Soil and Plant Analysis

Soil aeration problems in the field

Factors affecting the composition of soil air
Nature of soil

• Soil with more pore space volume will have more air.
• The drainage of excess water from soil macropores improves soil aeration.
• If the volumes of soil macropores are high, soil aeration will be good.
• Soil texture, bulk density, aggregate stability and organic matter contents influence the soil macropores.
• The quantity of oxygen in soil air is less than that in atmospheric air.
• The oxygen content of the air reduces with increasing depth of soil.
• Surface soil will have more oxygen than sub soil because of quick replenishment from the atmospheric air through diffusion.
• Light textured soil or sandy soil contains more air than the heavy soil.
• The concentration of CO2 is more in subsoil due to slow rate of replenishment and low aeration in lower layer than in the surface soil.
• High clay containing soils will have high soil moisture potential - which reduces the oxygen diffusion rate and increases the CO2 accumulation.

Type of crop

• Plant roots require oxygen, which they take from the soil air and deplete the concentration of oxygen in the soil air and release CO2.
• Soils on which crops are grown contain more CO2 than fallow lands.
• The amount of CO2 is more near the plant than farther away due to respiration by roots.

Microbial activity

• The microorganisms in soil require oxygen for respiration and they take it from the soil air depleting its concentration.
• Decomposition of organic matter produces CO2, because of increased microbial activity.
• Hence, soils rich in organic matter contain higher CO2.

Seasonal variations

• The oxygen content of soil air is higher in dry season than during the monsoon.
• During the dry season, most of the soil pores are filled with air and the exchange of gases between the soil air and the atmosphere is more.
• During monsoon seasons, most of the soil pores are filled with water.
• Temperature also influences the gaseous composition of the soil air.
• High temperature enhances the gaseous exchange between the soil air and the atmosphere.
• High temperature also increases the microbial respiration and releases more CO2 in the soil air.
• Under field conditions, poor soil aeration occurs due to two conditions: a) when the moisture content is too high occupying most of the pore space and b) when the exchange of gases with the atmosphere is slow.
• When the field is completely submerged with water, all the plants except some plants like rice, die for want of oxygen.

Exchange of gases between soil and atmosphere

• The exchange of gases between the soil air and the atmosphere is facilitated by two mechanisms.

Mass flow

• During the rain fall or during irrigation, a part of the soil air is replaced by water and the replaced air moves out into the atmosphere.
• When the soil moisture is lost due to evaporation, plant absorption, internal drainage etc., atmospheric air reenters into the soil pore space.
• The variations in soil temperature cause changes in the temperature of the soil air.
• Also, when the soil air gets heated during the day, it expands and the expanded air moves out into the atmosphere.
• When the soil cools during the night, the soil air contracts and the atmospheric air is drawn in.

Diffusion

• Most of the gaseous interchange in soils occurs by diffusion.
• Atmosphere and soil air contains gases such as nitrogen, oxygen, carbon dioxide etc., each of which exerts its own partial pressure in proportion to its concentration.
• These gases move from higher concentration (higher partial pressure) to lower concentration (lower partial pressure).
• Oxygen moves from the atmosphere into soil (air) and CO2 moves out of the soil air into the atmosphere through diffusion without the movement of entire air mass because of the difference in their concentration or partial pressure.
• This movement is called diffusion and this will continue till equilibrium is established.