SOIL AERATION BY MEENAKSHI ATTRI

1. Sher-e-Kashmir
University of Agricultural Sciences & Technology of Jammu
Division of Agronomy
Chatha Campus, Jammu - 180009
Incharge : Dr. Peeyush Sharma
Presented by: Meenakshi Attri
J -19-D-363-A
Ph.D. Agronomy
SOIL PHYSICS
SOIL AERATION

2. Soil Aeration
Soil aeration may be defined as the
exchange of carbon dioxide and oxygen
gases between the soil pore space and the
aerial atmosphere.

3. Definition of Soil Aeration:
 Soil aeration is phenomenon of rapid exchange of oxygen and
carbon dioxide between the soil pore space and the atmosphere,
in order to prevent the deficiency of oxygen and/or toxicity of
carbon dioxide in the soil air.
 The well aerated soil contains enough oxygen for respiration of
roots and aerobic microbes and for oxidation reaction to proceed
at optimum rate.
The rate of exchange of gases between soil air and outer
atmosphere {soil aeration}

4. Process of Air Aeration

5. Composition of average soil

6. Gaseous Exchange
The more rapidly roots and microbes use up oxygen and release
carbon dioxide, the greater is the need for exchange of gases b/w
the soil and the atmosphere. Gaseous exchange between
respiring sites (plant roots , microbes etc,) and outer
atmosphere takes place in 2 steps
1. Outer atmosphere and air filled soil pores (gaseous phase)
2. Air filled soil pores and the respiring sites (water film)
 Exchange of gases between soil and atmosphere is achieved through
two
a)Mass flow- gas exchange is due to fluctuations in water content of
soil that force air in and out.
b)Diffusion – gas exchange is by partial pressure.
Soil aeration process is achieved by gaseous
exchange

7. 1. Mass flow:
The mass flow occurs due to total pressure gradient of gas. The pressure gradient
causes movement of entire mass of air from a zone of high pressure to that of low
pressure. Such a flow of air occurs within the soil or from atmospheric air to soil air
or vice-versa.
 When the soil temperature is higher than the atmospheric temperature during mid-
day then the soil gases will expand and move out of the pore space into the
atmosphere.
 When the soil is cooler than the atmosphere during night, then the atmospheric gases
enter the soil. When the atmospheric pressure is high, the atmospheric gases will
enter in to the soil.
2. Diffusion:
The gaseous interchange between the soil and atmosphere takes place by diffusion.
 The partial pressure of O2 is higher in the atmospheres than in soil pore space and the partial
pressure of CO2 is higher in the soil pore space than the atmosphere. However, total pressure
both in the soil and the atmosphere may be the same. Thus O2 will move into the soil and the
CO2 will move out of the soil.
 The renewal of gases by mass flow is less important than the diffusion in determining the total
exchange that occurs between soil and the atmosphere.


8. Characterization of Soil Aeration:
:There are various parameters that can be used for characterizing soil aeration.
1. The volume percentage of soil air or air capacity is that part of the pore space
which is filled with air. This is generally determined by applying the tension
equivalent to a water column of 50 cm to a saturated soil on a tension table.
2. Gaseous composition.
3. The oxygen diffusion rate (ODR). It is determined by using the platinum
microelectrode technique where the diffusing oxygen is allowed to reduce at the
platinum electrode at a given electric potential. The rate of diffusion of oxygen to
the platinum electrode is used as an index of the rate of diffusion of oxygen
through the water film to the roots.
4. Oxidation-reduction potential, indicating the oxidized or reduced condition of the
soil.
5. Composition of the soil for its reduced components.

9. Factors Affecting Soil Aeration
1. Amount of air space:
The top soil contains much more pore spaces than the sub-soil, thus the opportunity for
gaseous exchange is more in the top soil than in sub-soil. Hence the oxygen content of the
top soil is greater that the sub-soil.
 The soil properties such as soiltexture, bulk density and aggregation affect the amount of pore
space and hence the soil aeration.
2. Soil organic matter:
When organic matter is added to the soil it is readily decomposed by the micro-organisms to
liberate the CO2 in soil air. Thus the concentration of both O2 and CO2 are affected by
microbial decomposition of the organic residues.
 Besides, the respiration of higher plants and the micro-organisms around the roots is also a
significant process affecting the soil aeration.
3. Soil moisture:
The macro-pores are filled up with water immediately after heavy rain or irrigation and level
of oxygen content falls to zero.
 When the soil is artificially drained again, the macro-pores are filled up with air and the
oxygen content of the soil increases.
4. Seasonal differences:
There is a considerable seasonal variation in the composition of soil air.
 In the spring time in temperate-humid regions the soils are wet and cold and the gaseous
exchange is poor.
 In summer months, when the soils are dry, the gaseous exchange will increase. This will result
in relatively high content of O2 and low CO2.

