10.2 Fertilisers

Plants make their own food by photosynthesis from carbon dioxide and water, but they also need other chemical nutrients for producing healthy leaves, roots, flowers and fruit. They get these chemicals from minerals in the soil. When many crops are grown on the same piece of land, these minerals get used up and have to be replaced by artificial fertilisers. Fertilisers enable plants to grow faster and provide large yields to feed the world’s growing population.

Ammonium salts (which contain the NH₄⁺ ion, e.g. ammonium phosphate, (NH₄)₃PO₄) and nitrates (which contain the NO₃^– ion, e.g. potassium nitrate, KNO₃) can be used as fertilisers.

Ammonium nitrate is the most important of the nitrogenous fertilisers, and provides both NH₄⁺ and NO₃^– ions. Ammonium nitrate contains 35% by mass of nitrogen and is produced when ammonia solution reacts with nitric acid:

NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq)

The ammonium nitrate can be crystallised into pellet form suitable for spreading on the land.

Ammonium nitrate is soluble in water, as are all other ammonium salts, for example ammonium sulfate, (NH4)2SO4. This solubility is important because plants need soluble nitrogen compounds that they can take up through their roots.

Ammonium salt tend to make the soil slightly acidic. To overcome this, they can be mixed with chalk (calcium carbonate), which will neutralise this effect. Calcium ammonium nitrate(CAN), also known as nitro chalk, is an example of a compound fertiliser.

Nitrogen(N) is important for producing the proteins needed for plant growth and also for the development of healthy leaves, but other additional elements that plants need are:

• phosphorus(P), especially important for healthy root
• potassium(K), which is important for the production of flowers and fruit.

Different plants need different combinations of these elements, which is why NPK fertilisers are produced. The NPK value informs the farmer how much of each element is present. Fruits such as apples and tomatoes need a lot of potassium, whereas leafy vegetables such as cabbage need a lot of nitrogen and root crops such as carrots need a lot of phosphorus.

A modern fertilizer factory will produce two main types of product:

• Straight N fertilisers are solid nitrogen-containing fertilisers soil n pellet form, for example, ammonium nitrate (NH₄NO₃), ammonium sulfate ((NH₄)₂SO₄), and urea (CO(NH₂)₂)
• NPK compound fertilisers are mixtures that supply the three most essential elements lost from the soil by extensive use: nitrogen(N), phosphorus(P) and potassium(K). They are usually a mixture of ammonium nitrate, ammonium phosphate and potassium chloride, in different proportions to suit different conditions. The numbers on a bag of NPK fertiliser correspond to the percentage of that nutrient in the make-up pf the fertiliser, e.g. 21:8:11 indicates 21% nitrogen, 8% phosphorus and 11% potassium, with the remaining 60% being filler ingredients that help disperse the chemicals. Nitrogen is sometimes omitted from these fertilisers because it washes into streams and rivers causing algal growth.