4. Soil Air
The soil spaces filled with gasses is called soil air.
Composition of Soil Air
• The Soil air contains a number of gases. Out of which oxygen, carbon-dioxide,
nitrogen, and water vapor are the most important.
• The composition of Soil air is more or less similar to that of the atmospheric air
except the content of carbon dioxide which is several times higher.
• The soil fauna and flora consume oxygen and gives out carbon dioxide.
• Organic matter addition and cropping increases the level of Carbon dioxide in
the soil. Carbon dioxide in soil air constantly moves from the soil pores into
atmosphere and vice-versa.
• The nitrogen content in soil air is almost the same as in atmosphere.
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5.
6. Factors Affecting the Composition of Soil Air
I. Nature of the Soil –
• Sandy Soils have macro pore, as a result of which, aeration is very
good in that soil.
• The soils that are water-logged contain small amount of oxygen as
the pore space is filled with water immediately after a heavy rains or
irrigation.
• The surface soil contains more macro pore than the sub-soil. As a
result, gaseous exchange is found to be more in surface soil than the
sub soil.
• The oxygen percentage of Soil air varies with the depth of the soil and
this is true in case of carbon dioxide also.
7. II. Soil organic matter –
• Soil organic matter is decomposed by microorganism present in the
soil.
• Microbiological decomposition leads to the production of carbon
dioxide and its content increases in the soil air. Hence soil rich in
organic matter contains higher percentage of carbon dioxide.
8. III. Season
• Season and temperature also influences the carbon dioxide content
of the soil. The activity of soil micro-organism increases at high
temperature during summer month which results in higher
production of carbon dioxide.
• The composition of soil air shows marked seasonal variation, the
intensity of which is affected by the texture of the soil and position of
water table.
9. IV. Soil moisture
• The oxygen content of a soil decreases when the macro pores are
filled with water. But when the soil is artificially drained again, the
macro pores are filled with air and the oxygen content of soil
increases.
V. Vegetation
• Soils on which crops are grown contain more carbon dioxide than
fallow land as a result of respiration of plant roots.
• The plant takes the soil oxygen and releases carbon dioxide. As a
result, the carbon dioxide content of the cropped land increases near
the root zone of the plant.
10. Soil Aeration and Plant Growth
• Soil aeration influences the plant growth. Oxygen is most important for
respiration of plant roots and soil microbes. Aerobic microorganism
functions properly if oxygen is present sufficiently in the soil.
• Soil micro-organism decomposes the organic matter in aerated soil and
nutrients are liberated for the growth of the plants. The symbiotic (e.g.
Rhizobium, Clostridium etc.) and non-symbiotic bacteria (e.g. Azotobactor,
etc.) function well in a soil having adequate aeration and they fix the
atmospheric nitrogen and enrich the soil with nitrogen.
• The nitrifying bacteria (e.g. nitrosomonas, nitrobactor etc.) works under
aerobic conditions and oxidizes ammonia to nitrites and/or nitrates, the
important form of nitrogen at which most of the plant takes nitrogen from
the soil.
• Under anaerobic condition, the denitrifying bacteria (Thiobacillus
denitrificans) works and converts the nitrites or nitrates into ammonia or
elemental nitrogen, and thus causes the loss of soil nitrogen.
11. • Aeration promotes root growth and absorption of water and nutrients by
plants. The absorption of nutrients by roots is one of the most important
physiological functions of living plants and this is dependent on the proper
aeration of soil. Absorption of water by plants roots takes place at a
greatly retarded rate in soil deficient in the supply of oxygen. In poorly
aerated soil, the carbon dioxide (CO2) gets accumulated which affects
water absorption process of the plant.
12. Soil air is also useful in increasing the nutrient availability of the soil
(i) By breaking down the insoluble minerals into soluble salts,
(ii) By decomposing plants and animals remains and
(iii) By bringing about nitrifying and nitrogen fixing process of bacteria.