11. Composition of Soil Air:
 The soil air contains a number of gases of which nitrogen, oxygen carbon dioxide
and water vapour are the most important. Soil air constantly move from the soil pores
into the atmosphere and form the atmosphere into the pore space.
 Although soil air and atmospheric air differ in the compositions, soil air contains a
much greater proportion of carbon dioxide and a lesser amount of oxygen than
atmospheric air. At the same time, soil air contains a far greater amount of water
vapour than atmospheric air. The amount of nitrogen in soil air is almost the same as
in the atmosphere

12. Factors affecting Composition of soil air
 Season
 Soil fertilty
 Soil water content
 Soil texture
 Soil aggregation
 Soil depth

13. Factors Affecting the Composition of Soil Air:
The composition of soil air is influenced by a number of factors such as nature
of soil, soil condition, type of crop, microbial activity, season etc.
Oxygen:
 The quantity of oxygen in soil air is less than that in atmospheric air. Plant
roots and various microorganism require oxygen which they take from the
soil air, thus, depleting the concentration of oxygen in the soil air.
 The amount of oxygen also depends upon the soil depth. The oxygen
content of the air in lower layer is usually less than that of the surface soil.
This is possibly due to the more readily diffusion of oxygen from the
atmosphere into the surface soil than in the sub-soil.
 The quantity of oxygen is usually higher in dry season than during the
monsoon. Because soils are normally drier during the summer months,
opportunity for gaseous exchange is greater during this period. This results
in relatively high O2 and low CO2 levels. Light texture soil i.e., sandy soil
contains much higher oxygen percentage than heavy soil.

14. Carbon dioxide:
 Decomposition of organic matter produces CO2. Hence, soils
rich in organic matter contain higher percentage of carbon
dioxide. Production of CO2 is associated with microbial
activity, CO2 increases with the increasing number and activity
of microorganism.
 High temperature during summer season encourages
microorganism activity which results in higher production of
CO2.
 The concentration of CO2 is usually greater in sub-soil
probably due to more sluggish aeration in lower layer than in
the surface soil.
Water vapour:
 Soil air, however, contains much more water vapour than
atmospheric air. Capillary water in the soil is used to saturate
the soil air with vapour.
 During crop growing period, when soil remains moist, the
amount of water vapour in the soil air would be more.

16. Aeration Status of Soils:
It can be determined in the three ways i. e:
(i) Percentage oxygen and carbon dioxide content of the
soil,
(ii) Oxygen diffusion rate, and
(iii) The oxidation reduction potential (Redox potential).

17. (i) Percentage composition of soil air:
 Well aggregated soils contain enough macrospores to keep
the soil aerated for proper growth and functioning of roots
and micro-organisms.
 After a heavy rain, the macrospores are filled up with water
but the soil may still contain some quantity of air dissolved in
water.
 So micro-organisms can grow for a short time only, after
which the soil must be drained so that the macrospores are
re-filled with air.

18. ii) Oxygen Diffusion Rate (ODR):
 It determines the rate at which the oxygen should be supplied to
the soil when it is being continuously used for the respiration of
roots and soil micro-organisms.
 The growth of roots of most crops ceases when the oxygen
diffusion rate decreases to about 20 x 10-8 gm./sq. cm/min. It
should be above 40 x 10-8 gm./sq.cm/min. for good growth of
most crops.
(iii) Oxidation-reduction potential of soil.
 The oxidation potential of chemical systems including soil is a
measure of the tendency of the oxidation reaction to occur in
that system, including soils.
 Highly reduced soils have a high oxidation potential of +0.50
volts.
 So a highly reduced soil which has a tendency to be oxidized has
a Reduction Potential or Redox Potential Eh of- 0.50 volts.
 Well drained and aerated soils which are highly oxidized usually
have a redox potential of +0.50 volts.
 The value of the Redox Potential increases when the oxygen
content of soils decreases.

19. Redox reaction
sequence of reductions that take place when well aerated soil
becomes saturated with water
Once oxygen is gone, the only active microorganisms are those
that can use substances other than oxygen as electron acceptors
(anaerobic)
Eh drops
Shows Eh levels at which these reactions take place
Poorly aerated soil contain partially oxidized products:Ethylene
gas, methane, alcohols, organic acids organic substrate
oxidized (decomposed) by various electron acceptors:
 O2
 NO3
-
 Mn+4
 Fe+3
 SO4
-2
 rates of decomposition are most rapid in presence of
oxygen

20. Oxidized form Reduced form Eh (v)
O2 H2O .38 - .32
NO3
-1 N2 .28 - .22
Mn+4 Mn+2 .22 - .18
Fe+3 Fe+2 .11 - .08
SO4
-2 S-2 -.14 - -.17
CO2 CH4 -.2 - -.28

21. Oxygen requirements of plants
 Plant require some minimum amount of oxygen for
their optimum growth and yield and the requirement
varies with the plant species.
 Foroptimum plant growth and yield soil aertaion must
be adequate
 The critical aeration below which plant growth is
adversely affected is atleast 10% air filled porosity or
10% oxygen concentration of soil air or 30×10-8 g/cm2-
min oxygen diffusion rate in the root zone.

