Soil Air

• Pore space and porosity

  Total volume of soil occupied by air and water
• Pore spaces

  • Allow water to move through the soil
  • Determine the amount of water a soil can hold
• Clay soil


  Can hold more water than sandy soil
• Clay soil

  Has low permeability
• Sandy soil


  Water drains faster than in clay soil
• Macro-pores

  • Large pores with bigger diameter
  • Drain freely under the influence of gravity
  • Allow easy movement of water and air
• Importance of macro-pores

  • Influence water air exchange
  • Stimulate plant roots penetration
  • Provide habitat for microorganism
• Micro-pores

  • Small pores with small diameter
  • Water will remain in these pores (unless suction forces occur)
  • Have high water retention capacity
• Bulk density

  Mass per unit volume of soil
• Calculating bulk density

  Bulk density = Mass of dry soil (g) / Volume of dry soil (cm3)
• Low bulk density

  • Indicates well drained
  • Suitable for cultivation
• High bulk density


  • Indicates compacted soil
  • Not suitable for cultivation
• Factors influencing bulk density of soil

  • Size of particle (soil texture)
  • Degree of compaction
  • Amount of organic matter
• Soil air

  • Volume of soil (pores) not occupied by liquids
  • Differs from atmospheric air (more water vapour and lower oxygen concentration)
• All living organisms need to get access to air to survive
• In the soil, living organisms must also get access to oxygen to survive
• Aquatic organisms must extract oxygen for them to survive
• Factors influencing the movement of soil air

  • Soil depth
  • Pore size
  • Aeration
  • Drainage
• Oxygen is replaced by carbon dioxide and used by roots and soil micro-organisms during respiration, lowering oxygen concentration
• Carbon dioxide is released by soil micro-organisms and roots, increasing its concentration in soil air
• Factors affecting the composition of soil air

  • Temperature
  • Microbial activity
  • Type of crop
  • Plant roots
• importance of oxygen

  1. Needed for respiration of plant roots and micro-organism
  2. .Needed for oxidation of minerals which contain iron
  3. Play a role in decaying of plant remains or organic matter
• importance of carbon dioxide

  1. It combines with water to form carbonic acid.
  2. Help to dissolve rocks and minerals to form carbonic acid stimulate weathering of rock to form soil
  3. It releases plant nutrients in a soluble form
  4. React with inaccessible compound to make them accessible to plant root
• importance of nitrogen

  1. Dilutes oxygen in the soil
  2. It slows down oxidation
  3. It is utilised by nitrogen-fixing bacteria in soil to make microbial protein.