Soil is composed of minerals, water, air and organic matter. It has distinct horizontal layers called horizons, including the O horizon of partially decomposed organic matter, the A horizon or topsoil containing humus and living creatures, the B horizon or subsoil rich in clay and minerals but also supporting large plant roots, and the C and D horizons of increasingly less weathered parent material and bedrock.

1. Soil profile

2. Soil
• Soil is a combination of minerals, water, air and organic matter. On average
there is
• 45% mineral matter
• 25% water
• 25% air
• 5 % organic matter

3. • Minerals come from rock that have been weathered
• Water and air from the atmosphere
• Organic matter from the biosphere

6. How is soil profile made

8. Horizon O
Partially decomposed organic
matter

9. Horizon A top soil
Contains humus living creatures and mineral matter from rocks

10. Horizon B subsoil
Rich in minerals and clay particles leached down from A. Roots of
large plants are found here

11. Horizon C (partially weathered rock )
Parent material that is being
weathered

12. Horizon D Bedrock
Parent material that has not
been affected by weathering

13. Questions
• Which type of weathering leads to the build up of humus. ?
• Write one characteristic of subsoil that would make it good for plant growth.
• Write characteristics of subsoil that makes it inappropriate for plant growth.