5.9 - 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 from ammonia

• 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 is made from ammonia

Nitric acid is made by reacting ammonia with oxygen and water, so ammonia is the starting material for both the ammonium and nitrate ions in ammonium nitrate.

Making fertilisers on an industrial scale

Industrial production of fertilisers differs significantly from laboratory preparation.

Several processes joined together

A fertiliser plant links several processes so the product of one becomes the raw material for the next:

1. Ammonia is manufactured first.
2. The acid is manufactured.
3. The ammonia and acid are reacted in a neutralisation stage.

Why the laboratory method does not scale

• In the laboratory, an ammonium salt is made by titration followed by crystallisation: acid is added from a burette, then the solution is evaporated and left to crystallise.
• This method cannot be scaled up directly:
  • Burettes only handle small volumes.
  • Evaporating and crystallising takes hours.
  • It is a slow batch process with high labour costs.
• An industrial plant carries out the reaction differently. Concentrated acid can be sprayed into ammonia gas, producing solid fertiliser powder immediately without needing an evaporation stage.