After reading this article you will learn about:- 1. Meaning of Porosity of Soil 2. Types of Pores in Soil 3. Factors Affecting Porosity of Soil.
Meaning of Porosity of Soil:
Pore-spaces (also called voids) in a soil consist of that portion of the soil volume not occupied by solids, either mineral or organic. The pore-space under field conditions, are occupied at all times by air and water. Pore-spaces directly control the amount of water and air in the soil and indirectly influence the plant growth and crop production.
Types of Pores in Soil:
In general there are broadly two types of pores in soils:
(i) Macro-pores and
(ii) Micro or capillary pores.
(i) Macro Pores:
Large-sized pores are referred to as macro-pores which allow air and water movement easily. Sands and sandy soils have a large number of macro-pores. It is found in between the granules.
(ii) Micro or Capillary Pores:
Smaller sized pores are generally referred to as a micro or capillary pores in which movement of air and water is restricted to some extent. Clays and clayey soils have a greater number of micro or capillary pores. It has got more important in the plant growth relationship. It is found within the granules.
Besides soil pores have been divided into following four categories based on the size groupings of soil separates:
1. Coarse Pores:
Greater than 0.2 mm or 200 microns (0.008 inch) average diameter.1 micron = 1 millionth of a metre.
2. Size of Medium Sands:
a. Medium Pores:
0.2-0.02 mm or 200-20 microns (0.008-0.0008 inch) average diameter.Size of coarse silt particles.
b. Fine Pores:
0.02-0.002 mm, which is 20-2 microns (0.0008 inch) average diameter?Size of fine silt particles.
c. Very Fine Pores:
Less than 2 microns (0.00008 inch) average diameter (Large clay particles are 2 microns in average diameter). Size of large clay particles. Porosity refers to the percentage of soil volume occupied by pore spaces. Size of individual pores, rather than total pore space in a soil, is more significant in its 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. Porosity of a soil can be easily changed.
Factors Affecting Porosity of Soil:
Wide difference in the total pore space of various soils occurs depending upon the following several factors:
(i) Soil Structure:
A soil having granular and crumb structure contains more pore spaces than that of prismatic and platy soil structure. So well aggregated soil structure has greater pore space as compared to structure less or single grain soil.
(ii) Soil Texture:
In sandy soils the total pore space is small whereas in fine textured clay and clayey loam soils total pore space is high and there is a possibility of more granulation in clay soils.
(iii) Arrangement of Soil Particles:
When the sphere like particles is arrangement 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 most close packing system resulting high amount of pore spaces.
(iv) Organic Matter:
Soil containing high organic matter possesses high porosity because of well aggregate formation.
Macro-organisms like earthworm, rodents, and insects etc. increase macro-pores in the soil.
(vi) Depth of Soil:
With the increase in depth of soil, the porosity will decrease because of compactness in the sub-soil.
Intensive crop cultivation tends to lower the porosity of soil as compared to fallow soils. The decrease in porosity may be due to reduction in organic matter content.
Due to puddling under sufficient soil moisture, the soil surface layer is made dense and compact. Eventually, the porosity of this surface soil is reduced by the infiltration of muddy surface materials.