Fundamentals of Soil Science (1+1)

Soil Porosity

Soil Porosity

• The pore space of a soil is defined as the portion of the soil volume occupied by air and water.
• It refers to the percentage of soil volume occupied by pore spaces.
• Pore-spaces directly control the amount of water and air in the soil and indirectly influence the plant growth and crop production.
• Size of individual pores, rather than total pore space in a soil, is more significant in its plant growth relationship.
• In general there are broadly two types of pores in soil.

• Macro pores:
• The diameter of these pores is larger than 0.06 mm. Macro-pores allow air and water movement readily. Sands and sandy soils have a large number of macro-pores.

• Micro or Capillary pores:
• The diameter of these pores is less than 0.06 mm. The movement of air and water is restricted to some extent in micro or capillary pores. Clays and clayey soils have a greater number of micro or capillary pores. It has got more importance in the plant growth relationship.
  

• For optimum growth of the plant, the existence of approximately equal amount of macro and micro-pores which influence aeration, permeability, drainage and water retention favorably is essential.

Factors Affecting Porosity of Soil

Soil structure:

• Particle arrangement determines the total pore space in the soil. When the sphere like particles are arranged in columnar form (i.e. one after another on the surface forming column like shape) it gives the most open packing system resulting very low amount of pore spaces.
• When such particles are arranged in the pyramidal form it gives the closest packing system resulting high amount of pore spaces. Thus, a soil having granular and crumb structure contains more pore spaces than that of prismatic and platy soil structure.
• The well aggregated soil structure has greater pore space as compared to structure-less or single grain soil.

Soil texture:

• Sandy soils have lesser total pore space and fine textured clay and clay loam soils have higher total pore space.

Organic matter:

• Addition of organic matter increases the volume of pore space by lowering the bulk density. It makes the soil more porous.

Compaction:

• If the soil is made more compact by movement of machinery and tillage implements and thus pore space is reduced.
• Intensive crop cultivation tends to lower the porosity of soil as compared to fallow soils.

Example: Find out the per cent pre space of a silt loam soil having bulk density (p_b) of 1.30 and particle density (p_s) of 2.65.

Solution: