Soil Aeration reduces compaction, oxidizes the soil, and allows the roots to take the appropriate nutrients and grow as vigorously as possible. With the help of better mechanization the soil is perforated with small holes to allow air, water and other nutrients to reach deeper. There is no easy simple way to determine whether the soil aeration potential is good or poor. Personal knowledge of the soil and the response of plants grown in it are the best sources of information for the grower. Poor soil aeration improves the environment for plant diseases so that their attacks are much more severe than good soil aeration conditions, so providing better aeration is a good condition for plant development.

2. ACHARYAN G RANGAAGRICULTURAL
UNIVERSITY
S.V.AGRICULTURALCOLLEGE,TIRUPATI
COURSENO : SOILS 501
COURSETITLE : SOIL PHYSICS
TOPIC : Role of SOIL Aeration for crop growth
and its development
SUBMITTEDTO:
Dr. G.P. Leelavathy
Assistant Professor
Department of Soil science and Agricultural
chemistry.
SUBMITTEDBY:
N.RANGASWAMY
TAM/2020-24

3. Flow of presentation
Soil air definition & it's importance
Composition of soil and atm air
Factors affecting the composition of soil air
Process of gaseous exchange
Factors affecting mass flow
Diffusion
Characteristics of soil aeration
Factors affecting soil aeration
Influence of soil aeration on plant growth
Air permeability
ODR( oxygen diffusion rate)
Management of soil aeration
Plant Adaptations-
Root Respiration
Conclusion

4. What is Soil Air?
 The three major components of soils are solid, liquid (water), and gas
(air).
 Soil solids are so arranged that about 50% of soil volume becomes void
or pore space.
 This void space is occupied by soil water and soil air but their proportion
varies
 So, soil air is that part of the soil that is not occupied by soil solids
and soil water.
composition of soil air shouldhave the following characteristics
Generally oxygen content decreases and the carbon dioxide content
increases with the depth of the soil.
 The higher content of carbon dioxide in subsoil than in topsoil is since
aeration of surface soil takes place between topsoil and atmosphere
while that of subsoil takes place between subsurface soil and surface
soil

6.  The carbon dioxide content of the soil shows a marked seasonal
variation, being higher in summer than in winter because of
greater root and microbial activity in summer.
 The oxygen and carbon dioxide contents of soil air depend upon
the application of organic matter, lime, and fertilizer in the soil
and vegetation on the soil.
 Oxygen content is lower and carbon dioxide content is higher in
manured, limed, fertilized and vegetated soils than in
unmanured, unlimited, unfertilized and bare soils because of
greater root and microbial activity in manured and vegetated
soils.

7.  The oxygen and carbon dioxide contents of the soil air depend
upon the porosity of the soil.
 Oxygen content is lower and carbon dioxide content is higher
in fine-textured, poorly aggregated and/or compact soils than
in coarse-textured, well aggregated and/or loose soils because
of the difference in diffusion rate.
• Rainfall has a remarkable influence on the oxygen and carbon
dioxide contents of soil air.
• Oxygen content is lower and carbon dioxide content is higher
in wet soils than in dry soils because of restricted diffusion.
• Thus during the period of rain the soil becomes wet causing
reduction in oxygen content and an increase in carbon dioxide
content of soil air

8. Average composition of atmospheric and well-aeratedsoil air
[NOTE: The content of CO2, in soil air, vary from 10-10,000 times]

9. Factors AffectingtheComposition of Soil Air
Soil texture
The amount of carbon dioxide increases and that of oxygen
decreases with an increase in the fineness of soil texture due to
variations of total porosity, macro pores, and water content of the
soil. For example, clayey soils usually contain more carbon dioxide
and less oxygen than loamy soils, and loamy soils contain more
carbon dioxide and less oxygen than sandy soils.
Soil structure
 Soil structure also affects the composition of soil air, due to
variation of total porosity, macro pores and moisture content of the
soil.

