Chemguide: Support for CIE A level Chemistry

Learning outcome 10.2(a)

This statement want you to know about the uses of calcium hydroxide and calcium carbonate in agriculture.

Before you go on, you should find and read the statements in your copy of the syllabus.

The use of calcium hydroxide and calcium carbonate in agriculture

The use of the word "lime" in agriculture can mean three entirely different things!

  • Powdered limestone (calcium carbonate) is often called lime.

  • On heating, calcium carbonate decomposes to make calcium oxide, which is known as "quicklime", but is often just called lime.

  • If you add water to calcium oxide, it reacts to form calcium hydroxide, which is known as "slaked lime" or "hydrated lime", but is also often just called lime.

To some extent, all three types of lime are used for exactly the same purpose - to raise the pH of the soil.

Many crop plants grow best at a pH of around 6 - 6.5. However, over time, soils tend to become more acidic than this. All of the various kinds of lime react with acids in the soil, and therefore raise the pH. Calcium carbonate and oxide and hydroxide all react with acids.

Most agricultural lime is calcium carbonate because it is cheaper and safer to handle. Because it isn't water soluble it will be slower acting than calcium hydroxide.

The use of lime and fertilisers together

A multiple choice question that I looked at asked about what would happen if a farmer treated a field with lime together with a nitrogen fertiliser containing an ammonium compound. If you knew about ammonium salts (which you will come across later if you haven't already done so), you would know that they react with bases to give off ammonia gas.

So you could work out what happens - but it is easier if you have met it before!

If you added a strong base like calcium hydroxide at the same time as the ammonium compound, then they would quickly react to release ammonia into the atmosphere - wasting the fertiliser.

If you just stir together a solid mixture of calcium hydroxide and an ammonium compound in the lab, you can smell (cautiously!) the ammonia being released.

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© Jim Clark 2010 (last modified May 2014)