The document discusses pores in soil, including: - Pore space varies depending on the relative amounts of water and air, and is influenced by the arrangement of solid particles. - There are three main types of pores - macro (visible), meso (visible at 10x magnification), and micro (not visible) - which influence water movement, aeration, and plant growth. - Factors like soil structure, texture, and particle arrangement impact porosity, the percentage of the soil volume made up of pores.

1. Pore space
• The amounts of water and air present in
pore spaces vary and depend upon their
relative amounts. If the amount of water
is more, air decreases and vice versa.
• The amount of pore space depends
upon the arrangement of solid particles.
• Pore spaces directly control the amount
of water and air in the soil and indirectly
influence the plant growth and crop
production.
• The quantity of pores in a soil and their
size distribution are useful general
indicators of the physical condition of
soils.
• Apart from quantity and distribution, the
tortuosity and continuity of pores are
important features influencing aeration

2. Types of pores
• Macro (Coarse) pores: Macropores have diameters
greater than 100 μm or 0.1 mm, and their main function is
aeration and drainage by gravity flow. They are also the pores in
which root proliferates. They are visible to the naked eye.
• Rapid water movement through soil and aeration at field
capacity takes place through soil pores.
• The coarse porosity are measured by estimating total
porosity of soil at field capacity. Coarse porosity equals
the total porosity less the volumetric water content at
field capacity.
• Additional measurements of hydraulic conductivity are
necessary for characterizing the spatial distribution of
coarse pores as channels for drainage.

3. – Medium (meso) pores: Medium pores have
diameters from 30 to 100 μm and their main function
is conduction of water by rapid capillary flow; they
are visible at times 10 magnification.
•
– Fine (micro) pores: Micro pores have diameters less
than 30 μm and their main function is water
retention and slow capillary flow.
– Micro pores are not visible, but their presence can be
inferred from observation of the face of aggregates:
when the aggregates have a rough surface, there are
many micropores.

4. • In general, soils should contain an adequate
number of pores of > 250 diameter to allow good
root penetration.; at least 10 % by volume of the
soil in the rooting depth should be composed of
interconnected pores > 50 μm in diameter to
allow free drainage.
• At least 10% by volume of the soil should consist
of pores with equivalent cylindrical diameter
between 0.5 and 50 μm to allow for storage of
available water.

5. Porosity and total porosity
• 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 favourably. Porosity in a soil
can be easily changed.
• Total porosity of soil is calculated from the dry bulk density and particle
density as shown below
Dry bulk density
• Total Porosity (% volume) = 1 - --------------------- x 100
Particle density
• Air filled porosity: Pores that are not filled with water are filled with air,
and an estimate of their volume can give an indication of the aeration
and drainage status of a soil. Generally, the higher the air filled porosity,
the better the drainage and aeration of a soil.

6. Factors affecting porosity of soil
• 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 structureless or single grain size.
• Soil texture: In sandy soils, the total pore space is small
whereas in fine textured clay or clayey loam soils total pore
space is high and there is a possibility of more granulation in
clay soils.
• Arrangement of soil particles: When the sphere like particles
are arranged in a 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.