10.  The carbon dioxide content is higher and oxygen content is less
in poorly aggregated soil than in well-aggregated soil or a platy
type of structure than the spheroidal (granular and crumby) type
of structure.
 For example granular soils Contain less than one-half as much
carbon dioxide as powdery soils.
Soil Compaction
As the compactness of a soil increases, the amount of total pore
space especially macro pores decreases causing a decrease in air
capacity and air permeability of the soil.
As a result the carbon dioxide concentration of soil air increases
and oxygen concentration of soil air decreases with an increase in
compactness of the soil.

11. Soil water content
 With the increase in soil water content, air capacity decreases due
to the reduction of pore volume for soil air.
 Air permeability also decreases with an increase in soil water
content due to the blockage of pores through which renewal of
soil air takes place soil.
 Carbon dioxide production increases due to an increase in
biological activity and decomposition of organic matter.
 Because of these, the carbon dioxide content of soil air increases
and the oxygen content of soil air decreases with an increase in
soil water content.
 For example, carbon dioxide content of soil air at 30 °C increases
about 10 times as soil water content changes from 6.8 to 26.8
percent

12. Organicmatter content
 The amount of carbon dioxide in the soil air is primarily
dependent upon the decomposition stage of soil organic
substances and modification of the physical properties such as
soil structure, porosity and soil water content.
 If soil organic matter contains undecomposed organic substances,
initially the carbon dioxide content of soil air increases due to the
microbiological decomposition of organic matter but when the
decomposition of organic substances is almost complete, carbon
dioxide content decreases due to the formation of granular and
crumby structures which are more porous, through which more
gaseous exchange takes place.

13. Soil depth
 Subsoils are usually more deficient in oxygen than surface soils
because of total pore space as well as macro pore space.
 Again aeration of surface soil takes place between surface soil
and atmosphere while that of subsurface soil takes place between
surface soil and subsurface soil.
 As a result the opportunity for gaseous exchange decreases with
increases in soil depth.
 so, carbon dioxide content increases and oxygen content
decreases with an increase in soil depth.
Environmentalcondition
(Rainfall and Temperature) Temperature influences the content
of oxygen and carbon dioxide in the soil air, because of high
temperature, carbon dioxide production is higher in summer than
in winter.

14.  For the same reason, soil air contains a higher proportion of
carbon dioxide under tropical conditions than under temperate
conditions.
 This is because higher temperature encourages biological activity
and increases the rate of organic matter decomposition.
 The concentration of carbon dioxide also generally increases
after rain probably because of increased biological activity and
decomposition of organic matter; reduction of pore volume for
soil air and decrease of air permeability due to blockage of air
passage
 Because of these, for a short period carbon dioxide content is
higher and oxygen content is less in manured field soil than an
unmanured field soil. Hence just after the application of
farmyard or any other organic manures, the oxygen content of
soil air decreases and the carbon dioxide content of soil air
increases

15. Seasonal variation
The carbon dioxide and oxygen content of the soil air varies
considerably from one season to another.
These variations are primarily due to changes in temperature and
soil water content.
The carbon dioxide content of the surface soil layers is lower in
colder months and higher in warmer months because of greater root
and microbial activities and a higher rate of organic matter
decomposition in the carbon dioxide content of soil air is higher and
oxygen content in warmer months, Because of it lower in summer
season than in winter season.

16. Plant root activity
 Roots of growing plants consume oxygen and release carbon
dioxide during respiration and tend to reduce the oxygen content
and increase carbon dioxide content or soil air.
 Because of this soil air of cropped land generally contains much
more (maybe about 1.5 to10 times more) carbon dioxide than soil
air of fallow land and this effect is most noticeable when crops
growing actively
Manuring and microbial activity
 Production of carbon dioxide is associated with microbial activity
which in turn depends on the amount and decomposition stage of
added manures.
 Manuring increases microbial activity causing reduction of oxygen
and increase of carbon dioxide content of soil air due to microbial
respiration and microbiological decomposition of organic matter.

