Composition of Soils (Cambridge International IGCSE Environmental Management)

Mineral particles in soil

• Mineral particles are small pieces of weathered rock

  • Soils contain a mixture of sand, silt and clay

• Sand—largest particles

  • Feels gritty and allows fast drainage of water

• Silt—medium-sized particles

  • Feels smooth and holds water better than sand

• Clay—smallest particles

  • Feels sticky when wet and holds lots of water and nutrients

Organic content in soil

• Organic content refers to living and once-living material in the soil

• Includes living organisms such as:

  • Plants and plant roots
  • Animals like insects and worms
  • Fungi that break down dead material
  • Bacteria that help with decomposition and nutrient cycling

• Also includes organic matter from decomposition

  • Dead plants and animals break down to form humus

• Organic content helps soil hold nutrients and water

Gases & water in soil

Gases in soil

• Soil contains air spaces filled with gases
• These air spaces provide oxygen for roots and soil organisms
• They also contain carbon dioxide released by respiration of organisms
• Good aeration helps maintain healthy biological activity in the soil

Water in soil

• Soil holds water between particles and in tiny pores

• Plants absorb this water through their roots

• Water carries dissolved nutrients like nitrates and phosphates

• The amount of water held depends on soil texture

  • Clay holds more water; sand holds less

Exam Hint Don’t forget gases when describing soil composition—many students leave this out, but soil must contain air spaces for plant roots to access oxygen for respiration.