The Gas Phase of Soil

1.1. Soil as a Three‑Phase System

From a physical perspective, soil is a three‑phase system: solid, liquid, and gaseous (Eash et al., 2016). The solid phase consists of mineral particles (sand, silt, clay) and organic matter (humus, plant residues). The liquid phase is the soil solution, containing water and dissolved minerals. The gaseous phase is the soil air, filling the pores not occupied by water.

In a typical well‑structured soil, the solid phase occupies roughly half the volume, while the other half is pore space. In turn, the pores are filled with air and water in variable proportions: the more water, the less air, and vice versa (Eash et al., 2016). It is this ratio that determines whether plant roots will receive enough oxygen or suffer from inadequate aeration.

1.2. Composition of Soil Air

Atmospheric air has a relatively constant composition: nitrogen (78.08–80.24 %), oxygen (about 20.9 %), carbon dioxide (0.03 %), and argon (about 0.93 %) (Mukha et al., 2003). Soil air differs significantly from atmospheric air in composition, and these differences have direct biological significance.

Compared with atmospheric air, soil air contains:

• less oxygen – its content can drop to tenths or even hundredths of a percent in poorly aerated soils;
• more carbon dioxide – CO₂ concentration can reach 10–20 % as opposed to 0.03 % in the atmosphere (Eash et al., 2016; Mukha et al., 2003);
• nitrogen content may vary either way: it can decrease when microorganisms bind it (nitrogen fixation) or increase during denitrification and decomposition of proteinaceous substances (Mukha et al., 2003).

This difference is explained by two opposing processes: on the one hand, plant roots and microorganisms consume oxygen and release carbon dioxide during respiration; on the other hand, the intensity of gas exchange between the soil and the atmosphere determines how quickly these gases equilibrate their concentrations (Weil & Brady, 2017; White, 2006).

Why is CO₂ content higher than in the atmosphere?

The main source of carbon dioxide in soil is biological processes. Oxygen and carbon dioxide in soil air have diverse effects on soil properties and directly or indirectly influence plant productivity (Mukha et al., 2003).

1.3. Why Does the Gas Composition Change?

The composition of soil air is not constant – it depends on:

• soil type (sandy soils are better aerated, clayey and structureless soils are worse);
• soil properties (physical – structure, density, porosity; chemical – organic matter content; biological – abundance and activity of microorganisms);
• season and weather conditions;
• agricultural use – on arable land, air composition depends on the cultivated crop and the applied agronomic practices;
• presence of vegetation – under vegetation, soils contain less oxygen and more CO₂ than fallow fields (Mukha et al., 2003).

In soils of normal moisture, oxygen content generally decreases from the upper horizons downward, while carbon dioxide increases. This is because the main sources of oxygen consumption – plant roots and microorganisms – are concentrated in the upper (humus or plough) horizon (Mukha et al., 2003).

Soil moisture and temperature have a particularly strong effect on soil air composition. With increasing moisture, air capacity decreases, the system of air‑conducting pores is disrupted, and gas exchange worsens (Mukha et al., 2003). Moreover, the intensity of biological processes – and therefore oxygen consumption and CO₂ production – depends on moisture and temperature.

1.4. Other Gases in Soil Air

Besides oxygen, carbon dioxide, and nitrogen, soil air constantly contains (in very small amounts) volatile organic compounds – aliphatic and aromatic hydrocarbons, aldehydes, alcohols. They are formed during the life activity of soil microorganisms and can be absorbed by roots, promoting plant growth (Mukha et al., 2003).

In bog and waterlogged soils, soil air may contain appreciable amounts of ammonia (NH₃), methane (CH₄), and hydrogen (H₂) (Mukha et al., 2003). Ethylene (C₂H₄) plays a special role – a gas that, at high concentrations, inhibits root growth (White, 2006; Weil & Brady, 2017).

Let us summarise so far. Soil air is not a static environment but a dynamic soil component whose composition is determined by the balance between gas influx from the atmosphere and their consumption/release during biological processes. It is precisely this difference – lower oxygen and higher CO₂ – that makes continuous gas exchange between soil and atmosphere critically important. We turn to this crucial process in the next part of our lecture.

2.1. What Is Gas Exchange?

Gas exchange in soil is the continuous process of exchange of soil air with atmospheric air. It is through gas exchange that fresh oxygen – necessary for root and microbial respiration – enters the soil, and excess carbon dioxide, which at high concentrations can be toxic, is removed. This process is also called soil aeration (Mukha et al., 2003; White, 2006).

