In dispersive soils, the structure deteriorates when the soil is wet. Dispersive soils are prone to compaction, temporary waterlogging and reduced seedling emergence. None of which are great for cropping.
Many soils in south-eastern Australia had naturally dispersive topsoils. These soils are high in sodium (Na) and are also known as sodic soils. Gypsum is the most common treatment for dispersive/sodic soils. It reduces dispersion by replacing some sodium (Na) ions with Ca.
In many cropping paddocks, repeated gypsum treatments have significantly improved the topsoil. Applying more gypsum is unlikely to further improve their structure.
Gypsum only improves soil structure in dispersive soils. If the soil is not dispersive, the only potential benefit of applying gypsum is to supply Ca and S.
A simple dispersion test can identify where gypsum isn’t needed to improve the soil structure. Anyone can carry out the Emerson aggregate stability test on soils other than sands or gravels.
How to test if your soil disperses:
- Take some air-dry soil peds (aggregates) about 5mm in diameter
- Put a few centimetres of distilled or rainwater into a clear jar, or dish. Do not use chlorinated tap water as this can interfere with the test
- Place the peds gently in the water.
- Check the peds after 30 minutes, 2 hours, and 24 hours.
What are we looking for?
If the topsoil is dispersive the water around the peds becomes cloudy or milky from clay in the water. The more cloudy the water, the more dispersive the soil is. See Agriculture Victoria’s time-lapse videos of dispersion. Gypsum makes these soils more stable and less dispersive.
If the ped has crumbled but the water is still clear, the soil is not dispersive. Adding gypsum to non-dispersive soils will not improve soil structure. If you don’t need to add Ca and/or S for nutrition, you don’t need gypsum.
What about lab tests?
Dispersive soils are often sodic, with an exchangeable sodium percentage (ESP) > 6%. This test tells us if the soil is likely to be dispersive. Usually, the higher the ESP, the more gypsum needed. But if an aggregate stability test shows the soil doesn’t disperse, think twice about applying gypsum.
Table 1. Relationship between degree of dispersion and ESP.
|Rating||Approximate ESP (%)||Comments|
|Non-sodic||<6||No dispersion evident after 24 hours. Aggregates slaked but not dispersed (milky) clay.|
|Slightly sodic||6-10||Dispersion (milky halo) evident after 24 hours. Soil aggregates slightly disperse.|
|Moderately sodic||11-15||Dispersion (milky halo) evident after several hours. Soil aggregates partially disperse.|
|Highly sodic||>15||Dispersion (milky halo) evident in less than 30 minutes. Soil aggregates completely disperse.|
Source: Farmnote 386, Nov 2009
It is harder to manage dispersive subsoils. Often, they also have other constraints such as a high pH, poor structure, and/or salinity. Other amelioration strategies might be needed as well, or be more important than applying gypsum.
Photo: The soil on the left is not dispersive. The soil on the far right is highly dispersive. Photo: Ehsan Tavakkoli