• Why is it important to me as a farmer?

• What is the best practice?
• How can you achieve this?

• Gypsum can alleviate the negative impacts on soil structure posed by sodic soils in the short-term
• Longer-term management strategies need to be in place to maintain and increase organic matter in soils

• Gypsum will have a quicker effect if it is incorporated into the soil
• Cloudy or muddy water in puddles is an indication that a soil may be dispersive and could respond to gypsum

• Gypsum can alleviate the negative impacts on soil structure posed by in hard setting clayey (sodic) soils
• Gypsum is a naturally occurring soft crystalline material which is a hydrated form of calcium sulphate. Deposits occur naturally in inland Australia
• It contains approximately 23% calcium and 18% sulphur. It is used to improve soil structure and reduce crusting in sodic soils
• Sodicity has serious impacts on farm production, as well as significant off-site consequences such as:
  • Surface crusting
  • Reduced seedling emergence
  • Reduced soil aeration
  • Increases run-off and erosion risk
  • Less groundcover and organic matter
  • Less microbial activity

• Sodic soils are known as dispersive clays and have a negative impact on seedling emergence
• Deep tillage may bring a sodic subsoil to the surface, where it will disperse on wetting by rain

• Gypsum dissolves slowly and changes the chemistry in two ways:
  • It creates a salt solution in the soil which reduces the degree of dispersion. This is a short-term effect, which occurs as the gypsum dissolves
  • The calcium in the gypsum replaces sodium attached to clay minerals. The displaced sodium cations (positively charged soil molecules) are leached below the plant root zone, thus making the soil less prone to swelling and clay dispersion

• It can remain unchanged
• It can swell
• It can fall apart into smaller fragments (it slakes, an indication of low organic matter)
• It can disperse into a fine milky suspension

• Air-dried aggregates 4-6 mm in diameter (size of a green pea)
• Rain water or distilled water
• Flat bottom dishes eg. shallow glass jar or transparent plastic cup

• The soil should be sampled from the horizons of interest, particularly the top 0 - 10 cm, and the subsoil. Allow the soil to air-dry (this is very important and may take a couple of days)
• Using a shallow glass jar or dish, gently place three air-dry aggregates about the size of a green pea into30 - 40 mm of rain or distilled water
• Watch the aggregates carefully for first few minutes to observe if slaking is to occur. Slaking will typically occur within the first hour and is an indication that the soils has a low organic matter content
• Leave the samples for 20 hours before checking for dispersion
• Dispersion is indicated by cloudiness or milkiness around the base of the aggregate. If dispersion is complete, a cloud will cover the bottom of the dish and the soil is categorised as �highly dispersive�

Figure 1 � Example of soils that do not disperse (left) and that highly disperse (right). � Source: Soil Types and Structures Module DEPI, Victoria  
[View larger image] 

• If the air-dry natural aggregates do not disperse, rework some soil in a moist (not saturated) state and make into three pea sized balls. Immerse into rain or distilled water once again. Observe the aggregates after 2 hours, and again after 20 hours
• The reworking simulates cultivation of the soil and opens up the aggregates to expose any potential sodicity issues

Figure 2 � Example of assessing aggregate stability on natural (left) and reworked (right) aggregates of soil from three horizons of a soil profile: surface soil, upper subsoil and deeper subsoil. � Photograph: Mark Imhof, DEPI Victoria  
[View larger image] 

• Figure 2 shows the upper subsoil to be the most dispersive horizon of this soil profile, classified as �Highly Dispersive�. The shallow surface soil of this profile is �Non-Dispersive�, but when moist surface soil was reworked it became �Dispersive after Moist Reworking�. The deeper subsoil would be classified as �Moderately Dispersive�
• This soil is likely to be responsive to gypsum

• 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

• ESP is calculated as the proportion of the cation exchange capacity occupied by the sodium ions and is expressed as a percentage
• The ESP value is commonly provided in a traditional soil test
• In Australia, sodic soils are categorised as soils with an ESP of 6-14% and strongly sodic soils have an ESP of 15% or greater
• Predictions for gypsum responsiveness based on ESP alone may be inaccurate. Therefore, a soil dispersion test should also be done

Figure 3 � ESP for this example on bottom left. Total number of cations (positive ions) is 16; total number of Na is 3 � so 3/16 is 0.18 X100 = 19% - i.e strongly sodic. � Source: Soil Types and Structures Module DEPI, Victoria

