - 2:08pm

Dispersive soils are common in the agricultural areas of Western Australia, where they occur mainly as duplex or gradational profiles. Soils with more than about 18% sodic clay are susceptible to dispersion when wet. Sodic duplex soils are particularly susceptible to waterlogging because they are commonly on broad, flat landscapes with poor drainage. These soils are difficult to manage and have several constraints to crop and pasture growth.

If you are looking for information about managing dispersive soils, go directly to Managing dispersive (sodic) soils. Otherwise, read on.

What are dispersive (sodic) soils?

A dispersive soil is so-called because when the clay comes into contact with non-saline water, water molecules are drawn in between the clay platelets, causing swelling, then separation of the platelets and loss of soil structure: the clay disperses into the water. The water then appears milky. This collapse of structure causes the soil to slump, lose porosity and become denser, thus restricting root growth of annual crops and pastures. Clay that disperses in fresh water is 'sodic'.

Sodic soils can be identified by their chemistry. A large number of the exchangeable cations on clay particle surfaces are sodium ions, which do not strongly bond clay particles. The percentage of cation exchange sites occupied by sodium is termed the exchangeable sodium percentage (ESP). Soils with ESP of 6 to 10 are slightly dispersive, and with ESP greater than 15 they are highly dispersive. These values can be calculated from soil analysis by the main soil testing laboratories.

For more detailed information refer to the dispersive (sodic) soils science page.

What do dispersive soils look like?

Dispersive clay dams

Fresh water in these dams appears milky because of the clay platelets in suspension. If the water is saline, dispersion does not occur, even with sodic soils.

Topsoil characteristics

  • The topsoil is often grey in colour in the higher rainfall areas, and may be red in the lower rainfall grainbelt.
  • When dry, their surface is usually smooth and often has widely spaced, large cracks connected in a polygon shape. When wet, their smoothness glistens and ponding fresh water has a milky or muddy appearance, which is caused by clay particles suspended in water.

Subsoil characteristics

  • The subsoils are generally 5 to 20 cm below the surface.
  • The subsoils are generally massive clays (that is, they have few voids and no clear structure). In the higher rainfall areas, dispersive subsoils are yellow or yellow–grey, often with a mottled pattern (Figure 1) and are usually pH neutral to acid. In the low rainfall areas, dispersive subsoils typically have hard calcium carbonate nodules and are alkaline.
  • The massive clayey subsoils are very dense or compact; cracks only appear in them after they are (a) exposed, (b) relieved of the weight (overburden pressure) of the soil that is normally above them, or (c) dried.
  • Yellow and grey colours in subsoils indicate the soil is poorly drained.
Photograph of a dispersive soil profile in a pit showing the mottled zone below the dense grey clay top layer.
Figure 1 Dispersive (sodic) grey clay profile

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Where dispersive soils are found

Geographically

  • The largest areas of sodic clays are in the eastern half of the Great Southern district of Western Australia; they are less commonly found in the upper Great Southern, central and eastern parts of the grainbelt (see Figures 2 to 5, click on the image to expand it).
  • Sodic clays occur as small areas scattered across the landscape or farms; they rarely occur in areas large enough to cover whole farms.
  • Soil surveys and mapping indicate they cover about 1.5 million hectares or 8 to 10% of the grainbelt.

Figures 2 to 5 show maps of soils with different levels of ESP at different depths. Click on each map to expand the view, or you can copy and view these maps at full scale in any graphics program.

Map of soils in the south west of Western Australia with sodic soil to 30cm
Figure 2 Soils in south-west Western Australia with more than 18% clay and exchangeable sodium percentage (ESP) greater than 6, at 30 cm depth
Map of soils in the south west of Western Australia with sodic soil to 30cm
Figure 3 Soils in south-west Western Australia with more than 18% clay and exchangeable sodium percentage (ESP) greater than 15, at 30 cm depth
Map of soils in the south west of Western Australia with sodic soil to 50cm
Figure 4 Soils in south-west Western Australia with more than 18% clay and exchangeable sodium percentage (ESP) greater than 15, at 50 cm depth
Map of soils in the south west of Western Australia with sodic soil to 80cm
Figure 5 Soils in south-west Western Australia with more than 18% clay and exchangeable sodium percentage (ESP) greater than 15, at 80 cm depth

Landscape position

Sodic clays are usually found in low-slope sections of the landscape, often on valley floors, and this distribution is obvious in Figures 2 to 5. However, in broad flat areas of low slope, they can be relatively high in the landscape.

