Lime gypsum and dolomite are used to improve garden soil

The actions of lime gypsum and dolomite are important to the way we use them to improve garden soil. It is not enough to simply say: “Calcium is low, so we will add some”. The form in which Calcium (or Magnesium) is added will affect the result achieved.

Also, it is important to remember that applications of lime gypsum and dolomite should be made at least 3 months (and preferably six months) before any fertilizer. They need some time to improve the soil and some nutrients can be converted into unavailable forms by reacting with either lime or dolomite.

However, it is most important to bear in mind that Lime Gypsum and Dolomite are used to adjust the proportions of exchangeable cations in the soil. This means that application rates must be calculated relative to the Adjusted Cation Exchange Capacity of the soil .

Lime Gypsum and Dolomite should never be applied according to a simple 'Rule -of-thumb' or simply to alter the soil pH - these approaches simply cannot give a reliable result for every soil.

Gypsum

Gypsum is Calcium Sulphate, which adds Calcium in a reasonably soluble form (2.5 kilograms will require 1,000 litres of water to dissolve it). The Calcium it provides is exchanged on the soil particle surface for any excess Sodium or Magnesium that may be present. These are then removed as Sodium sulphate and Magnesium Sulphate through leaching.

For this reason, Gypsum is used when exchangeable Calcium is low and there is an excess of exchangeable Magnesium (Mg) &/or Sodium (Na). The amount applied depends on how much Mg &/or Na needs to be removed. However, moisture is an essential requirement for this process – first to dissolve the Gypsum and then to leach out the Sodium Sulphate and Magnesium Sulphate.

In low rainfall areas without irrigation, the required amount of Gypsum may need to be divided into a number of smaller applications, depending on the rainfall. But remember – Gypsum should not be applied at all where there is a shallow, saline water table and no subsurface drainage. The risk here is that after the excess Sodium and Magnesium are leached down to the watertable, capillary action may return them to the root zone – making the situation worse rather than better .

Lime and Dolomite

Lime is Calcium Carbonate and Dolomite contains both Calcium carbonate and Magnesium Carbonate. Carbonates of any kind will have a very different action in the soil from that of Sulphates like Gypsum. Both Lime and Dolomite are essentially non-soluble, so reactions occur at the surface of the particles, making finely ground products essential for good results this makes ENV (Effective Neutralizing Value) the best measure of lime quality, as it accounts for both purity and particle size.

The Carbonate from Lime and Dolomite reacts with Exchangeable Hydrogen to produce Carbon Dioxide and water – leaving Calcium (& Magnesium) behind in exchange. The amount of Lime &/or Dolomite used depends on the amount of exchangeable Hydrogen to be replaced.

For this reason, the Mikhail System uses Lime and Dolomite when exchangeable Calcium &/or Magnesium are low and Exchangeable Hydrogen is high – not simply when the pH is low.

Other materials

Apart from Lime Gypsum and Dolomite, there are other materials that can be used in special circumstances. For instance, in high Calcium soils with little exchangeable Hydrogen, but low Magnesium, the use of Dolomite will (at best) be very slow to produce results. For these soils, Magnesium Sulphate (Epsom salt) is preferred. Its action is similar to that of Gypsum, but its much higher solubility makes it important to be a little bit careful about how you use it. For this reason, if a heavy application is required, the full amount is never applied all at once. Always stick to a maximum of 250 kg/ha per application.

In cases where a large amount of Magnesium is required and there is a significant proportion of exchangeable Hydrogen, Magnesium Oxide can be a useful alternative to Dolomite, as it provides no additional Calcium. It is also used as an alternative where Dolomite is hard to obtain, or where available sources are not of sufficiently high quality.

Magnesium Oxide is also used as an alternative to Magnesium Sulphate where the cost of this material is prohibitive or it is simply unavailable. Magnesium Oxide works under circumstances where Magnesium Sulphate would normally be used (due to its high reactivity), but a slow response should still be anticipated. Again, if a heavy application is required, a better result can be obtained by splitting this into several applications with a maximum of 250 kg/ha each. Its application immediately after the use of a Sulphate fertilizer could also help.

Further Reading:

Soil problems that can be improved using Lime Gypsum and Dolomite
Lime Gypsum and Dolomite are useful in fixing a wide range of soil problems, so long as you choose the right ones and the appropriate application rates to use.

Lime Gypsum and Dolomite are needed to improve Soil Balance
Calcium and Magnesium are essential plant nutrients, but here we are focusing only on their exchangeable forms and the role these play in improving soil structure and friability.

An important aspect of soil balance is the Calcium Magnesium Ratio
Many people talk about this, but few really understand how it works. It has no direct affect on plant nutrition and must be assessed in conjunction with the amounts of exchangeable Calcium, Magnesium and Sodium - as percentages of the Cation Exchange Capacity.