7.3 - Fertilisers & Their Production

Why crops have to be fed, and the three nutrients

• Growing crops take nutrients from the soil as they grow.
• Harvesting removes these nutrients from the field instead of letting them decompose back into the soil.
• Over time, the soil becomes depleted, which slows plant growth and reduces crop yield.

What a fertiliser does

• A fertiliser is a substance added to soil to replace missing nutrients or supply more than the soil naturally contains.
• Replacing nutrients allows the same land to be cropped repeatedly and increases crop yields.

Why fertilisers matter

• The world's population is rising, but agricultural land is limited, so crop yield per hectare must increase.
• Most manufactured fertilisers are made using ammonia.

The three elements a plant needs most

• Plants need three main elements in large amounts: nitrogen (N), phosphorus (P), and potassium (K).
• If crops lack any of these, growth slows and yields fall.

The form they must be supplied in

Plants cannot absorb these elements directly; they must be dissolved in soil water and taken up through the roots as soluble compounds:

• Nitrogen - Ammonium salts and nitrates
• Phosphorus - Soluble phosphates
• Potassium - Soluble potassium salts

NPK fertilisers

• An NPK fertiliser is a formulation supplying nitrogen, phosphorus, and potassium.
• It's a mixture of salts, not a single compound.
• The proportions of N, P, and K are varied to suit different crops and soil types.

Making a fertiliser salt, and the Ostwald process

• Ammonia dissolves in water to form an alkaline solution.
• Reacting ammonia with an acid neutralises it, forming an ammonium salt:
  • Ammonia + acid → ammonium salt
• The salt contains the ammonium ion (NH₄⁺).
• The acid used determines which salt is made.

Nitric acid gives ammonium nitrate

Ammonia + nitric acid → ammonium nitrate

NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq)

Ammonium nitrate contains nitrogen from both the ammonia and the nitric acid, so it delivers a high percentage of nitrogen by mass.

Sulfuric acid gives ammonium sulfate

Ammonia + sulfuric acid → ammonium sulfate

2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)

Two ammonia molecules react per molecule of sulfuric acid because sulfuric acid provides two hydrogen ions.

Why ammonium salts work as fertilisers

Ammonium salts are very soluble in water, so they dissolve in soil water and are easily absorbed by plant roots.

Nitric acid with other bases

Nitric acid can be neutralised with other bases to make fertilisers, such as reacting with potassium hydroxide to form potassium nitrate.

The Ostwald process

Nitric acid is manufactured from ammonia by the Ostwald process in three stages:

1. Ammonia and air are passed over a hot platinum catalyst to react and produce nitrogen monoxide and water.
2. Nitrogen monoxide reacts with more oxygen to form nitrogen dioxide.
3. Nitrogen dioxide is dissolved in water to produce nitric acid.

Notes on the process:

• The platinum acts as a catalyst, speeding up the first stage without being used up.
• Ammonia is therefore the starting material for both ions in ammonium nitrate.