Soil Aeration

Soil aeration is a vital process because it largely controls the soil level of two life-sustaining gases:

Oxygen (O2) and carbon dioxide (CO2).

Component Soil Air (%) Atmosphere (%)
N2 79.2 79.0
O2 20.6 20.9
CO2 0.25 0.03
Source: Russel, E.J., and A. Appleyard. 1915. The atmosphere of the soil, its composition, and causes of variation. J. Ag. Sci. 7:1–48.

What controls the composition of the soil atmosphere?

C6H12O6 + 6O2 ↔ 6CO2 + 6H2O + energy

organic matter + oxygen ↔ carbon dioxide + water + energy

These reflect the overall direction of two major processes.

Respiration ⇆ Photosynthesis

Below the soil surface, respiration dominates. Aerobic respiration from roots, fungi, and bacteria likely overshadow respiration of the meso-fauna and macro-fauna.

Factors affecting aeration (oxygen availability) are:

• Microbial activities
• consumption of O2
• production of CO2
• results in soils greater CO2 contents
• Macropore content
• a function of texture
• structure
• bulk density
• compaction decreases bulk density

Mostly, soil pores are not full of water and aerobic organisms can live. Aerobic organisms require free oxygen for respiration.

However, saturation conditions can exist and the duration of saturation impacts O2 availability. Impermeable layers (rock, or otherwise) and horizons can impede water movement and causing pores to be full of water for extended periods.

Saturated conditions can cause a shift from dominantly aerobic metabolism to anaerobic metabolism. Anaerobic metabolic byproducts such as methane (CH4), nitrous oxide (N2O), and hydrogen sulfide (H2S) dominate as gasses under these conditions. Methane (CH4) and nitrous oxide (N2O) that move into the above ground atmosphere contribute to global warming.

Two mechanisms are responsible for gas movement: mass flow and diffusion.

Rarely does equilibrium in concentrations exist. Metabolism product gasses move toward the surface, and O2 moves away from the surface into the soil. They flow down a concentration gradient moving from high concentration to low concentration.

Diffusion is more important in soils than mass flow except, perhaps, at the soil surface and/or where macro-pores and cracks exist.

Macropore content is a function of texture, structure, bulk density, compaction, etc.

Soil water content affects diffusion coefficient (D) because gas moves slowly through pores filled with water.

Mechanisms of gas exchange:

Mass flow is generally due to pressure differences between soil air and the atmosphere

Example: During a rain, water forces air out of pores evapotranspiration removes water from soil pores and air is sucked into the soil.

Diffusion is a flow of gas molecules due to differences in gas concentrations (most important!)

This equation relating the flow of gas is similar to the flow of water.

The mathematical formula to describe the movement of gases by diffusion is:

Gas flow in soil needs some definition. Flow then is a quantity in volume, $$Q$$, that moves over a crosssectional area, $$A$$, in time, $$t$$.

Thus, flow is represented as $$\frac{Q}{A\cdot t}$$.

$$\frac{Q}{A\cdot t} = D \cdot \frac{\Delta C}{\Delta x}$$
• $$\Delta C$$ is the difference in concentration between two points.
• $$\Delta x$$ is the change in the distance, $$x$$, between the two points
• $$D$$ = the diffusion coefficient (a measure of the speed at which gases move through a material). This is empirically determined.

• Previously defined above were:
• $$Q$$ = volume
• $$A$$ = area
• $$t$$ = time

Diffusion coefficient $$\left(D\right)$$ is determined by: the arrangement of empty (air-filled), continuous pores (primarily macropores). Flow is slowed by increased bulk density due to compaction, high clay content (too few macropores, though likely high in total porosity), and high water content

Why measure diffusion?

Diffusion can be slower than the consumption of oxygen at the biological point of use/uptake. Consequences of low oxygen is a switch to anaerobic (no oxygen present) respiration.

• Reduced root growth and activity
• Determines the form of inorganic elements (denitrificationdenitrification: NO3 → + N2 or N2O; and reduction of iron Fe3+ + e- –> Fe2+ )
• Root rotting organisms are often associated with poor aeration.
• Reduced organic matter decomposition rates when anaerobic shift to fermentation (anaerobic) microbial pathway:
• C6H12O6 → 3CO2 + 3CH4 (methane)
• Soil color influenced by reduction of iron (Fe3+ [red] + e- → Fe2+ [blue/green])

Management for good soil aeration

Proper irrigation and drainage (water management).

Maintain stable soil structure (maximize macropores), add organic matter and amendments if necessary to promote stable soil structure.

Cultivation to break up compacted layers (lower the ρb) & Increase macropores).

If all else fails, plant crops tolerant of low oxygen (e.g., hydrophytes = rice & wetland plants).

Wetlands

Soils that are water saturated near the surface for prolonged periods when soil temperatures and additional conditions are conducive to biological growth such that O2 is utilized faster than it can be replaced leading to anaerobic conditions.

Three Conditions of Wetlands

1. A wetland hydrology or water regime
2. Hydric soils
3. Hydrophytic plants

Wetlands are protected in the United State of America by the Federal Government. To determine if a wetland exists two of the three conditions of wetlands must exist.

Terms

Aerobic
Living or active only in the presence of oxygen. ()
Anaerobic
Without molecular oxygen. The opposite of aerobic. ()
Bulk density
In soils, the dry mass (weight) of soil per unit bulk volume. ()
Continuity
Length of unbroken, continuous, or coherent soil pores. ()
Concentration (of material)
The amount of the material per unit volume of solution, gas, or solid. Also, the process of increasing the concentration at a certain place. ()