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Grade 10 Intermediate About 20 min Version 1 Awaiting review Premium

Fertilisers, pesticides and the river

Plants need nitrogen to grow, so farmers add fertilisers: chemical ones such as urea, or organic ones such as compost and farmyard manure. The first kilograms of nitrogen raise the harvest a lot, but each extra kilogram helps less. Nitrogen the crop cannot use washes off in heavy rain into streams and rivers, where it feeds algae; the algae block light and use up oxygen when they rot, so fish die (eutrophication). Compost releases nitrogen slowly and feeds soil life. Pesticides kill pests, but they can also kill bees, earthworms and fish.

What you will learn

  • Interpret data on how fertiliser affects crop yield.
  • Explain how fertiliser runoff causes eutrophication.
  • Weigh up the benefits and harms of chemical fertilisers, compost and pesticides.

Before you start

  • Know that plants need mineral nutrients such as nitrogen.

Materials

  • Data from field trials (simulated)

Equipment

  • None

Procedure

  1. Record the harvest with no fertiliser, then with 80 and 200 kg of nitrogen from urea.
  2. Record 200 kg of urea nitrogen in a heavy monsoon and look at the river.
  3. Use compost for the same nitrogen and compare.
  4. Spray pesticide and record what happens to the bees and earthworms.

How this simulation is simplified

  • Yield = 2 + 4 × (1 − e^(−usable nitrogen ÷ 40)) tonnes per hectare; a simplified model based on typical rice trials.
  • Urea: 50% of the nitrogen is taken up, 30% (normal rain) or 50% (heavy monsoon) washes off. Compost: 40% taken up, 10% or 20% washes off.
  • Pesticide raises the yield by 8% by saving the crop from pests, but harms bees and earthworms.