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Grade 10 Beginner About 15 min Version 1 Awaiting review Premium

Soil erosion and how to prevent it

Soil erosion is the removal of the fertile top layer of soil by water or wind. In Nepal's hills, monsoon rain running down steep slopes can carry away a lot of soil, causing landslides, silting up rivers and leaving fields poor. The faster the water runs, the more soil it carries, so steep and bare slopes lose the most. Plant roots bind the soil and leaves break the fall of raindrops. Mulch (a layer of straw or leaves) protects the surface. Terraces, the steps cut into hillsides that you see all over Nepal, turn one steep slope into many flat ones so the water slows down and soaks in.

What you will learn

  • Measure how slope and rainfall affect soil erosion.
  • Compare methods of soil conservation.
  • Explain why terraces and plant cover are used in Nepal's hills.

Before you start

  • Know that soil is a natural resource that forms very slowly.

Materials

  • Garden soil, grass turf, straw, water

Equipment

  • Shallow trays, wooden blocks, protractor, watering can, beakers, filter paper, balance

Procedure

  1. Pour 40 mm of rain onto a tray of bare soil at 10°, 20° and 30°. Collect and weigh the soil washed off each time.
  2. At 20° with 40 mm of rain, repeat with grass, straw mulch and terraces.
  3. Repeat the bare tray at 20° with only 10 mm of rain.

How this simulation is simplified

  • On bare soil the soil washed away is 0.2 g for every millimetre of rain and every degree of slope. Grass leaves 5% of that, mulch 15% and terraces 30%.
  • Of 10 mL of rain per millimetre falling on the tray, 60% runs off bare soil, 30% off terraces, 25% off mulch and 20% off grass.