18.  Tillage - The exchange of gases is faster in tilled soils. A
shallow tillage encourages CO2, in the topsoil in comparison to
a deep tillage.
 Puddling required for growing rice decreases the macro pores
and results in poor aeration for succeeding crops like wheat.
Deeptillage Shallowtillage

19.  Khonke (1968) it refers to the "air capacity of the soil.
 The volume of pore space filled with air when the soil is under a
tension of 50 millibar.
 This value is also called "non-capillary porosity’’.
 It corresponds to a pore size of 0.06 mm or larger in diameter
 Oxygen is required for the respiration of plant roots, microbes, and
the soil fauna.
 The CO2, helps in increasing the availability of nutrients to plants.
 The N, serves as a substrate for the production of plant-utilizable
(available) nitrogen by symbiotic and non’ symbiotic bacteria.
 Water vapor prevents the desiccation of soil and helps in the
movement of water within the soil.
Importance of Soil Air

20.  A constant supply of O2, essential, and is concentration should be
at least 10 percent for normal growth of the plants. Lack of O2, is
more injurious to plants than an excess of CO2, within the
reasonable limits (20%).
 An excess of O2, is also undesirable because it oxidises the
organic matter rapidly and dries the soil quickly.

21. Sufficient Oxygen
Brown mushy roots Bunchy white roots
Oxygen deficiency

22. Processof Gaseous Exchange
The exchange of gases between soil and the atmosphere
(renewal of soil air) is a natural process
It involves two mechanisms namely,
 Mass flow
 Diffusion.
 The mass flow of air occurs due to the total pressure gradient of
gases which causes the movement of an entire mass of air from a
region of higher pressure to a region of lower pressure.
 Mass flow of air may occur from atmosphere to soil and from
one location to another in the soil.
 The difference in total pressure may arise due to meteorological
factors such as temperature, pressure, wind and air replacement
due to irrigation.
Mass flow

23. Mass flow: Ions move in the soil
solution to the plant roots as a
function of transpiration of
loss the water from the leaves
Ex:-Ca, Mg and anions
Diffusion: Nutrient ions move from
high to low concentrations
(Concentration gradient)
Ex:- K& P

24.  Diffusion is the molecular transfer of gases.
 The molecules of gases are in a state of movement in all
directions.
 Through this process each gas tends to move in a direction
determined by its partial pressure.
 Diffusion allows extensive movement from one area to another
even if there is no overall pressure gradient.
 Diffusion is the predominant process of soil aeration.
 In diffusion, individual gas constituents move separately due to
partial pressure gradient but the total pressure of air may be the
same.
 When the partial pressure of CO2, in soil air increases due to root
and microbial activities, the CO2, diffuses from the soil to the
atmosphere.
Diffusion

26. Factors affecting the mass flow
 Temperature - As per Charles' law air pressure is directly
proportional to the air temperature at a constant volume.
 So, whenever there is a temperature gradient between two points, a
pressure gradient develops which causes gases to move
 Pressure - As per Boyle's law, the volume of air is inversely
proportional to the pressure at a constant temperature.
 With the increase in pressure in the atmosphere, the volume of
soil air decreases resulting in the movement of air from the
atmosphere to soil.
 As the pressure in the atmosphere decreases, the volume of soil air
increases which causes the movement of soil air to the atmosphere

27. Rainfall or irrigation
 Rainfall and irrigation displace the
soil air as such to the lower depths.
 When water is lost from the soil by
evaporation, plant uptake, or deep
drainage the air moves from the
atmosphere to the soil.
Wind
Pressure and suction effects of high
wind cause the exchange of gases
between the soil and the atmosphere
but the effect is restricted to surface
soil only

28. Similarly, when O2, in soil air is consumed for respiration, its partial
pressure is reduced and the O2, diffuses into the soil.
Diffusion increases with an increase in temperature.
 Soil aeration status may be characterized by determining the
concentration of O2, and CO2, in soil air using chemical or
gas chromatographic methods.
 The air-filled porosity and air permeability can also be used
to determine the aeration status of soil.
 Other methods include the determination of the diffusion
coefficient of gases and the redox potential of the soil.
 However, the best method for characterization of aeration
status is to measure the oxygen diffusion rate (ODR) in the
soil.
Characterization of Soil Aeration Status

29. Factors AffectingSoil Aeration
Soil water content :
 Immediately after heavy rain or irrigation both macro and micro
pores are almost filled with water and removal of excess water
from the soil through drainage and/or evaporation determines
the aeration status of the soil.
 Air can enter the soil pores only when some of the water is
removed.
 Diffusion of any gas through water-filled pores is much less
than through air-filled pores.
 Therefore, soil aeration decreases with an increase in soil water
content due to a reduction in pore volume for soil air and
blockage of pores through which renewal of soil air takes place.

