Understanding lime and gypsum better

I have come across this question very often: What is the difference between calcitic lime, dolomitic lime, hydrated lime, granulated lime and gypsum? They are definitely not the same thing and should never be used interchangeably. There is a reason why a specific product is recommended in a specific context.

Lime and gypsum are broadly grouped as they are calcium-containing minerals which are used for soil amendment on agricultural soils.

First, a note on how effective a specific source of lime is

There are two things that need to be considered in understanding the effectiveness of a specific lime source: 1) neutralising value and 2) fineness. The neutralising value gives an indication of how effective the lime will be at neutralising soil acidity, i.e. increasing soil pH. Ideally the neutralising value of lime should be greater than 95. The fineness gives an idea of the distribution of particle sizes of the lime. The finer the particles, and the greater the proportion of finer particles, the more effective the lime will be.

A third factor, which will be discussed in greater detail below, is the mineral content of the lime in terms of calcium and magnesium levels.

Lime vs gypsum

Let us start with the easiest distinction. Lime is a carbonate, oxide or hydroxide of calcium. It is used to increase soil pH and provide calcium ions in the soil. Gypsum is calcium sulphate. It is also used to provide calcium ions in the soil, but does not have the effect of increasing soil pH. Gypsum is in fact a neutral salt, and therefore is used on soils with pH’s of greater than 7. It is also used to improve soil sulphur levels.

The impact of Gypsum on sodic soils

Calcitic vs dolomitic lime

Calcitic and dolomitic lime are very similar in that they are calcium carbonate based. They are also used for similar purposes, i.e. to neutralise soil acidity, and to add calcium ions to the soil. There is one significant difference though. Dolomitic lime contains magnesium, whereas calcitic lime does not. This is an important distinction, as certain soils require additional magnesium, and other soils would be harmed through the addition of magnesium. It is important that the correct lime is used for the correct context.

Quick lime

You may also come across quick (also called burnt) lime. This is calcium oxide. It is more concentrated than calcitic lime, but is unpleasant to handle, so is very seldom used in agriculture.

Hydrated lime

Hydrated lime, also known as slaked lime, is calcium hydroxide, as opposed to calcitic and dolomitic lime. It has a higher neutralising value than calciltic lime, but there are some challenges associated with using it.

A note on hydrated lime

I found a document discussing hydrated lime from a perspective of whether it can be used on organic farms in the USA. Their conclusion was that it cannot be used as it is a synthetic fertiliser.

Some of the comments that came up about the negatives of using it are that it:

• is a corrosive material, and is very unpleasant for people to handle;
• is corrosive towards plants;
• can potentially directly harm soil micro-organisms, although there is no evidence on this;
• raises the pH of the soil too quickly, which can potentially have a negative effect on soil life.

The positive perspective on using it is that it raises the pH quicker than calcitic lime, which is positive purely from a soil fertility perspective.

For these reasons, I would advise farmers to rather use other sources of lime

Granulated lime

Granulated lime has become a popular product. Basically it is just lime that has been finely ground and made into a pelleted form. This has two advantages. Firstly, it is much easier to handle and spread than other forms of lime. Secondly, the particle fineness is much lower than other forms of lime, and therefore it reacts much quicker in the soil.

A note on granulated lime

As wonderful as this sounds, it usually comes with a hefty price tag, often costing about ten times more than normal lime. This is probably why it is also often quoted as being ten times more effective, and therefore you would only need to use a tenth of the recommended rate of normal lime. When considering whether to use granulated lime, the question just needs to be asked: what is the goal of applying lime? If the goal is to purely raise the pH, then granulated lime is effective at doing this. If the goal also includes adding calcium to the soil, building soil calcium levels and increasing calcium base saturation percentage, then I would not recommend granulated lime. Granulated lime does not contain any more calcium than normal lime. If you apply ten times less granulated lime than you would normal lime, then you will be applying ten times less calcium. Keep this in mind before getting caught up in the marketing buzz of granulated lime.

Conclusion

The usefulness of each form of lime is relative to the context within which it is going to be used, and the goal of applying it. Understanding the difference between types of lime is important in using the correct form at the correct time.

Resources

http://www.bvsde.paho.org/bvsacd/cd27/hydroxide-crops.pdf – Accessed 15 November 2018

Agricultural_uses_of_lime_and_gypsum.pdf – Accessed 19 March 2019

http://www.ccmaknowledgebase.vic.gov.au/shkb/brown_book/23_Lime_Comparison.htm – Accessed 19 March 2019

https://www.dpi.nsw.gov.au/agriculture/soils/acidity/publications/best-lime – Accessed 20 March 2019