Aeration is not merely the presence of air in the pores. It is a dynamic process – the constant renewal of soil air. If gas exchange is disrupted, even if air is present in the soil, its composition rapidly shifts toward oxygen depletion and CO₂ enrichment, and roots begin to suffer from oxygen starvation. Therefore, for plants, it is not so much the air capacity of the soil (how much air is in the pores) that matters, but rather air permeability – the soil’s ability to transmit air through itself and exchange it with the atmosphere (Mukha et al., 2003).

2.2. Why Is Gas Exchange Necessary?

Gas exchange provides three vital functions:

1. Oxygen supply to roots. Plant roots respire – they consume oxygen to oxidise organic substances and obtain the energy needed for growth, water uptake, and ion absorption. Without a constant supply of O₂, respiration declines, and roots die (Eash et al., 2016; Weil & Brady, 2017).

2. Removal of excess CO₂. Carbon dioxide is continuously released during root and microbial respiration. In a well‑aerated soil its concentration stays at 0.1–1.5 %, but when gas exchange is impaired it can rise to 10 % or more (White, 2006). At such concentrations, CO₂ begins to inhibit root processes.

3. Creation of oxidising conditions. The presence of oxygen in soil air maintains a high redox potential (Eh), which is necessary for the proper course of many chemical transformations – for example, the conversion of ammonium nitrogen to nitrate form (nitrification), which is available to plants (Weil & Brady, 2017; Mukha et al., 2003).

2.3. How Is Gas Exchange Accomplished?

Gas exchange between soil and atmosphere occurs by two main mechanisms: mass flow (convection) and diffusion. Although both operate, they are not of equal importance.

2.4. Why Is Diffusion in Soil Slower Than in Air?

In free air, gas diffusion is very fast. But in soil, gas molecules collide not only with each other but also with solid particles, and they must navigate tortuous pores. As a result, diffusion through soil is always slower than in free air – by estimates, 2 to 20 times (Mukha et al., 2003).

The rate of gas diffusion in soil depends on two main factors:

1. Air‑filled porosity – that is, the volume of pores occupied by air rather than water. The more such pores, the faster the gas exchange. At an air‑filled porosity of about 20 %, gas exchange proceeds normally; when it drops to 8–12 %, it slows markedly (Mukha et al., 2003; White, 2006).

2. Tortuosity of pores – the more tortuous the path, the slower the diffusion. In structured soil, where there are large inter‑ and intra‑aggregate pores, diffusion is faster than in structureless soil (Eash et al., 2016; Weil & Brady, 2017).

Water has an especially strong effect on diffusion. When pores are water‑filled, gas exchange through them is practically impossible. The diffusion coefficient of oxygen in water is about 10 000 times smaller than in air (Weil & Brady, 2017; White, 2006). Therefore, waterlogging is the strongest factor disrupting gas exchange.

2.5. What Happens Under Normal Gas Exchange?

If the soil has good air‑filled porosity and no excess moisture, the concentration gradients of O₂ and CO₂ drive their counter‑flow, which maintains:

• O₂ content in soil air close to atmospheric levels (especially in upper horizons);
• CO₂ concentration not exceeding a few percent;
• active respiration of roots and microorganisms;
• normal course of oxidative chemical reactions.

When gas exchange is impaired, for example due to waterlogging, the entire system fails – and the consequences are discussed in Section 6.

Section conclusion. Gas exchange is the continuous exchange of gases between soil and atmosphere, providing oxygen inflow and CO₂ outflow. The main mechanism is diffusion, driven by concentration gradients and dependent on porosity, structure, and moisture. Diffusion rate is the key indicator determining whether the soil will be well aerated or not. In the next section, we examine in detail how gas diffusion in soil works and what equations describe it quantitatively.

3.1. What Is Gas Diffusion in Soil?

Diffusion is the spontaneous movement of gas molecules from a region of higher concentration (or partial pressure) to a region of lower concentration. In soil, diffusion is the primary mechanism of gas exchange, continuously supplying oxygen to roots and microorganisms and removing excess carbon dioxide (Weil & Brady, 2017; White, 2006).

Quantitatively, the diffusive flux is described by Fick's first law. For a one‑dimensional vertical gas flux in soil:

where:

• J_g – diffusive flux density (mass or volume of gas passing through a unit area per unit time);
• D_p – diffusion coefficient of the gas in soil (also called the effective diffusion coefficient);
• ∂C/∂z – concentration gradient of the gas with depth (Rolston & Moldrup, 2012; White, 2006).

The minus sign indicates the direction of flow: gas moves toward decreasing concentration.