• Gypsum can be spread to manage surface soil problems
• Subsoil that is not exposed is very difficult to treat with gypsum: the problem lies in getting the gypsum down to the sodic layer. Such treatments may not be economic, and soil management may be the best way to manage these soils
• Gypsum will have a quicker effect if it is incorporated into the soil � where with more recent methods (injection into subsoils) this process is used in combination with ripping
• Gypsum is only effective in stabilising chemically unstable soils in the short term and does little to improve the structure of clays that are not sodic� where in this case, farmers have commented on the benefits of adopting zero tillage to improve soil structure
• Generally, soils with less than 15% clay show little response to gypsum. The soils that do have more than 15% clay are the light-to-heavy clays and many of the loams
• Note that where soil crusting is due to slaking alone, gypsum may not be effective (see organic matter below). Slaking is a physical process resulting in surface crusting in contrast to dispersion (surface crusting due to a chemical process
• If there is a positive response to gypsum you will notice:
  • less surface crusting
  • increased soil friability
  • improved infiltration of rain
  • better seedling emergence
• This will reduce your fuel bills, as there will be less draught
• It will also increase trafficability, so that there will be a wider window for tillage
• The duration of response to gypsum is about 2 to 3 years. Therefore gypsum is not a long-term solution
• Although the application of gypsum can effectively counter sodicity in the short run, longer term management strategies need to be in place to maintain and increase organic matter in soils
• Increased organic matter can improve hard-setting soils, and it can also enhance the effect of gypsum
  • Organic matter binds soil aggregates together and helps soil resist physical breakdown resulting from cultivation and rainfall impact. It can combat the severity of both slaking and dispersion. Minimum tillage techniques and long term pasture/lucerne phases in crop rotations will help maintain/improve organic matter levels

Table 1 � Suggested gypsum application rate with respect to soil ESP. � Source: DPI NSW

• Predictions for gypsum responsiveness should be based on a soil dispersion test and ESP values
• In southern Victoria, typical application rates of gypsum are around 2.5 t/ha and applied on a 3 to 5 year basis

Figure 4 - Difference in soil structure on a sodic soil (prone to dispersion) treated with gypsum at 2.5 t/ha (left) compared to untreated soil (right) on grey cracking clay. � Source: Department of Primary Industries, NSW

• Possible drawbacks of gypsum application include:
  • Gypsum sold as an industrial by-product* may contain heavy metals (cadmium) and other contaminants (fluoride)
  • Mined (lakebed) gypsum* tends to have very little heavy metal impurity, but can contain substantial amounts of lime, sodium chloride, sand and/or clay
  • Financial returns can be highly variable
  • Application of calcium may cause magnesium deficiency

*Note: Check label for product purity, and conduct soil testing to identify actual needs.

• How do I manage the impact of sodic soils?
• How do I manage waterlogging?

• Dispersive soils. - Clarkson T (2003). South West Victoria SoilSmart series: CCMA Soil Health


• Sodic Soil Management - Chapter D5- SOILPak series for Vegetable Growers Farmers - Department of Primary Industries, NSW


• Does my soil need gypsum? - Section B4.1- SOILPak series for Southern Dryland Farmers - Department of Primary Industries, NSW

• Amelioration of dense sodic subsoil using organic amendments increases wheat yield more than using gypsum in a high rainfall zone of southern Australia.  - Sale P, Gill J, Peries R and Tang C (2008). Department of Agricultural Sciences, La Trobe University, Bundoora, Victoria 3086, Australia.

• Clarkson T (2003), Dispersive soils. South West Victoria SoilSmart series: Department of Primary Industries Victoria
• Thompson, A Soils - sodic and acidic. Nature and Society Forum.
• Soils Glossary. Victorian Resources Online - Department of Primary Industries, Victoria.
• Johnston T (2011), Soil Types and Structures Module. Victorian Department of Primary Industries.
• Soil Health Knowledge Bank. Department of Agriculture, Fisheries and Forestry.

• SOILPak series for Vegetable Growers (see D5) Department of Primary Industries, NSW.
• SOILPak series for Southern Dryland Farmers (see B4.1) Department of Primary Industries, NSW.
• Sale P, Gill J, Peries R and Tang C (2008). Amelioration of dense sodic subsoil using organic amendments increases wheat yield more than using gypsum in a high rainfall zone of southern Australia.. Department of Agricultural Sciences, La Trobe University, Bundoora, Victoria 3086, Australia.

Page Updated: September 2013
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