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Soil

Radiation and the atom · level 1, lesson 5 of 6

A question to start

Dig up a handful of soil from a garden. It looks like plain brown dirt. What is it made of?

The idea

Soil is a mixture. It has:

  • tiny bits of rock, worn down over a very long time by rain, wind and ice, from big pebbles down to fine dust;
  • bits of old plants and animals, rotting away, which make it dark;
  • water and air in the gaps between the bits;
  • living things: worms, insects and huge numbers of tiny living things too small to see.

You can see the parts by shaking soil in a jar of water. Stones sink first, then sand, then fine mud. Bits of old plants float on top.

bits of plantswaterfine mudsandstones

Worked examples

1. The jar test. Big, heavy bits settle first, so stones end up at the bottom and fine mud on top of the sand.

2. Dark soil. Soil with lots of rotted plants is dark and good for growing.

3. Sandy soil. Soil near a beach is mostly sand. Water drains through it fast.

Try it

  1. Name three things soil is made of.
  2. Where do the bits of rock in soil come from?
  3. In the jar test, what settles first?
  4. What makes some soil dark?
Answers
  1. Any three: bits of rock, bits of old plants and animals, water, air, living things.
  2. Bigger rocks worn down by rain, wind and ice.
  3. The stones, as they are the biggest and heaviest.
  4. Lots of rotted plants and animals in it.

Common mistake

"Soil is just dirt." Soil is a busy mixture of rock, water, air and living things. It takes hundreds of years to make a few centimetres of it.

Where it leads