This document discusses soil air and aeration. It notes that soil air is important for root respiration and decomposition by microorganisms. Well-aerated soil allows for rapid oxygen exchange between the soil and atmosphere. Factors like pore space, temperature, depth, and wetting/drying influence gas exchange rates. Pore space occupies the volume not taken up by solids and is made up of spaces between aggregates and those created by roots, microbes, and expanding gases. Pore size and soil texture impact aeration. Aeration is the replacement of soil air by atmospheric air through diffusion and mass flow. Poor aeration can result from excess moisture or slow gas exchange. Improving drainage, tillage, and adding

1. SOIL AIR

2.  Importance of soil air :-
 it is used for the respiration by the root
 Decomposition of the organic matter by the microorganisms.
 Most desired condition is well aerated soil –
where oxygen exchange between soil air and
athmospheric air is rapid.
 Factors which influence rate of gases exchange
 pore spores
 Temperature`
 Depth of soil
 Wetting and drying
 covering

3. PORE SPACE
 Called void – have a portion of the soil volume
not occupied by solid.
 It made by:
 Irregular shapes of primary aggregates and their aggregation.
 Forces of penetrating roots
 Microorganism
 Expanding gases entrapped by water.
 Pores occupied by water and air
 %PS depends on :-
 Soil texture
 Aggregate arrangement
 Structure

4.  Sandy soils : 35-50%
 Medium to fine textured soil : 40-60%
 Good for plant growth : 50%
 Compacted soil : 25-30%

5. SIZE OF PORES
Macro pore space Micro pore space
Diameter > 0.06 mm Diameter < 0.06 mm
Primaly found between aggregates Occur within structural aggregates
Allow ready movement of air and •Mostly filled with water in moist soil
perlocating water •Not permit much air movement into/ out
of soil and water movement also slow

6. SOIL AERATION
 Process- air in soil replaced by air from
atmosphere.
 In well – aerated soils, the soil is similar to
the atmosphere, while poor- aerated soils
usually contain < o2 and > co2 than the
atmosphere.
 Vital process – largely controls the soil levels
of o2 and co2.

7.  Poor soil aeration under 2 conditions :
 excess moisture
 Very slow gaseous exchange
2 mechanisms:
 Mass flow
 diffusion

8. MASS FLOW
 due to pressure differences between
atmosphere and soil air.
 Achieved by fluctuation in soil moisture
content.
 Modified by temperature, wind, barometric
pressure.

9. DIFFUSION
 More important than mass flow.
 Involve great bulk of gaseous exchange.
 Each gas moves in a direction determined by
its own partial pressure.
 Diffusion allows extensive movement of air
from one area to another even though there
is no overall pressure gradient.

10. MEANS OF CHARACTERIZING SOIL AERATION
 Content of O2
 Atmospheric air – O2 = 21%, CO2 = 0.03%, N2 = 79%
 Soil air – O2 = 20.9%, CO2 = 0.035%, N2 => 78%
 O2 diffusion rate
 ODR determines the rate at which oxygen can be
replenished when it is used by roots/microorganisms or
when it forced out by water.
 If the rate of the replenishment is high, ODR is good.
 ODR decrease with soil depth
 Top growth : ODR = 30-40x 10-8 g/cm2

11.  Oxidation and reduction potential
 Measure of tendency of a substance to
accept or donate electrons.
 If a oxidation and reduction potential is high,
the substance is called oxidizing agent.

12. FACTORS AFFECTING SOIL AERATION
Soil
texture
Organic
matter
content
factors Bulk
density
Aggregate
stability

13. INFLUENCES OF SOIL AERATION ON
Soil Plants
Through its effects on the Activities of higher plants
redox potential, the level of are adversely effected in Soil microorganisms
soil O2 largely determines three ways by poor aeration Organisms influences many
the forms of several : soil reaction and inturn, soil
inorganic elements. The - Growth of plants, properties.
oxidised forms of these especially roots.
elements are readily Most obvious is microbial
utilizable by plants. The - Absorption of nutrients breakdown of organic
reduced forms of these and water. matter.
elements are so soluble that - Formation of inorganic
toxicities may occur. compounds toxic to plants

14. WAYS TO IMPROVED SOIL AERATION
Improve
drainage
Plowing/
tillage
Incoperate organic
matter