Crops need nitrogen, phosphorus and potassium compounds

1. Plants take mineral ions from the soil through their roots, and harvesting a crop removes these nutrients from the field.
2. Nitrogen compounds, taken up as nitrate or ammonium ions, are needed to make proteins for growth, especially of leaves.
3. Phosphorus compounds, taken up as phosphate ions, are needed for healthy roots and for respiration.
4. Potassium compounds are needed for healthy flowers and fruit and to help the plant resist disease.
5. An NPK fertiliser contains compounds of all three elements, so it replaces the main nutrients a crop removes.
   1. The compounds must be soluble in water, because roots can absorb only dissolved ions.

The Haber process makes ammonia for fertilisers

1. Nitrogen makes up about 78% of the air, but it is very unreactive, so plants cannot use it directly.
2. The Haber process combines nitrogen with hydrogen to make ammonia: $$N_2+3H_2\rightleftharpoons 2{NH}_3$$
   1. The reaction is reversible, so not all the gases react, and the ammonia is separated out while the unreacted gases are passed through again.
3. It uses an iron catalyst at about $450{}^{\circ }C$ and 200 atmospheres.
4. Ammonia made this way is the starting point for almost all nitrogen fertilisers, such as ammonium nitrate and ammonium sulfate.
5. Before the process began in the early 1900s, nitrogen fertilisers came from limited supplies such as manure and mined nitrates, so the Haber process allowed far more food to be grown.

Industrial fertiliser production uses several linked processes

1. Air supplies the nitrogen, and natural gas and steam supply the hydrogen for the Haber process.
2. Some ammonia is oxidised using air to make nitric acid, which then reacts with more ammonia to make ammonium nitrate: $${NH}_3+{HNO}_3\to {NH}_4{NO}_3$$
3. Sulfuric acid, made from sulfur, reacts with ammonia to make ammonium sulfate: $$2{NH}_3+H_2{SO}_4\to ({NH}_4{)}_2{SO}_4$$
4. Phosphate rock is mined and treated with nitric, sulfuric or phosphoric acid to make soluble phosphate compounds.
5. Potassium compounds are mined, for example at Boulby in North Yorkshire, and need little processing before use.
6. The products are blended in the right proportions to make NPK fertilisers for different crops.
7. The plants on one site are often linked, so that the products and waste heat from one process feed the next.

Making fertiliser salts in the laboratory by neutralisation

1. In the laboratory, ammonium sulfate is made by reacting ammonia solution with dilute sulfuric acid: $$2{NH}_3(aq)+H_2{SO}_4(aq)\to ({NH}_4{)}_2{SO}_4(aq)$$
   1. Ammonia solution is an alkali, so this is a neutralisation reaction that forms a salt.
2. A titration with an indicator finds the volume of acid that reacts completely with a measured volume of ammonia solution.
3. The titration is repeated without indicator, using the same volumes, so the salt is not contaminated.
4. The solution is partly evaporated and left to crystallise, giving crystals of ammonium sulfate.

Comparing industrial and laboratory fertiliser production

1. The laboratory uses batch production, while industry uses a continuous process that runs for months without stopping.
2. The laboratory starts from ready-made ammonia solution and sulfuric acid, while industry makes its ammonia and acids from air, natural gas, water and sulfur.
3. A laboratory makes a few grams in glassware, while a factory makes thousands of tonnes in large steel reactors.
4. The laboratory method needs several steps by hand, including titration, evaporation and crystallisation, which makes it slow.
5. Industry uses automatic control and reuses heat from exothermic reactions, so its cost per tonne is far lower.
6. Both methods make the same salt, because the neutralisation reaction at the heart of each is the same.