Association with vegetation

Sodic clays have historically been referred to as ‘Moort soils’ in the southern agricultural areas because of the close association of Moort trees (Eucalyptus platypus – FloraBase) with sodic, grey-clay soils. However, Moorts – and other tree species associated with sodic grey clays – are also found on other soils, and are therefore not a reliable indicator of sodic or dispersive soils. In the lower rainfall areas, dispersive soils are associated with red morrel (Eucalyptus longicornis – FloraBase) and salmon gum (Eucalyptus salmonophloia – FloraBase).

Vegetation on a site is a reasonable indicator, but not a confirmation, of sodic soils.

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What makes dispersive clays difficult to manage?

Dispersive clays have several characteristics that make them difficult to manage:

  • poor infiltration
  • sealing or crusting
  • surface ponding
  • waterlogging
  • poor traction and trafficablity
  • compaction and hardsetting
  • salt accumulation in dry environments (less than 200 mm growing season rainfall).

Surface sealing and poor infiltration

Rain on bare dispersive clays causes the surface clods and aggregates to break down and form a ‘crust’ or seal, by dispersion. The surface appears smooth and continuous, and has few, if any, large pores. Such seals are composed of very fine grains of soil, which limits the rate at which water and air can move into the soil.

Consequently, infiltration of rain to the soil is restricted, and rain that does infiltrate is absorbed slowly and does not penetrate deeply. Shallow soil moisture quickly evaporates, and plant available water is restricted.

Locally, these soils have been called ‘Sunday soils’. They are workable for only one day: the day before, they are too wet; the day after, they are too dry.

Surface ponding and waterlogging

Once a dispersive clay has formed a seal or crust a millimetre or two thick, the rate at which additional rain can be absorbed is very slow and rainwater quite quickly begins to pond on the surface.

If rainfall is very light and persists for several days, the surface soil (which is shallow and underlain by subsoil that is dense and impermeable) will saturate and waterlog. Waterlogged soil lacks enough oxygen for plant roots to grow.

Poor traction and trafficability

The dispersion and shallow infiltration of rainwater in dispersive clays makes their surface very slippery when wet and farm vehicles lose traction and steering control. This makes cultivation, spraying and seeding operations difficult, if not impossible.

When seasonal conditions remain wet for a considerable time, the full depth of the topsoil, and perhaps the subsoil, becomes saturated. The slippery conditions of wet dispersive clays means they have little or no friction between soil particles, and have little or no capacity to carry or support heavy loads and consequently farm vehicles often become bogged.

Slumping, compaction and hardsetting

The dispersive nature and consequent lack of load-bearing capacity of dispersive clays makes them highly prone to settling or slumping under their own weight when wet. This form of subsidence transforms a loose and friable seedbed into a dense mass. The lack of load-bearing capacity that dispersive clays have when wet also guarantees they will compact very easily under the weight of farm vehicles and animals.

The typical sealed and slumped winter condition of dispersive clays transforms in summer into a very hard mass that is smooth and dense, with only a few large cracks.

In this condition, dispersive clays are impossible to cultivate or seed, and such operations must await a gentle, early break to the season with low rainfall and cool temperatures if seeding is to be undertaken without difficulty, risk or delay.

Salt accumulation

Salt tends to accumulate in low rainfall (less than 200 mm growing season rainfall) sodic clays because:

  • Low levels of salt fall in rainfall every year
  • Leaching of these salts below the root zone is reduced by:
    • the low rainfall is not enough to saturate soil below the root zone in most years
    • sodic clays disperse in fresh water, reducing hydraulic conductivity and leaching
    • transpiration by vegetation increases salt concentration in the root zone
    • high evaporation rates concentrate salt in the surface layers.

As a consequence, salts accumulate in the root zone, the salinity of the soil solution increases, and plant growth and performance is reduced.

