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Farmnote 57/1990 [Reviewed July 2006]

by Fionnuala Frost, Research Officer, National Soil Conservation Program and Garry Orr, Technical Officer, Division of Resource Management, Merredin

Soils containing dispersible clays are often problem soils. A dispersible clay is a clay that does not stay stable when wetted, but slakes or disperses easily.

The major problem with dispersed clay is that it can block soil pores and reduce the permeability to water. The clay also acts as a cement that hardens the soil when it dries.

Gypsum applied to soils with dispersible clays improves the permeability to water by reducing the dispersion of the clay. Reducing the dispersion allows more of the rainfall to enter the soil, reducing run-off and erosion risks and improving drainage after heavy rains.

The action of the dispersed clay in hardening the soil (or increasing soil strength) is also decreased by applying gypsum. The lower soil strength allows for more timely cultivation and seeding. Energy inputs and machinery maintenance can be reduced, while decreased soil strength also allows improved crop performance from rapid emergence, improved aeration and efficient water use.

The benefits from applying gypsum will vary, depending on the season. Apply gypsum with the aim of adopting more sustainable, reduced tillage, rather than continuing with multiple workings and having to reapply gypsum a few years later.

Several tests to help identify gypsum-responsive soils have been developed through research, including trials conducted by the Agriculture Western Australia. Testing is necessary because soils vary in chemical composition and physical properties as a result of their inherent condition and management practices imposed on them.

Two of the simplest assessments are described here. These are:

• using field observations, and
• using a simple dispersion test.

Field observations

Soils likely to be structurally degraded (lose their crumb structure when wet), and likely to be gypsum-responsive may:

• be hard when dry or have a surface crust (note that not all crusting soils are gypsum-responsive);
• become sticky or non-trafficable after light rainfall; puddles of water will have a milky appearance from the suspended clay;
• be difficult to cultivate because they are too hard or too wet;
• collapse after heavy rainfall;
• have low water infiltration and high run-off,
• produce patchy crop emergence and early growth, particularly in poor seasons.

Visual dispersion test

Apart from looking at the condition of the soil and crop, you can do a simple test on the farm to determine if your soil is likely to respond to gypsum. The dry aggregate dispersion method described below includes photographic standards for providing a scale of the degree of dispersion (see Figure 1).

Faigure 1. Varying degrees of clay dispersion in soils. The higher amounts of dispersal (4 and 5) indicate a soil's suitability for gypsum application. No.0 displays slaking (breaking off of soil particles), compared to 1 to 5 which show clay dispersion

The process of clay dispersion is a reliable indicator of unstable soil structure. Highly dispersive (structurally unstable) soils are likely to be more gypsum-responsive than those soils that are less dispersive.

Materials

• 100 mL beakers or 500 g glass jars (Vegemite jars are suitable)
• distilled water or fresh rainwater (scheme water is unsuitable)
• dry soil aggregates (small clods, or clumps)
• black surface (bench, cloth or paper)

Method

1. Take about ten dry soil samples from the area to be tested, half from the soil surface and half from a depth of 15 cm. Highly dispersive topsoil is far more limiting in terms of potential productivity than dispersive subsoils.
2. Label each soil sample and break the sample into aggregates, or amounts about 5 mm in diameter.
3. Pour 50 mL of distilled water into ten separate, clean containers and place on the flat, dark surface. (If you are using 500 g Vegemite jars, add 100 mL of distilled water to each. You will then be able to place five aggregates in a jar without affecting the results.) However, make sure you do not have samples of both topsoil and subsoil in the same jar.
4. Label the containers to identify each sample.
5. Gently place one aggregate into the centre of each beaker or jar and allow to stand for 24 hours without disturbance. (Make at least two tests for each sample to ensure the results are consistent.)
6. Rank the degree of dispersion on a scale from 0 to 5. Use Figure 1 to estimate this.

The results of the test are explained in Table 1.

Table 1. Likely response to gypsum of soils with varying levels of clay dispersion

A soil is likely to respond to gypsum if from field observations it shows the characteristics associated with poor soil structure and will readily disperse (ranking of 4 or 5).

All clays swell on wetting and the process of swelling causes particles to mechanically break off from the aggregate. This process is called slaking. This is a different observation to dispersion where the clay disperses to form a milky cloud around the aggregate.

It is the dispersiveness of a soil that determines its likely response to gypsum.

The recommended rate for applying gypsum is 2.5 t/ha. Test strips are recommended on the targeted paddocks in the year preceding a blanket application to observe any responses to gypsum. If there is no response, continue observing the strip for a further season to account for seasonal variation.

Some soils are non-trafficable after rainfall and yet will not be dispersive. The sign to look for in this situation is a sandy to sandy loam-surfaced soil with a massive tight clay subsoil. Waterlogging is invariably the problem on these soil types. The clay subsoil prevents surface water from draining deep into the profile. Where this happens, to minimise degradation you can take the following steps:

1. Reduce tillage.
2. Avoid working the paddock if it is saturated.
3. Reduce the volume of surface water flowing onto the paddock, for example, by installing recommended earthworks.

Further reading

• Farmnote No. 99/84 'Direct drilling on the contour to control erosion' (Agdex 572).
• Farmnote No. 32/85 'Gypsum improves soil stability' (Agdex 514).
• Journal of Agriculture (1987). 'Gypsum use in the wheatbelt'. Vol. 28, No. 2.