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AIR AND HEAT MOVEMENT IN SOIL
Movement in soil :
The constant movement or circulation of air in the soil mass resulting in the
renew of its component gases is known as air movement in soil .
Soil air constantly moves from the soil pores space into the atmosphere and
from the atmosphere into the pore space.
An ideal soil (by volume) consists of about 25% air.
• Soil air has composition similar to that of atmosphere but differs in amount of
constituents gases .Air movement in soil
Soil air have >Co2 than atmospheric air .
Even in well- aerated soil CO2 is 10x that of atmosphere .
• O2 content of soil ranges from 2-10% depending on soil condition.
• In Soil air-higher level of CO2 & lower level of O2 as compared
to atmosphere.
• Higher moisture content in soil produces CH4, H2S & C2H4
• Amount & composition of air in soil are dynamic & to a large degree are
determined by water content & activity of soil organisms.
• O2 is required for respiration of plant roots, microbes & soil fauna.
• O2 helps to dissolve nutrient & make them available to plants .The specific heat of a dry soil is 0.2 cal g-1 .
Increasing water content in soil increases the specific heat of the soil and
hence a dry soil heats up quickly than a moist soil.
• Thus, adequate level of O2 ( & water) must be maintained in soil air
through proper aeration .
O2 availability in the field is regulated by three principal Factors :
– Soil macro-porosity (as affected by soil texture & structure, compaction)
– Soil water content (as it affects proportion of porosity that is filled with air)
– O2 Consumption by respiring organisms including plant roots and
microorganism .
Soil aeration method :
• There are two method of soil aeration :
1. Mass flow
2. Diffusion
In mass flow and diffusion process always flow form their high
concentration to low concentration .Impact of poor aeration are :
1. Root elongation is particularly sensitive to aeration.
2.O2 def. disturbs metabolic processes in plants, resulting
- accumulation of toxic substances in plants
- low uptake of nutrients & water (due to stomatal closure) & thus early wilting )
3. Induces series of reduction reactions
- denitrification, (NO3- to N2O↑ and N2↑
-CH
-4
Fe3+ to Fe2+, Mn4+ to Mn2+ , SO42- to H2S, CO2 to CH4
4. Production of growth hormones is also stopped.
Impact on Plant growth
• Lack of O2 is main problem for plant growth in flooded or saturated soils
• Stagnant water much worse than flowing water that supplies dissolved O2.
• Under anaerobic condition, plants respond by closing leaf stomata, which reduces
o Photosynthesis,
o Sugar translocation,
o Water and nutrient uptake,
o Hormone activity.
• Symptoms include Wilting, Nutrient deficiencies & Death.
• Under flooding, many plants develop aerenchyma to aerate their root system Induced
by the presence of ethylene (C2H4).Heat movement in soil :
Transport of heat within soil can occur by conduction and
convection process.
1. Conduction : The transfer of heat from one
particle of matter to another by direct particle to
particle contact or vibration of particle.
2. Convection : The transfer of thermal energy (heat) through the bulk
movement of matter.
– Convection occurs in FLUIDS (liquids and gases).
– Convection produces CURRENTS in both gases and liquids
The sources of heat for soil are :
-Solar radiation (external),
-Heat released during microbial decomposition of organic matter
- Respiration by soil organisms including plants .
Soil temperature strongly influences biological processes;
- Seed germination
- Seed emergence
•-Plant growth, root development
• Microbial activity in soils
• Moisture supply e.g. transport of water & nutrients in soil is
lowered by low temperature
• Soil biological & chemical processes: Availability of plant
nutrients like N, P, S.
• Soil aeration & soil structure
• Soil drying & evaporation.