22. Importance of soil air
It is used for the respiration by the root
Decomposition of the organic matter by
the microorganisms.

23. Importance of Soil Aeration:
Soil aeration affects the availability of some nutrients elements to plant roots.
Manganese and iron occurs in the well aerated soil in their higher valent forms
(Mn++++, Mn+++, Fe+++) and in poorly aerated soils in their lower valent forms (Mn++,
Fe++). They are available to plants only in their lower valent forms.
Ferric phosphate would be reduced to ferrous phosphate. Carbon dioxide produced
from the decomposition of organic matter reacts with water to form carbonic acid which
slowly dissolves insoluble phosphate. So the availability of phosphates (would be
increased to the plant roots.
Sulphur occurs as sulphate in well aerated soil. Plant roots assimilate sulphate. Sulphate
is reduced to sulphide in poorly aerated (water logged) soils. Hydrogen sulphide is toxic
to plant a root which suffers from it in water logged soil.
Organic matter is decomposed by aerobic bacteria in well aerated soil when complex
organic nitrogen and phosphorus compounds are decomposed to their respective simple
inorganic compounds which plant roots readily assimilate symbiotic and non-symbiotic
nitrogen fixation takes place only in well aerated soils.

24.  Nutrient absorption is an energy consuming process. Energy is available from
respiration is expended in absorbing nutrient ions from the soil. Hence nutrient
absorption is retarded in poorly aerated soils.
 If an excessive amount of readily decomposable organic matter has been added to
the soil, then it would decompose to evolve high amounts of carbon dioxide to the
soil. Consequently root growth and germination of seeds would be adversely
affected.
 Some crops become infested with pathogens in poorly aerated soils. The incidence
of will disease caused by the fungus (Fusarium sp) has been attributed to poor
aeration. Citrus and suffers from die-back in poorly aerated soils have also reviewed
the works of some investigators who have observed that poorly aerated soils
(waterlogged soils) has an effect on the pathogenicity of root infesting fungi.
Nitrates are reduced to oxides of nitrogen and nitrogen gases in poorly aerated
soils. These gases escape to the atmosphere, long light coloured roots develop in
well aerated soils.
Root hairs develop best under well aerated condition.

25. Some conclusions about aeration:
1. Forms/mobility
2. Roots
3. Decomposition
1. Forms and Mobility
Soil aeration determines which forms of chemicals are present and how mobile they are
 Redox colors in Poorly and Well-Aerated Soil
 Nutrient elements
A) Poorly aerated soils
reduced forms of iron and manganese Fe+2, Mn+2
Reduced iron is soluble; moves through soil, removing red, leaving gray, low chroma colors
(redox depletions)
B) Well-aerated soils:
Oxidized forms of iron and manganese Fe+3 Mn+4
Fe precipitates as Fe+3 in aerobic zones or during dry periods
Reddish brown to orange (redox concentrations)

26. C. Nutrient Elements
 Plants can use oxidized forms of nitrogen and sulfur
 Reduced iron, manganese
 Soluble/”good” in alkaline soils
 More soluble in acid soils; can reach toxic levels
2. Root respiration
Good aeration promotes root respiration
Poor aeration: water-filled pores block oxygen diffusion into soil to replace what is
used up in respiration
3. Decomposition
In aerated soils, aerobic organisms rapidly oxidize organic material and decomposition is
rapid
In poor aeration, anaerobic decomposers take over and decomposition is slower

27. Soil Air in Relation to Soil and the Crop Management:
Soil Management:
 The maintenance of a stable soil structure is an important means of
augmenting good aeration.
 . Maintenance of organic matter by addition of Farm yard Manure and crop
residues and by growth of legumes is perhaps the most practical means of
encouraging aggregate. Stability, which, in turn, encourages good drainage and
better aeration.
 In heavy textured (clay) soil, it is very difficult to maintain optimum aeration.
Aeration in the soil can be added by controlling weeds and tilling the heavy
soil.
 Consequently, no tillage (zero tillage) or minimum tillage practices, which the
quite satisfactory on well-drained soils, have limitations on poorly drained
soils.
Crop management:
 Selection of crop is important criteria for adaption of crop in the soil. Alfalfa,
fruits and forest trees and other deep-rooted plants require deep, well- aerated
soils, such plants are sensitive to a deficiency of oxygen, even in the lower soil
horizon.
 In contrast, shallow-rooted plants, such as grasses, clovers etc. do well on soils
that tend to be poorly aerated, especially in the subsoil. The rice plant
flourishes even when the soil is submerged in water.

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cause of variation. Journal of Agricultural Sciences,7:1-48.
•Verma, S and Sharma, P.K, 2008.Long term effects of organics, fertllizers and cropping
systems on soil physical productivity evaluated using a single value index. Soil and
Tillage Research,98:1-10
•A Textbook of Soil Physics by Dr. PK Sharma
•A Textbook of Soil Physics by Dr. AK Saha and Dr. Anuradha saha
References

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30. Thank you