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 Interactions Among Soil Components:

Soil Water & Air Interactions

5.1 Pore Space  
  5.1.1 Types of soil pores  
  5.1.2 Relationship between water and air in pores  
5.2 Aerobic and Anaerobic Conditions
   5.2.1 Aerobic and anaerobic organisms
5.3 Effect of water on thermal properties

 

5.1 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.  

5.1.1 Types of soil pores

·        Macropores (d>0.08mm) 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 (d<0.08mm) 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.  

Macro- vs. Micro-pores in the Soil

Source: Lesley Dampier

Pore size distribution is related to soil structure.

See the Interactions- Soil Structure web section.

 

5.1.2 Relationship between water and air in pores

Pore space can be filled with either water or air.  The volume of soil water and soil air, however, cannot exceed the total soil porosity.  As soil water increases, soil air must decrease and vice versa.   

 

 

5.2 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 SO42-, are used by soil organisms carrying  out this process.  Wet soils are oxygen limited and anaerobic (sometimes called anoxic) respiration will take place. 

 

 

5.2.1Aerobic 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 sulfates as electron acceptor

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

See the SoilWeb Soil Components - Soil Organisms web section

 

 

5.3 Effect of water on thermal properties

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

Thermal Conductivity (λ)

Property

Units

Definition

Impact of water content

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)
 

Thermal Conductivity

Source: Lesley Dampier

 

Soil Heat Capacity (Cv)

Property

Units

Definition

Impact of water content

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

 

Heat Capacity

Source: Lesley Dampier

 

 

Thermal Admittance (λ/Cv) 1/2

Property

Units

Definition

Impact of water content

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.

 
 

Thermal Admittance

Source: Lesley Dampier

 

 

Thermal Diffusivity (λ/C)

Property

Units

Definition

Impact of water content

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

 
 

Thermal Diffusivity

Source: Lesley Dampier

 

                                         

 

 

Faculty of Land and Food Systems
THE UNIVERSITY OF BRITISH COLUMBIA