Calcium performs many functions in the soil. It provides a key nutrient for plant growth. The quantity of Calcium required by soils, and the form it is applied in, can lead to remarkably different outcomes. In addition, the amount of calcium in soils must be balanced against other cations (Magnesium, Sodium, Potassium and Hydrogen) to achieve desirable results. The wrong decision can literally cost you the earth!

Most of us know that calcium is provided to soils in large amounts – in the form of Lime, Gypsum or Dolomite. But do you know why? Soils often require two or all three of these amendments to be properly balanced.
Gypsum is arguably the most important of the big three due to its ability as a Calcium source to do many “jobs” for the grower. Gypsum is Calcium Sulphate (CaSO4.2H2O), containing 23.5% Calcium and 18.6% Sulphur. When it’s applied to the soil, gypsum will begin to dissolve- releasing Calcium and Sulphate. The solubility of gypsum is quite low, 0.24% however with large amounts of irrigation water or rainfall it does dissolve, and when it does it behaves in a very powerful way.
Clay soil is defined by its texture, and is measured by rubbing the soil between fingers and thumb when wet. Clay particles are the smallest particles in the soil, and the higher the percentage of these smaller particles the heavier the soil texture. Clay is not necessarily a bad soil feature. Heavier soils have a larger Cation Exchange Capacity compared to sandier soils due to the larger soil colloid (particle) surface area, but clay soils do become problematic for farming when they either set very hard when dry, or become very sloppy when wet.

The cause of this is two-fold: (1) the soil is low in organic matter and (2) the exchangeable cation percentage in the soil (CEC) becomes unbalanced with exchangeable magnesium (Mg) or sodium (Na) being too high (- in this case, the soil hardens when it dries).

When Gypsum is applied to the soil, the Calcium will exchange with the excess Exchangeable Mg and/or Exchangeable Na. The released Mg and/or Na will react with the sulphate to produce Mg/Na Sulphate and will be leached from the soil profile.

Salt, or excess sodium in the soil, is a perennial problem in Australian farming. Australian soils containing more than 5% exchangeable sodium is classified as sodic soils (excessive sodium), as opposed to USA soils where, for example, for a soil to be sodic the soil must hold an exchangeable sodium percentage of 15% or more. Australian soils are different to soils of other countries. They are generally much older
(some of the oldest soils in the world), have often been leached of nutrients for millions of years and are more weathered and made up of soil particles derived from different parent rock materials. Therefore it is important to know, when interpreting the results of Australian soils, will yield different recommendations than when interpreting the results of USA soils.

So, is Gypsum always good for managing salt in your soil?
The answer is Yes, but only if the sodium and or magnesium are high. You need to determine whether:

  1. The soil is saline? (High salts and low magnesium and sodium- will be called saline soil) These soils do not require any application of Gypsum – only leaching;
  2. The soil is saline and high in sodium? (Exchangeable sodium greater than 5% with high salt – will be called saline sodic soil) These soils require leaching and applications of Gypsum;
  3. The soil is saline with high sodium and high magnesium? (If the above is true and the exchangeable magnesium is greater than 20% – will be called saline sodic high magnesium soil) These soils also require leaching and applications of Gypsum.

(Lime is needed if Exchangeable Calcium is low (<65% CEC).
Lime is calcium carbonate (CaCO3) containing 40% calcium and is essentially insoluble in the soil. For this reason the finer the lime particles, the better the results, as the reactions occur on the particle surfaces. ENV (effective neutralising value) is the best measure of lime quality as it accounts for both lime purity and particle size. The higher the ENV of the lime the more effective it will be in doing its job.
The Calcium in lime will displace hydrogen from the soil particle (colloid) surface. The hydrogen reacts with carbonate, provided by the lime, and forms carbon dioxide and water. This mechanism is ideal to assist in increasing the soil pH to neutral levels where trace elements and nutrients can be more available and can also be used simply to remove excessive hydrogen from the soil.


(Dolomite is needed if Exchangeable Magnesium is less than 12%)
Dolomite is magnesium carbonate (MgCO3.) containing 11% magnesium and calcium carbonate (62.5) with 25% Ca. It behaves in a very similar way to lime, however, the Exchangeable Hydrogen will be replaced with magnesium and calcium, producing carbon dioxide and water. Some of the hydrogen will react with calcium carbonate or magnesium carbonate to produce calcium bi-carbonate and magnesium bi-carbonate. Since the bicarbonate is more soluble than carbonate, the calcium will replace part of excess sodium.


Written by Ted Mikhail