LFS:SoilWeb/Interactions Among Soil Components/Soil Water & Soil Air Interactions

Pore Space

The organization of the solid components in soil determines the geometric characteristics of pore space. These configurations determine how water and air are transmitted and retained.

Types of Soil Pores

Macro- vs. Micro-pores in the soil

Macropores (diameter > 0.08 mm) occur between aggregates (interped pores) or individual grains in coarse textured soil (packing pores) and may be formed by soil organisms (biopores). They allow ready movement of air and the drainage of water and provide space for roots and organisms to inhabit the soil.

Micropores (diameter < 0.08 mm) occur within aggregates. They are usually filled with water and are too small to allow much movement of air. Water movement in micropores is extremely slow and much of the water held by them is unavailable to plants.

Pore size distribution is related to soil structure. See Soil Structure for more information.

Relationship between Water & Air in Pores

Pore space can be filled with either water or air. The volume of soil water and soil air, however, cannot exceed pore volume (Vf). As soil water increases, soil air must decrease and vice versa.

Aerobic and Anaerobic Conditions

Respiration is a metabolic process in which an organism obtains energy by oxidizing carbohydrates (e.g., sugars). An oxidizing agent is required.

Under aerobic conditions, oxygen is available as an oxidizing agent for respiration by organisms. Dry soils have adequate oxygen supply for aerobic respiration.

Under anaerobic conditions, oxygen is not available as an oxidizing agent for respiration by organisms. Instead, other agents, such as Fe³⁺, Mn⁴⁺, NO₃^- or SO₄^2-, are used by soil organisms carrying out this process. Wet soils are oxygen limited and anaerobic (sometimes called anoxic) respiration will take place.

Aerobic and Anaerobic Organisms

Microorganisms differ in their aeration requirements.

Aerobic organisms require the presence of oxygen. Examples: Some fungi, some bacteria, actinomycetes.

Obligatory anaerobic organisms function only in the absence of oxygen. Examples: Bacteria that use sulfate as an electron acceptor.

Facultative anaerobic organisms can function with or without molecular oxygen. Examples: Bacteria that use nitrate as an electron acceptor.

See Soil Organisms for more information.

Effect of Water on Thermal Properties

Different water contents will have a strong influence on soil thermal properties.

Property	Units	Definition	Impact of Water Content	Graph
Thermal conductivity (λ)	J/ms°C	Measure of the ease with which a soil transmits heat. It describes heat flow in response to a temperature gradient.	Water is a GOOD conductor (has large λ) and air is a POOR conductor (has low λ). Wet soils will conduct heat better than dry soils (non-linear).

Property	Units	Definition	Impact of Water Content	Graph
Soil heat capacity (Cv)	J/m³°C	Amount of heat needed to cause a 1°C change in temperature of a unit volume of soil.	Water has HIGH heat capacity, while air has almost none. Soils with high Cv are buffered against temperature change. It is much easier to raise soil temperature by 1°C in a dry soil than wet soil

Property	Units	Definition	Impact of Water Content	Graph
Thermal admittance (λ/Cv)^1/2	kJ/m²°Cs	Represents ability of soil to accept and release heat.	Soils with low thermal admittance have extreme surface temperature fluctuations. Because water has a HIGH heat capacity and is a GOOD conductor, wet soils will have a HIGH thermal admittance.

Property	Units	Definition	Impact of Water Content	Graph
Thermal diffusivity (λ/C)	m²/s	An indication of subsurface temperature response to surface temperature change.	Soils with high thermal diffusivity undergo large and rapid subsurface temperature responses to surface temperature change. Does not change much with water content in organic soil, but in mineral soils, the peak thermal diffusivity occurs near field capacity.