30. Macro porosity of the soil
 The volume of macro pores of soil is the most important factor
influencing soil aeration.
 Macro pores affect the total air space and gaseous exchange
between soil air and atmospheric air.
 Soil texture, soil structure especially aggregate stability, and soil
compaction i.e. bulk density determine the macro pore space and
in turn soil aeration.
 Soil aeration i.e. gaseous exchange between soil air and
atmospheric air is more in coarse-textured soil than fine-textured
soil, in spheroidal (granular and crumby) type structure than
platy structure, in well-aggregated soil than poorly aggregated
soil, in less compacted soil than highly compacted soil and in
soils having high humus content than in soils having low humus
content because of increase in aeration porosity

31. Soil heterogeneity
 Any impeding soil layer such as a heavy textured or compacted
soil layer or the presence of any pan in a soil profile limits ready
air exchange between soil air and atmospheric air which in turn
decreases soil aeration.
Tillage
 Tillage generally increases air-filled porosity causing an increase
in soil aeration i.e. the rate of gas diffusion in the topsoil for a
short period after ploughing. Deep ploughing and mould board
ploughing to normal depth generally yield the highest rates of
gas diffusion.
 Tillage with heavy implements also may form a compact layer
below the ploughing depth.

32. Drainage
Drainage of waterlogged lands or uplands having high water table
improve soil aeration.
Improvement of drainage conditions in the soil increases the soil’s
oxygen status and reduces denitrification losses.
So, air permeability is greater in general in ploughed topsoil for a
short period than in the unploughed topsoil but similar or less
after a long period of ploughing in the ploughed topsoil than in
unploughed topsoil and in soil layer below the ploughing depth in
the ploughed field than in the unploughed field,

33. Influence of Aeration on Plant Growth
 Different types of plants prefer to grow in different levels of soil
aeration.
 Hydrophytes grow well with continuous flooding but
mesophytes and xerophytes normally grow better in well-aerated
conditions.
The effect of aeration on the growth of upland higher plants can be
divided into two categories:
 The effect on the soil constituents which in turn influences growth
 The direct effect on the physiological condition of plants.

34. The effect of aeration on the soil constituents
 Oxygen content of soil air determines the nature as well as the rate
of microbial activity in the soil.
 When oxygen is present in adequate amounts aerobic organisms
become active and oxidation reactions take place, If sugar is
considered as an example of an organic compound, the following
oxidation reaction occurs :
(Sugar) C6H12O6 + 602------- 6CO2 + 6H2O
 The presence of adequate oxygen in the soil is conducive to the
production of highly oxidized soil constituents, which usually
provide a physical and chemical environment suitable for good
plant growth.
 The oxidized states of nitrogen and sulfur are readily utilized by
higher plants.

35. The carbon dioxide produced by biological activity and water form
carbonic acid which increases the solubility of many soil minerals
for which plant nutrition and fertility status of soil is increased.
 Carbon dioxide dissolved water usually increases the solubility of
natural phosphate minerals in soil.
 the minerals are converted to various forms of calcium phosphate
by the action of carbon dioxide.
 In the absence of adequate oxygen in the soil, anaerobic
organisms become active, and organic matter is reduced to
alcohols, aldehydes, or methane.
 The reduced reactions occur at a much slower rate one example of
which is stated below :
C6H1206 ------3CO2 + 3CH4

36.  So under anaerobic conditions, biological reactions that take
place in soil reduce the oxygen of many soil constituents,
thereby creating chemical conditions that are not favorable to
the growth of most higher plants.
Air Permeability
 Air permeability of soil is a parameter that indicates its
readiness to transmit gases when a difference in pressure builds
up.
 The percentage, size, and continuity of pores within the soil and
occupied by the air determine the permeability of the soil.
 Special instruments like an air Permeameter" and a gasometer"
are used to determine the air permeability.