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Dispersive soils make cropping difficult

Conditions that make cropping difficult or less productive include:

  • Abrupt wet starts to the season: surface ponding or waterlogging reduces germination and retards early growth.
  • Late dry starts to the season: cloddy seedbeds result in poor seed–soil contact and moisture uptake by the seed and seedling.
  • Dense, massive subsoil limits rooting depth.
  • Rapid change in the root zone from too wet (waterlogging) to too dry (droughting).

Conditions that favour sodic soils

  • Mild, moist conditions in spring allow dispersive clay soils to provide crops with more plant-available mositure than is possible with lighter, sandier soil types.
  • Farmers in the northern and eastern grainbelt report that in wet years, heavy textured sodic alkaline soils are the best for cropping, and in dry years, they are the worst.

Interaction with rainfall

The generally moderate and reliable amount, and winter-dominant distribution of rainfall (Table 1 and Figure 6) interacts with dispersive clays to increase the challenge facing farmers trying to manage them in ways that maximise production.

Table 1 Long-term rainfall records for locations with dispersive clays
Location Median rainfall (mm/y) Percentile 1 (10% of records lower than) Percentile 9 (10% of records higher than)
Beverley 417 305 545
Quairading 366 262 490
Pingelly 444 339 598
Kulin 351 261 463
Wagin 428 309 549
Lake Grace 348 234 457
Katanning 480 370 595
Pingrup 353 256 471
Tambellup 447 344 557
Ongerup 390 278 489
Jerramungup 421 277 539
Merredin 326 235 419
Kalannie 291 234 363

While there is certainty about rain falling in the growing season, the fact that it exceeds evaporation in winter (Figure 6) means that soils must have the capacity to absorb and store this excess without becoming saturated and lacking oxygen.

Line drawing of typical seasonal distribution of rainfall and potential evaporation at Ongerup
Figure 6 Typical seasonal distribution of rainfall and potential evaporation. Note the excess rain over potential evaporation in the winter months; soils need the capacity to store in excess of 100 mm of the rain that falls between May and July.

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Plant-available soil water

Dispersive clays have difficulty in absorbing and storing excess rainfall. As the season cools and evaporation falls rapidly, the soil profile wets up at a rate faster than indicated by the atmospheric evaporation data shown in Figure 6. This is because soil, particularly when it is only just moist, evaporates moisture much more slowly than the atmospheric rate. Consequently, when rainfall exceeds atmospheric evaporation in winter, soils with limited capacities to absorb rain, like dispersive clays, will waterlog.

The interaction between dispersive clays and rain often causes problems with crop establishment. If rain falls quickly, the surface of dispersive clays will seal and excess rain will pond on the surface causing the seedbed to waterlog. If the rain comes slowly but in substantial quantities, the capacity of the topsoil and subsoil to store it is easily surpassed and profile waterlogging results. In these circumstances, crop establishment will be reduced.

Conversely, if there is a dry start to the growing season, the hard cloddiness of seedbeds will not facilitate good seed–soil contact and crop germination and establishment will be restricted. The interaction between dispersive clay and rainfall in spring can also adversely affect crop and pasture growth. Such late-season impacts on crop growth and production are a consequence of the density of the subsoil of dispersive clays – very dense clays have a low porosity or water-holding capacity and stored water is difficult for plants to extract.

In a dry season, when crop demand for water to support active growth is high, dispersive clays have insufficient plant-available water in their compact subsoils to allow crops to finish well.

Waterlogging

In a wet spring, when plants are actively growing, the topsoil of a sodic duplex soil can be unsaturated while the subsoil is waterlogged. Plants in this situation produce much fewer, if any, roots in the saturated and anaerobic subsoil, and more than normal amounts of roots in the shallow topsoil. Plant roots below about 15 cm have been ‘pruned’ by waterlogging, which makes the plants very susceptible to droughting and a poor finish as rainfall diminishes and temperatures rise in late spring and early summer.

For more information

Use the 'See also' links on the right hand side of this page and the contacts below.

Contact information

David Hall
Tim Overheu
+61 (0)8 9892 8533