🌬 Soil Air

• Soil air is a continuation of the atmospheric air. Unlike the other components, it is constant state of motion from the soil pores into the atmosphere and from the atmosphere into the pore space.
• This constant movement or circulation of air in the soil mass resulting in the renewal of its component gases is known as soil aeration.

Composition of Soil Air

• The soil air contains a number of gases of which nitrogen, oxygen, carbon dioxide and water vapour are the most important. Soil air constantly moves from the soil pores into the atmosphere and from the atmosphere into the pore space. Soil air and atmospheric air differ in the compositions.
• Soil air contains a much greater proportion of carbon dioxide and a lesser amount of oxygen than atmospheric air. At the same time, soil air contains a far great amount of water vapour than atmospheric air. The amount of nitrogen in soil air is almost the same as in the atmosphere.

Composition of soil and atmospheric air

Factors Affecting the Composition of Soil Air

• Nature and condition of soil: Field air capacity is the fractional volume of air in a soil at field capacity. lt depends upon soil texture. Soil air constitutes in sandy soil, more than or equal to 25% (i.e. > 25%), loamy: 15-20% and in clay less than 10% of the total soil volume. Clay soils retain more water hence is the lower air capacity.
• Type of Crop: Plant roots require oxygen, which they take from the soil air and deplete the concentration of oxygen in the soil air. Soils on which crops are grown contain more CO₂ than fallow lands. The amount of CO₂ is usually much greater near the roots of plants than further away. It may be due to respiration by roots.
• Microbial activity: The microorganisms in soil require oxygen for respiration and they take it from the soil air and thus deplete its concentration in the soil air. Decomposition of organic matter produces CO₂ because of increased microbial activity. Hence, soils rich in organic matter contain higher percentage of CO₂.
• Seasonal variation: The quantity of oxygen is usually higher in dry season than during the monsoon. Because soils are normally drier during the summer months, opportunity for gaseous exchange is greater during this period. This results in relatively high O₂ and low CO₂ levels. Temperature also influences the CO₂ content in the soil air. High temperature during summer season encourages microorganism activity which results in higher production of CO₂.

Exchange of Gases between Soil and Atmosphere

👉🏻 The exchange of gases between the soil and the atmosphere is facilitated by two mechanisms:

• Mass flow: With every rain or irrigation, a part of the soil air moves out into the atmosphere as it is displaced by the incoming water. As and when moisture is lost by evaporation and transpiration, the atmospheric air enters the soil pores. The variations in soil temperature cause changes in the temperature of soil air. As the soil air gets heated during the day, it expands and the expanded air moves out into the atmosphere. On the other hand, when the soil begins to cool, the soil air contracts and the atmospheric air is drawn in.
• Diffusion: Most of the gaseous interchange in soils occurs by diffusion. Atmospheric and soil air contains a number of gases such as nitrogen, oxygen, carbon dioxide etc., each of which exerts its own partial pressure in proportion to its concentration.
  • The movement of each gas is regulated by the partial pressure under which it exists. If the partial pressure on one of the gases (i.e. carbon dioxide) is greater in the soil air than in the atmospheric air, it (CO₂) moves out into the atmosphere. Hence, the concentration of CO₂ is more in soil air.
  • On the other hand, partial pressure of oxygen is low in the soil air, as oxygen present in soil air is consumed as a result of biological activities. The oxygen present in the atmospheric air (partial pressure of O₂ is greater) therefore, diffuses into the soil air till equilibrium is established. Thus, diffusion allows extensive movement and continual change of gases between the soil air and the atmospheric air. Oxygen and carbon dioxide are the two important gases that take in diffusion.
• Plant growth suffers when oxygen diffusion rate (ODR) below 40 x 10_-8 g per cm₂ per minute.
• The growth of root is ceased at 20 x 10_-8 g/cm₂/ minute ODR.

Importance of Soil Aeration

• Plant and root growth: Soil aeration is an important factor in the normal growth of plants. The supply of oxygen to roots in adequate quantities and the removal of CO₂ from the soil atmosphere are very essential for healthy plant growth. When the supply of oxygen is inadequate, the plant growth either retards or ceases completely as the accumulated CO₂ hampers the growth of plant roots. The abnormal effect of insufficient aeration on root development is most noticeable on the root crops. Abnormally shaped roots of these plants are common on the compact and poorly aerated soils. Germination is inhibited in the absence of oxygen. Cereals except rice are intermediate in oxygen requirement and rice can tolerate very low level or even complete absence of oxygen in the soil.
• Microorganism population and activity: The microorganisms living in the soil also require oxygen for respiration and metabolism. Some of the important microbial activities such as the decomposition of organic matter, nitrification, Sulphur oxidation etc. depend upon oxygen present in the soil air. The deficiency of air (oxygen) in soil slows down the rate of microbial activity. For example, the decomposition of organic matter is retarded and nitrification arrested. The microorganism population is also drastically affected by poor aeration.
• Formation of toxic material: Poor aeration results in the development of toxin and other injurious substances such as ferrous oxide, H₂S gas, CO₂ gas etc. in the soil.
• Water and nutrient absorption: A deficiency of oxygen has been found to check the nutrient and water absorption by plants. The energy of respiration is utilized in absorption of water and nutrients. Under poor aeration condition (this condition may arise when soil is water logged), plants exhibit water and nutrient deficiency
• Development of plant diseases: Insufficient aeration of the soil also lead to the development of diseases. For example, wilt of gram and dieback of citrus and peach.

Influence of soil temperature and air on plant growth

👉🏻 Effect of soil temperature on plant growth

• Soil temperature requirements of plants: The soil temperature requirements of plants vary with the species.
  • The temperature at which a plant thrives and produces best growth is called optimum range (temperature).
  • The entire range of temperature under which a plant can grow including the optimum range is called growth range.
  • The maximum and minimum temperatures beyond which the plant will die are called survival limits.

• Availability of soil water and plant nutrients: The free energy of water increases with temperature. Up to wilting point limit, warming of soil increases water availability beyond which it decreases. Low temperatures reduce the nutrient availability, microbial activities and root growth and branching. The ability to absorb nutrients and water by plants reduces at low temperatures.

Soil temperature management

• Use of organic and synthetic mulches: Mulches keep soil cooler in hot summer and warm in cool winter.
• Soil water management: High moisture content in humid temperate region lowers soil temperature.
• Tillage management: Tilling soil to break the natural structure reduces the heat conductance and heat loss. A highly compact soil loses heat faster than loose friable soil.
• Methods of measuring soil temperature: Mercury soil thermometers of different lengths, shapes and sizes with protective cover are buried at different depths to measure the temperature. Thermo couple and thermistor based devices are also available. Infra-red thermo meters measure the surface soil temperature. Automatic continuous soil thermographs record the soil temperatures on a time scale. The International Meteorological Organization (IMO) recommends standard depths to measure soil temperatures at 10, 20, 50 and 100 cm.