It is important to understand: diffusion occurs not because of a difference in total air pressure, but because of differences in partial pressures of each individual gas. Thus, oxygen can enter the soil and carbon dioxide can leave it simultaneously, even if the total air pressure in the soil and atmosphere is the same (Weil & Brady, 2017).

3.2. Why Is Diffusion Slower in Soil Than in Free Air?

In free air, gas molecules collide mainly with each other, and their mean free path is about 10⁻⁵ cm. In soil pores, molecular motion is restricted by solid particles, and the pores themselves are tortuous. This leads to the effective diffusion coefficient in soil (D_p) always being smaller than the diffusion coefficient in free air (D₀) (Rolston & Moldrup, 2012; Marshall et al., 1996).

The ratio Dₚ/D₀ is called the relative diffusion coefficient and is always less than 1. The two main reasons for the reduction are:

1. Reduction of cross‑sectional area for diffusion. In soil, gas moves only through air‑filled pores. The volume of these pores is the air‑filled porosity (εₐ). The smaller εₐ, the smaller the area available for diffusion (Weil & Brady, 2017).

2. Increased path length (tortuosity). Solid particles and aggregates force gas molecules to go around obstacles, so the actual path a molecule travels from one point to another is significantly longer than the straight‑line distance. This effect is accounted for by the tortuosity (impedance) factor – α (or b), which is always less than 1 (White, 2006; Shukla, 2023).

Thus, the effective diffusion coefficient in soil can be written as:

where α is the impedance factor, accounting for pore tortuosity and connectivity (White, 2006). In well‑structured soil, α can reach 0.6, while in compacted or structureless soil it may fall to 0.1 or below (White, 2006).

3.3. How Do Moisture and Structure Affect Diffusion?

Water is the main enemy of gas diffusion. When pores are filled with water, gas exchange through them practically stops. The diffusion coefficient of oxygen in water is about 10 000 times smaller than in air (Weil & Brady, 2017; White, 2006). Even a thin water film on pore walls can significantly slow diffusion, because gas molecules must pass through it to reach the root or microbial surface (Weil & Brady, 2017).

Soil structure has a dual effect. In well‑aggregated soil there are:

• macropores (between aggregates) – they are large, well‑connected, and allow rapid diffusion;
• micropores (within aggregates) – they are small, often water‑filled, and diffusion in them is severely restricted.

Therefore, in aggregated soil, the main gas exchange occurs through macropores, while the interior of large aggregates may remain anaerobic even if the soil as a whole is well aerated (Weil & Brady, 2017; White, 2006). This phenomenon has important consequences for denitrification and other reductive processes.

3.4. Empirical Models for Estimating Dₚ

Since direct measurement of the diffusion coefficient in soil is laborious, empirical and semi‑empirical models have been developed to estimate Dₚ from more readily available parameters (porosity, moisture, bulk density). Some of the best‑known models include:

• Penman (1940): Dₚ/D₀ = 0.66εₐ – assumes a constant impedance factor of 0.66 (Marshall et al., 1996; Shukla, 2023).
• Marshall (1959): Dₚ/D₀ = εₐ^1.5 – accounts for the fact that pore connectivity improves with increasing air‑filled porosity (Marshall et al., 1996; Rolston & Moldrup, 2012).
• Millington (1959): Dₚ/D₀ = εₐ^4/3 – close to Marshall's model (Marshall et al., 1996).
• Millington and Quirk (1961): Dₚ/D₀ = εₐ^{10/3} / Φ², where Φ is total porosity. This model accounts for water blocking diffusion in narrow pores (Marshall et al., 1996; Shukla, 2023).

For undisturbed soils, more complex models that consider pore‑size distribution (e.g., Moldrup’s model using the Campbell b parameter) have been proposed (Rolston & Moldrup, 2012). However, for understanding the general picture, it is sufficient to know that the relative diffusion coefficient increases with increasing air content, and this relationship is non‑linear: at low εₐ values (below 0.1), diffusion slows sharply (Weil & Brady, 2017).

3.5. Diffusion Through Water Films and Within Aggregates

We noted earlier that diffusion in water is about 10 000 times slower than in air. This has two important implications:

1. For oxygen to reach roots and microorganisms, air‑filled pores are critically important. If a root or bacterium is surrounded by a continuous water film, oxygen must diffuse through water, creating a serious limitation (White, 2006).

2. Anaerobic zones may develop inside large aggregates. If an aggregate is wet, oxygen cannot diffuse to its centre faster than it is consumed by microorganisms. The critical radius of an aggregate at which the centre becomes anaerobic depends on the respiration rate and the diffusion coefficient in water (White, 2006). This critical radius can range from a few millimetres to a centimetre (Marshall et al., 1996).

