Gypsum works in two ways. Both depend upon the gypsum being dissolved by rain or irrigation water and entering the soil solution. The first is called the electrolyte effect (“electrolyte” means salt solution). This effect is based on the fact that swelling and clay dispersion decrease as the salinity of water infiltrating the soil increases. It occurs with all types of salts, not only gypsum. It is short-term in nature because it ceases when all the applied gypsum has disappeared.
The second effect is specific to calcium salts, including gypsum. It is based on the fact that cations in the soil, such as calcium (Ca2+), Magnesium (Mg2+), sodium (Na+) and potassium (K+), are bound to clay particles by electrical forces and are exchangeable. This means that if a high concentration of calcium cations are introduced to the soil solution (by adding gypsum) they will exchange for other cations, particularly sodium, on the clay. By this process a sodic clay is changed to a calcic clay (that is, one dominated by exchangeable calcium), thereby reducing the swelling and clay dispersion. At the same time, the sodium cations are released into the soil solution, but are leached below the root zone where their presence is less important than nearer the surface. This second effect is a long-term benefit, lasting well after all the applied gypsum has disappeared from the root zone.
Gypsum improves the structure of hardsetting or crusting clay topsoils, not only by reducing swelling, but also by preventing clay dispersion. The result is a more friable soil that has lower strength when dry. Such a soil forms a better seedbed that gives a higher plant density due to increased seedling emergence. This factor alone may be sufficient to economically justify the use of gypsum. It is most apparent when rainfall follows sowing, resulting in little or no emergence in untreated areas treated with gypsum. Figure 3 shows the effect of gypsum on the emergence of wheat on a grey clay in north-western New South Wales.
Other likely benefits of gypsum application to sodic clay topsoils include:
Increased water intake, resulting in improved water storage for subsequent plant growth* and reduced risk of soil erosion
Less wear on implements
Reduced fuel consumption
Improved trafficability after heavy rain or irrigation
Frequency of application
A question often asked is: If I apply gypsum to my soil at, say, ½kg per sqm how long will it last? There is no simple answer to this question because a number of factors are involved, for example, seasonal conditions (particularly rainfall). The chemical composition of the irrigation water, the quality of gypsum used, the method of application, the severity of the problem, and whether topsoil or subsoil or both are being treated.
Gypsum vs Lime to Improve Soil Structure
Lime is another calcium compound – calcium carbonate (CaCO3). Whether you should use gypsum or a mixture of lime and gypsum for improving the structure of sodic clay soils depends on the pH of the soil.
Gypsum can be used to treat soils of any pH because it only has to dissolve in water to produce benefits. Gypsum itself has little effect on soil pH (generally a decrease of 0.1 or 0.2 units).
However, if the pH (0.01 M CaCl2)+ of the soil is less than 5 it is preferable to use a mixture of lime and gypsum because the soil benefits in two ways. Not only is structure improved by the addition of calcium, but also pH is increased, thereby helping to overcome the nutritional problems associated with strongly acidic soils.
Lime is not recommended for soils with pH (0.01 M CaCl2) above 5, because lime is very insoluble in all but strongly acid soils.
Fineness and neutralising value are two important factors that determine how quickly and how well lime reacts with acid soils. A fine grade of agricultural lime is recommended.