Permeability of soils
Soil aeration maintains oxygen levels in the plants root zone, needed for microbial and root respiration, and important to plant growth. Additionally, oxygen levels regulate soil temperatures and play a role in some chemical processes that support the oxidation of elements like Mn2+ and Fe2+ that can be toxic.
There is great variability in the composition of soil air as plants consume gases and microbial processes release others. Soil air is relatively moist compared with atmospheric air, and CO2 concentrations tend to be higher, while O2 is usually quite a bit lower. O2 levels are higher in well-aerated soils, which also have higher levels of CH4 and N2O than atmospheric air.
It was studied by Allen Hazen that the coefficient of permeability (k) of a soil is directly proportional to the square of the particle size (D). Thus permeability of coarse grained soil is very large as compared to that of fine grained soil. The permeability of coarse sand may be more than one million times as much that of clay.
Impurities in waterEdit
the presence of impurities in soil decreases the permeability of soil
Void ratio (e)Edit
The coefficient of permeability varies with the void ratio as e/sup>/(1+e). For a given soil, the greater the void ratio, the higher the value of the coefficient of permeability. Here 'e' is the void ratio.
Based on other concepts it has been established that the permeability of a soil varies as e2 or e2/(1+e). Whatever may be the exact relationship, all soils have e versus log k plot as a straight line.
Degree of SaturationEdit
If the soil is not fully saturated, it contains air pockets. The permeability is reduced due to the presence of air which causes a blockage to the passage of water . Consequently, the permeability of a partially saturated soil is considerably smaller than that of fully saturated soil. In fact, Darcy's Law is not strictly applicable to such soils.
Fine grained soils have a layer of adsorbed water strongly attached to their surface. This adsorbed layer is not free to move under gravity. It causes an obstruction to the flow of water in the pores and hence reduces the permeability of soils. According to Casagrande, it may be taken as the void ratio occupied by absorbed water and the permeability may be roughly assumed to be proportional to the square of the net voids ratio of (e - 0.1)
Entrapped air and organic matterEdit
Air entrapped in the soil and organic matter block the passage of water through soil, hence permeability considerably decreases. In permeability tests, the sample of soil used should be fully saturated to avoid errors.
Coefficient of permeability
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