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Gypsum – what is it good for?

Gypsum can help stabilise aggregate structure in some soils. Use of gypsum in other soils will not improve soil physical or chemical properties, so it is important to understand the processes that occur when gypsum is added to soil.

Water, air and roots move between soil particles and aggregates (also called peds). The pores between aggregates are fairly large, whereas the pores within aggregates and between particles are often too small for effective water movement or root penetration. Soil aggregate formation and stability, also called soil structure, is one of the most important manageable soil physical properties. In all but the sandiest soils, good aggregate structure is needed for water infiltration and soil drainage.

Figure 1: Physical relationship between water, air and roots in the soil.

Soil aggregates are held together by cations (positively charged atoms), among other things. Soil particles are negatively charged, and so repel each other. Cations act as a bridge, essentially pulling soil particles together into aggregates, in a process called flocculation.

Not all cations are created equal when it comes to flocculating soil particles or stabilising soil structure. The relative flocculating ability of common soil cations is shown in Table 1. Because sodium is a very poor flocculator, it takes a lot of sodium to cause soil aggregate formation. Calcium, on the other hand, is a very good flocculator, so just a little is enough to improve soil structure.

We can get a rough idea of how stable a soil’s structure is by looking at the relative amounts of these cations. This is done with the sodium adsorption ratio (SAR), where cation concentrations (indicated by brackets) are in mmol/L:

SAR = [Na⁺] / √([Ca²⁺] + [Mg²⁺])

An alternative equation for expressing the impact of sodium on aggregate stability is the exchangeable sodium percentage (ESP):

ESP = [Na⁺] / cation exchange capacity

In addition to the relative proportions of flocculating cations, it is also important to know the total concentration of soluble salts in the soil. Cations are always accompanied by negatively charged ions (anions), which together form salts. Salts dissolved in water conduct electricity, so we can measure the electrical conductivity, or EC, of a soil + water mixture to determine the amount of salt.

Together, the SAR and EC of a soil can be used to predict its aggregate stability, as shown in Figure 2. If a soil has a combination of high SAR and low EC, the aggregates will tend to disperse. If it has a high EC and low SAR, the soil particles will be aggregated.

Figure 2. Soil electrical conductivity (EC) and sodium adsorption ratio (SAR) determine aggregate stability.

It is easy to conduct a simple soil test to determine soil stability. Put about 1 tablespoon of soil and 2 cups of water in a jar and shake. Let it settle for at least 15 minutes. If the soil settles out of the water, the soil particles are aggregated. If the soil does not settle out of the water, the soil aggregates are unstable and susceptible to dispersion.

In soils with unstable structure, gypsum (CaSO₄·2H₂O) can help promote and stabilize structure by providing a good source of calcium. Calcium acts as a “glue” that holds soil particles together into aggregates and stabilizes soil structure. Powdered gypsum is the least expensive form. Prilled gypsum is effective at reducing dust, but is somewhat more expensive. Regardless of the form of gypsum used, it is important that the quality of the gypsum be determined by a reputable laboratory. This is because knowing the calcium content of gypsum is not enough, as calcium may be present in other forms such as lime (CaCO₃), which is not an effective calcium source in soils with high pH.

The amount of gypsum that should be applied is best determined by a soil analysis. Soil analyses should be conducted to determine soil sodium status, as either SAR or ESP. Either provides a good indication of the need for gypsum, and the two measures are roughly equivalent. Approximate amounts of gypsum to add, based on soil analyses, are shown in Table 2. Gypsum can also be added directly to irrigation water; amounts should be determined by laboratory water analysis.

Another consideration when deciding whether gypsum is the correct material to apply is the ratio of calcium to magnesium in the soil, usually written as Ca:Mg. Extensive research indicates that the ideal ratio of calcium to magnesium ions in the soil for plant growth is between 3 and 6 parts calcium to every 1 part magnesium. Gypsum is an effective method of raising low calcium, but the Ca:Mg balance must be maintained. Table 3 shows the role of gypsum and other soil ameliorants in balancing Ca:Mg and pH. Note that gypsum does not alter pH.

Alternatives to gypsum include sulphuric acid and elemental sulphur, although these are effective only in calcareous soils that contain solid-phase calcium carbonate (e.g. lime). Sulphuric acid dissolves calcium carbonate in the soil, forming gypsum and thereby releasing calcium:

H₂SO₄ + CaCO₃ → CO₂ + H₂O + CaSO₄

Note: Sulphuric acid is both toxic and corrosive and requires specialised storage and handling procedures.

Elemental sulphur is converted to sulphuric acid by sulphur-oxidizing bacteria, ultimately producing the same effect as sulphuric acid. This can take several weeks to several months, depending on soil conditions.

With any of the soil additives described here, getting the amendment into the soil can be problematic. Gypsum is slightly soluble in water, and will slowly move into the soil with rain or irrigation water, but physical incorporation will improve soil structure more rapidly than unincorporated surface applications. Sulphuric acid is much more soluble and is rapidly transported into the soil with infiltrating water. For a soil with poor texture or poor aggregation, moving water into the soil can be extremely difficult, and in these cases soil amendments may have to be physically incorporated.

If your soil does not have stable structure, then gypsum, sulphuric acid or elemental sulphur can help to improve soil physical properties. However, if your soil has good structure, adding these amendments will not be beneficial.

All irrigation water contains salts. It is critical to know the SAR of your irrigation water because this can change the SAR of the soil, and convert a stable soil to an unstable, dispersed soil over time. Irrigation water may require treatment with sulphuric acid, gypsum or other amendments to prevent the formation of problem soils. Soil structure problems are much easier to prevent than to cure!

Consult the MSDS for all substances you work with to learn what special storage and handling procedures may be necessary