20 Soil Aeration

What is aeration?

Aeration is the degree of mixing of soil air with atmospheric air. Much like the air feels fresher in your home with all of the windows wide open, the faster the air moves into and out of the soil, the better aerated the soil is. The better the soil aeration, the more oxygen will be in the soil.

Aeration can be measured 3 ways:

• Percent oxygen: calculate the %O₂ in the soil, or the ratio of O₂:CO₂. A higher %O₂ or higher O₂:CO₂ ratio signifies better aeration.
• Air-filled porosity: This is the percentage of soil pores that are filled with air, out of the total pores. The greater the air-filled porosity, the greater the aeration. Air-filled porosity can be calculated as:

AFP = TP – θ_v

Where:
AFP = air-filled porosity
TP = total porosity
θ_v = volumetric water content = water-filled porosity

• Redox potential (E_h): a measure of how easily chemicals can acquire or lose electrons; proportional to the amount of energy redox reactions will yield given the chemical species present.^[1] Learn more about this in the Redox Processes and Wetlands chapters. (This concept is frequently considered intermediate instead of introductory, it so may be optional in introductory soil science courses.)

Importance of aeration

Aeration is integral to plant root growth and affects numerous soil processes. Plant roots require oxygen to perform aerobic respiration (i.e. to “breathe”) and so over time, plants without sufficient root aeration will suffocate and die.

Like plant roots, aerobic microbes need oxygen gas (O₂) to perform aerobic respiration. When aeration is slow, therefore, microbes tend to grow more slowly, leading to slower organic matter decomposition. Furthermore, when aerobic microbes run out of oxygen, their growth stops, and other microbes that use other compounds for respiration begin to grow instead. These microbes frequently use plant nutrients and important components of soil minerals, and/or produce greenhouse gas emissions. Hence, aeration also affects soil nutrient content, mineral composition, and soil greenhouse gas emissions. Abiotic reactions are also affected by the presence or absence of oxygen. Therefore, due to both biotic and abiotic processes, soil development fundamentally changes when aeration changes.

How aeration works

What causes air to flow in and out of soil? It comes down to the two ways that gases can move: mass flow and diffusion.

Mass flow is the bulk movement of fluids and solutes (including gases) from high to low pressure, i.e. movement along a pressure gradient. A pleasant breeze or water flowing in a stream are both examples of mass flow, even if you can’t sense the pressure difference. You’ve also observed mass flow if you’ve ever let air out of a balloon: the air in the balloon has a higher pressure than outside the balloon, so when you open up the balloon or pop it, the air exits the balloon via mass flow, from high to low pressure. In soil, both soil particles and water-filled pores block the mass flow of gases.

Diffusion, by contrast, is the movement of a specific solute – including gases – from areas of high to low concentration (movement along a concentration gradient). Typically the atmosphere has a greater oxygen concentration than the soil, so oxygen diffuses from the air to the soil. Diffusion is much slower than mass flow. Diffusion of dissolved O₂ gas is additionally 10,000x slower through water than through air!

Because water impedes the flow of O₂ and other gases so drastically, increasing moisture dramatically reduces aeration.

Factors affecting aeration

Based on how the flow of O₂ gas works, and our understanding of what organisms use O₂ gas, we can predict what factors will affect aeration, whether it’s measured as air-filled porosity or O₂ concentration. These factors are all related to soil structure, soil moisture, and/or soil organisms.

Soil structure and drainage

Any aspect of soil structure that improves soil drainage will also improve soil aeration, because the faster water flows out of the soil, the faster the water-filled pores empty out, making room for air and increasing the air-filled porosity. Air, then, can flow through the air-filled pores, increasing aeration.

Here is a brief summary of soil physical properties affecting aeration (these are very much interrelated with each other):

• Texture: Other factors remaining equal (especially aggregation), smaller particles will have smaller pores between them, slowing the flow of water and drainage, and reducing aeration.
• Aggregation: better aggregation increases macroporosity, increasing drainage and aeration.*
• Porosity: greater macroporosity leads to better drainage and aeration
• Tortuosity: the more winding (less direct) the flow path for air or water, the slower the drainage and the poorer the aeration. Conversely, more direct flow paths created by better macropore connectivity enhance drainage and aeration.
• Soil structure type: Some structures, such as platy structures, have very high tortuosity and hence lower aeration. However, other structures such as granular structures, have low tortuosity and better aeration. More in the infiltration and drainage chapter.

*Inside an aggregate, aeration decreases as you approach the center of the aggregate. However, aggregation still increases soil aeration as a whole due to how macroporosity aids both air and water flow.

Organism respiration rates

As mentioned earlier, both plant roots and aerobic microorganisms perform aerobic respiration. To review, the reaction for aerobic respiration is:

C₆H₁₂O₆ + 6O₂ ↔ 6CO₂ + 6H₂O

As you can see, the reaction has O₂ gas in the reactants and CO₂ gas in the products. Consequently, aerobic respiration leads to lower aeration. This is why soil has lower O₂ and higher CO₂ than the atmosphere, and why mixing atmospheric air with soil air increases aeration.

Root respiration

Microbial respiration

Consequently, anywhere near soil roots or large concentrations of aerobic soil microbes will have lower aeration.

Location in the soil profile and landscape

Within a soil profile, aeration normally decreases with increasing depth. This is simply because the farther away the point of interest is from the surface, the further air has to travel to get there. The effect of soil depth is compounded by moisture and structure – so aeration will decrease with increasing depth much faster in a soil with poor aggregation, low microporosity, and/or poor drainage.

Regarding landscape position, you may recall from the soil formation factors that topography plays a major role in influencing clay content. Going downslope, clay content tends to increase because a) clay erodes downhill, and moves between larger particles downhill, and b) soil at the bottom of the hill receives more water from uphill, causing it to weather into smaller particles (like clay) faster. Given that increasing clay content decreases drainage and aeration, we would expect aeration to also decrease as we go downslope (i.e. as elevation decreases within a landscape). So, we’d expect to find the poorest aeration on toeslopes and in closed depressions.

Lastly, the position of the groundwater table interacts with the effect of both location within a soil profile and the landscape to affect aeration. As you go down within a soil profile, you get closer to the groundwater, so soil moisture increases and aeration decreases. Going downhill also brings you closer to the groundwater – which, as you may recall, is saturated with water, and alternatively called the “saturated zone” – thereby decreasing aeration as well. At the bottom of a hill, where the groundwater is close to the surface, aeration decreases the most rapidly with increasing soil depth.

Climate

Comparing soil aeration between regions, both average precipitation and temperature, and their variation across seasons, affect soil aeration.

Precipitation

Areas with higher precipitation, and hence soil moisture, experience lower aeration on average. Not only do soils in wetter regions receive more water at the surface, but also usually have higher water tables on average. Given that water flows from high to low water potential, and the matric potential of the saturated zone (groundwater) is zero, this higher water table reduces drainage. All other factors remaining the same, aeration and precipitation are therefore inversely related.

Temperature

Temperature has two contrasting effects. First, an increase in temperature leads to an increase in evaporation, leading to decreased soil moisture and increased aeration. Second, when controlling for moisture (i.e. when effects on evaporation are negligible) and other factors, increasing soil temperature increases microbial respiration, and this decreases aeration.

Vegetation density and transpiration

Unlike proximity to roots, increasing density of vegetation at the landscape scale actually increases aeration. This occurs because as plants transpire, they decrease soil moisture and lower the groundwater table, thereby increasing air-filled porosity and drainage.