10.07.2 Fertilisers

Syllabus

Compounds of N, P and K are used as fertilisers to improve agricultural productivity.
NPK fertilisers contain compounds of all three elements.
Production of NPK fertilisers uses a variety of materials in several integrated processes.
NPK fertilisers are formulations of various salts containing appropriate percentages of the elements.
Ammonia can be used to manufacture Ammonium salts and Nitric acid.
Potassium Chloride, Potassium Sulphate and Phosphate rock are obtained by mining, but Phosphate rock cannot be used directly as a fertiliser.
Phosphate rock is treated with Nitric or Sulphuric acid to produce soluble salts for use as fertilisers.

Students should be able to:

recall the names of the salts produced when Phosphate rock is treated with Nitric acid, Sulphuric acid and phosphoric acid
compare the industrial production of fertilisers with laboratory preparations of the same compounds, given appropriate information.

What does this mean?

Fertliser

Growth requires Protein - we get this from food

Plants make their own proteins and this takes Nitrogen - so if we add Nitrogen to soil plants grow faster and stronger.

But they can't take it from the air, so they have to take it in via their roots.

Which means the Nitrogen compounds they need have to be able to dissolve in water.

One source of soluble Nitrogen is Ammonia - an alkaline gas.

Dissolved in water it forms Ammonium Hydroxide - but this is very alkaline so we wouldn't want to spray it on plants

NH3 (g) + H2O (l) --> NH4OH (aq)

This can be neutralised by acids to produce Ammonium salts.

Beirut Harbour before and after 3,000 Tonnes of ammonium Nitrate detonated

Fortunately, Ammonium salts are soluble, which means they can be spread on fields and will dissolve in rain water to be absorbed through the roots of plants.

So Ammonium salts make good Nitrogenous fertilisers.

Ammonia can also be made into Nitric Acid.

Neutralising Ammonia solution with Nitric acid will produce a fertiliser with two moles of Nitrogen in each mole of salt.

Ammonium Hydroxide + Nitric Acid --> Ammonium Nitrate + Water

NH4OH (aq) + HNO3 (aq) --> NH4NO3 (aq) + H2O (l)

Ammonium Nitrate makes a good, cheap fertiliser for soils that only lack Nitrogen.

But it can be explosive. So it must be stored with care

NPK Fertilisers

Plants don't only need Nitrogen to grow.

They also need other elements but the reason why is Biology and so of no interest to anyone

Like any fertilisers, NPK Fertlisers contain Nitrogen but they also contain Phosphorus and Potassium (K on the Periodic Table).

NPK fertilisers contain different % of each element depending on the need of the farmer - in other words, a formulation (Year 9)

Potassium Chloride and Potassium Sulphate can be mined in some parts of the world and simply blended with Ammonium salts.

You don't need to learn how they process Potassium Salts first

But Phosphate rock are insoluble and have to be processed first - and you do need to learn this.

Generally, it is crushed and reacted with Acids

Try to learn the box to the right

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2017

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