37. ODR(OxygenDiffusionRate)
 The rate at which O2, is replenished when it is used by plant
roots or microorganisms.
 The ODR meter is used to measure ODR (Platinum
microelectrode technique).
 The critical value of ODR of soils is 20x10-8g/cm2/min below
which the growth of roots of most plants ceases.

38. Scheme of soil oxygen dynamics. J is gas density flux, c is gas concentration (g m -
3), t is time, x is distance, Sm is microorganism respiration, Sp is plant
respiration, and Sr is redox reactions in the soil.

39.  The optimum ODR range for most crops lies between 30x10-8 to
40x10-8g/cm²/min.
 The ODR decreases with moisture and depth of soil.
 A soil condition where the ODR is at least 30x10-8 g/cm²/min and
O2, the concentration of the soil air is at least 10% in the root zone
is considered as having “adequate aeration"

40. Management of Soil Aeration
 The soil aeration can be optimized by managing soil structure, soil
temperature, proper drainage, tillage, plant adaptation and
regulation of plant roots.
Soil Structure
 Soil structure may be improved by the addition of farmyard
manure, green manuring, crop residues, and the growing of
legume crops to increase the volume of macro pores.
 If more macro pores are present in the soil which condition
facilitates good aeration in the soil.
Drainage
 The drainage will decrease the moisture content and provide an
aerobic environment in the soil.

41. Cultivation
 A shallow cultivation of the soil and inter-culture operations
control weeds and help in the exchange of gases, especially in
poorly drained heavy-textured soils
 After rain, if crust forms it may hinder the gaseous exchange, a
light cultivation will break it and help in improving soil
aeration.
Temperature
 An increase in soil temperature increases the oxygen diffusion
rates, enhances microbial activity, and raises CO2, production in
the soil.
 The net result of the increase in soil temperature on the partial
pressure of O2, may, therefore, be either positive or negative.

42.  In such situations mulching plays an important role as mulch
protects the soil from the impact of raindrops and therefore,
helps in retaining the tilth.
 Thus mulching facilitates aeration but it also keeps the soil
moist and restricts the soil aeration.
 The positive or negative effect of mulching on oxygen
diffusion rate depends on the soil’s situation.
 However, it has been observed that mulching usually decreases
the O2, diffusion into the soil.
Plant Adaptations
 Plant roots, in general, are adapted to aerobic conditions.
 However, some of the plant species develop mechanisms such
as an increase in the air space of roots (more porosity) or
internal aeration through leaves and cortex cells and grow well
even in oxygen-deficient soil.

43. Submerged Roots

44. [NOTE: Rice thrives well in submerged conditions and soybean
crops can tolerate temporarily waterlogged conditions better
than crops like maize, pigeon pea and other deep-rooted Tops)
Regulationof Root Respiration
 Soil aeration may be managed by regulating the respiration of
roots and microbes by fertilization, cultural practices, plant
population, and incorporation of organic residues into the soil.
 Crops of lower O2, requirement or shallow-root systems may
be grown in situations where only surface soil has some degree
of aeration.

46.  Soil Aeration reduces compaction, oxidizes the soil, and allows
the roots to take the appropriate nutrients and grow as
vigorously as possible.
 With the help of better mechanization the soil is perforated with
small holes to allow air, water and other nutrients to reach
deeper.
 There is no easy simple way to determine whether the soil
aeration potential is good or poor.
 Personal knowledge of the soil and the response of plants
grown in it are the best sources of information for the grower.
 Poor soil aeration improves the environment for plant diseases
so that their attacks are much more severe than good soil
aeration conditions, so providing better aeration is a good
condition for plant development.
CONCLUSION