9.4 - Fertilisers & Their Production

Why crops have to be fed

• 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.

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.

What fertilisers do to rivers and lakes

Rain washes unabsorbed fertiliser from fields into streams, rivers, and lakes.

The chain of eutrophication

1. High levels of dissolved nutrients enter the water.
2. Algae take up the nutrients and multiply rapidly, forming an algal bloom on the surface.
3. The algal bloom blocks sunlight, preventing aquatic plants from photosynthesising, causing them to die.
4. Microorganisms feed on the dead plants and multiply rapidly.
5. The microorganisms respire aerobically, using up dissolved oxygen in the water.
6. Dissolved oxygen levels drop so low that fish and other aquatic animals suffocate.

Comparing natural and manufactured fertilisers

• Natural fertilisers (e.g. manure, compost):
  • Bulky and expensive to transport.
  • Low and variable nutrient content.
• Manufactured fertilisers:
  • Known, precise composition and concentration.
  • Can be produced on a large scale.
  • Cause eutrophication if excess runs off into waterways.