A farmer grows the same crop in the same field every year without adding fertilisers. After several years, the crop yield decreases significantly. Explain why the crop yield decreases and state two elements that a fertiliser would need to supply to restore the soil.

Cambridge International IGCSE Chemistry (0620) — 10.2 Fertilisers · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The crop yield falls because the plants keep taking up mineral ions from the soil, but the farmer does not add fertilisers, so the nutrients are not replenished. After several years the soil becomes deficient in essential minerals, reducing plant growth and yield.

A fertiliser would need to supply at least two of the main macronutrients: nitrogen and phosphorus (or potassium).

Examiner tips

  • Use the word ‘deficient’ to show understanding of nutrient loss. Mention that plants absorb mineral ions. State two different macronutrients. Keep the answer concise to fit the 4‑mark limit.

Common mistakes

  • Writing only one element or repeating the same element twice. Using vague terms like ‘nutrients’ without naming specific elements. Failing to explain that the loss is due to repeated cropping without replacement.

Mark scheme (4 marks)

  1. Plants absorb / remove mineral ions (nutrients) from the soil as they grow
  2. Repeated cropping means minerals are not replaced / soil becomes deficient in minerals
  3. First correct element named (nitrogen / phosphorus / potassium)
  4. Second correct element named, different from the first (nitrogen / phosphorus / potassium)

Key terms in this question

fertiliser · crop yield

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