Gas Phase and its flow in Soil

  Water Vapor & other Gases

Terrestrial organisms live in a gaseous medium composed mostly of nitrogen and oxygen. Water vapor is present in varying amounts, and carbon dioxide and other gases, in trace amounts

Carbon dioxide is a substrate for photosynthesis and oxygen is a product; while oxygen is a substrate for respiration and carbon dioxide is a product.

Water vapor almost always moves from the organism to the environment. The humidity of the organism is near 100 percent, while the surroundings are nearly always much drier. The organism must remain in a highly hydrated state in order for biochemical reactions to occur, so the constant loss of water is a threat to survival, and frequent access to liquid water is a necessity for most terrestrial organisms.

The intake of liquid water and the loss of water vapor to the environment are usually the most important components of the water budget of an organism.

  Evaporative cooling

Environmental temperatures are often higher than can be tolerated by biological systems. If there were not some mechanism for cooling the organism, it would perish. As water evaporates, roughly 44 kilojoules of energy are required to convert each mole to the vapor state. This is called the latent heat of vaporization.

Evaporative cooling

Evaporative cooling is a natural way of controlling temperatures of organisms in hot environments. The amount of cooling available to the organism depends on the concentrations of water vapor at the organism surface and in the environment, and on the conductance to water vapor of the organism surface and boundary layer

  Exchange of Gases between Soil and Atmosphere

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.


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.

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.

  Water Vapor: Saturation Conditions

If a container of pure water is uncovered in an evacuated, closed space, water will evaporate into the space above the liquid water. As water evaporates, the concentration of water molecules in the gas phase increases. Finally, an equilibrium is established when the number of molecules escaping from the liquid water equals the number being recaptured by the liquid. If the temperature of the liquid was increased, the random kinetic energy of the molecules would increase, and more water would escape.

The equilibrium vapor pressure, established between liquid water and water vapor in a closed system is known as the saturation vapor pressure for the particular temperature of the system.

The saturation vapor pressure is the highest pressure of water vapor that can exist in equilibrium with a plane, free water surface at a given temperature.

The influence from any other gases present in the space above the water is negligible, the vapor pressure above the water surface is essentially the same whether the closed space is initially evacuated or contains air or other gases.

Condition of Partial Saturation

Ambient vapor pressure is simply the vapor pressure that exists in the air, as opposed to saturation vapor pressure, which is the maximum possible vapor pressure for the temperature of the air.

Relative humidity is the ratio of ambient vapor pressure to saturation vapor pressure at air temperature

Relative humidity is generally multiplied by 100 to express it as a percent rather than a fraction

  Dew Point

Vapor Deficit

is the difference in vapor pressure or mole fraction between saturated and ambient air

Dew Point

If the temperature gets cold, the air gets to the point that it is holding the most water vapor it can hold. The relative humidity for this temperature would be 100 percent. This is also known as the dew point temperature.

In other words, the saturation vapor pressure at dew point temperature is equal to the ambient vapor pressure

  Spatial and Temporal Variation of Atmospheric Water Vapor

  • During the day, vapor pressures are highest near a soil or plant surface and decrease with height, at night, vapor pressures tend to be lowest near the surface and increase with height.
  • Vapor pressures tend to be a bit higher in the day than at night and typically reach a minimum at the time temperature is at the minimum.
  • As with temperature, the surface acts as a source of water vapor in the day and a sink at night (when condensation and dew formation occur), and is therefore responsible for the shape of the vapor pressure profiles.
  • The magnitude of spatial and temporal vapor pressure variation is much smaller than for temperature, and is usually small enough that it can be ignored in comparison with other sources of uncertainty in the measurements.
  • If only the average vapor pressure for a day is known, the best estimate of hourly vapor pressures is that they equal the average for the day.

Changes in vapor pressure with height are relatively small, so vapor pressures in an organism microenvironment are similar to the vapor pressure at measurement height

  Estimating Vapour Pressure

One of the most common measurements of atmospheric moisture is relative humidity.

Instrument : Psychormeter / Hygrometer

Humidity is bad in communicating any useful information by itself, averaging individual humidity measurements destroys any possibility of obtaining useful information from the original data because the average humidity depends on the pattern of temperature variation (which is lost in the averaging process).

It is best to immediately convert humidity data to vapor pressure or dew point. Then record, average, and process these data

The minimum daily temperature can often be taken as the dew point temperature

Vapor deficit gives an estimate of the driving force for evaporation, and is useful in relating transpiration and biomass production in plant communities

Vapor pressure or humidity normally is measured in weather stations. Typically such measurements are taken 1 .5 to 2 m above the ground in an open area. The humidity of microenvironments in plant canopies or near leaves can be quite different