A number of factors affect the permeability of soils, from particle size, impurities in the water,

void ratio The void ratio of a mixture is the ratio of the volume of voids to volume of solids. It is a dimensionless quantity in materials science, and is closely related to porosity as follows: :e = \frac = \frac = \frac and :\phi = \frac = \frac = \f ...

, the degree of saturation, and

adsorbed Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...

water, to entrapped air and organic material.

Background

Soil aeration maintains

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

levels in the plants' root zone, needed for microbial and root respiration, and important to

plant growth Important structures in plant development are buds, shoots, roots, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plant a ...

. Additionally, oxygen levels regulate

soil temperature Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former t ...

s and play a role in some chemical processes that support the

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

of elements like Mn²⁺ and Fe²⁺ that can be toxic.Encyclopedia of Soil Science
/ref>

Determination of the permeability coefficient

#Laboratory experiments: ##Constant Head Permeability Test, ## Low-level permeability test, ##Horizontal permeability test. # Field experiments: ##Free aquifer, ##Pressured aquifer.

Composition

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.

Particle size

It was studied by

Allen Hazen Allen Hazen (August 28, 1869 – July 26, 1930) was an expert in hydraulics, flood control, water purification and sewage treatment. His career extended from 1888 to 1930 and he is, perhaps, best known for his contributions to hydraulics with the ...

that the coefficient of permeability (k) of a

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...

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 soil

The presence of fine particulate impurities in a soil can decrease its permeability by progressive clogging of its porosity.

Void ratio (e)

The coefficient of permeability varies with the

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 ''e''^''2'' or ''e''^''3''/(1+e). Whatever may be the exact relationship, all soils have e versus log k plot as a straight line.

Degree of saturation

If the

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 Darcy's law is an equation that describes the flow of a fluid through a porous medium. The law was formulated by Henry Darcy based on results of experiments on the flow of water through beds of sand, forming the basis of hydrogeology, a branch of e ...

is not strictly applicable to such soils.

Absorbed water

Fine grained soils have a layer of

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 Casagrande is a surname. Notable people with the surname include: * Alan Casagrande de Moura (born 1987), Brazilian footballer *Arthur Casagrande (1902–1981), Austrian-American civil engineer * Caroline Casagrande (born 1976), American politicia ...

, 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 matter

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.

References

{{reflist Soil science Pedology Soil mechanics