Thus, even in a well‑aerated soil, micro‑sites with anaerobic conditions – “hot spots” of reductive processes – may exist (Weil & Brady, 2017).

3.6. Practical Significance of Diffusion for Agronomy

Knowledge of gas diffusion principles helps the agronomist understand why:

• structural soils (with good aggregation) are better aerated than structureless ones;
• compaction and crust formation severely impair gas exchange, even at moderate moisture;
• waterlogging is the most dangerous factor, because water not only displaces air from pores but also creates a barrier to diffusion.

Diffusion is the “silent engine” of gas exchange, operating continuously as long as a concentration gradient exists. Understanding how structure, moisture, and porosity affect diffusion allows correct decisions on tillage, drainage, and crop selection for specific conditions.

Section conclusion. Diffusion is the main mechanism of gas exchange in soil. Its rate is determined by the diffusion coefficient, which depends on air content, pore tortuosity, and moisture. Water slows diffusion by thousands of times, so maintaining optimal moisture and good structure is key to ensuring root oxygen supply. In the next section, we turn to an integrated indicator – soil respiration – which reflects the total intensity of biological processes that consume oxygen and release CO₂.

4.1. What Is Soil Respiration?

Soil respiration is the sum of all processes of oxygen consumption and carbon dioxide release by all living organisms inhabiting the soil: plant roots, microorganisms (bacteria, fungi, actinomycetes), and soil fauna (White, 2006; Eash et al., 2016). In essence, soil respiration is an integral indicator of biological activity of the entire soil ecosystem.

In a well‑aerated soil, respiration involves the oxidation of organic matter to CO₂ and water, releasing energy used by organisms for life processes:

This process is called aerobic respiration – it requires a constant presence of molecular oxygen (White, 2006; Mukha et al., 2003).

4.2. Who Breathes in the Soil?

The main consumers of oxygen in soil are (Mukha et al., 2003):

1. Plant roots – they consume considerable oxygen during active growth, water uptake, and ion absorption. Roots, especially in the absorption zone, have high respiratory activity. Moreover, roots release organic substances into the rhizosphere, which stimulate microbial respiration (Eash et al., 2016).

2. Microorganisms (bacteria, fungi, actinomycetes) – they are the main oxygen consumers in soil. Their abundance in the upper horizon can reach billions per gram of soil, and they continuously decompose organic matter, consuming oxygen and releasing CO₂ (Eash et al., 2016; Foth, 1990).

3. Soil animals (earthworms, nematodes, mites, insects) – although their total biomass is smaller than that of microorganisms, they also contribute to soil respiration (Foth, 1990).

It is important to emphasise that root and microbial respiration are closely interrelated. Roots release up to 25 % or more of photosynthetic products (organic acids, sugars, amino acids) into the rhizosphere, which serve as food for microorganisms. In return, microorganisms decompose organic matter and release mineral elements available to plants (Foth, 1990; Eash et al., 2016).

4.3. Respiratory Quotient (RQ)

To characterise the type of respiration, the respiratory quotient (RQ) is used – the ratio of the volume of CO₂ released to the volume of O₂ consumed (White, 2006; Weil & Brady, 2017):

During aerobic respiration, when carbohydrates are oxidised, RQ is close to 1 (White, 2006; Mukha et al., 2003). If organic acids or fats are oxidised, RQ may be less or greater than 1, but overall, for normally aerated soils, RQ is near unity.

Under anaerobic respiration (in oxygen‑deficient conditions), RQ becomes greater than 1, because CO₂ continues to be released while O₂ is no longer consumed (or consumed to a lesser extent) (White, 2006; Weil & Brady, 2017). This is an important diagnostic sign: an increase in RQ indicates a shift toward anaerobic processes.

4.4. What Determines the Intensity of Soil Respiration?

The intensity of soil respiration (rate of O₂ consumption or CO₂ release) depends on several factors acting together (White, 2006; Weil & Brady, 2017):

1. Temperature. Soil respiration strongly depends on temperature. In the range 0 to ~30–35 °C, the respiration rate roughly doubles for every 10 °C increase (Q₁₀ ≈ 2 rule) (White, 2006; Weil & Brady, 2017; Eash et al., 2016). This means that in summer, in warm weather, soil respiration is significantly more intense than in winter or early spring. The relationship is described by the equation:

where Rᵣ – respiration at temperature T, R₀ – respiration at 0 °C, Q – temperature coefficient (usually 2–2.5) (White, 2006).

2. Moisture. Respiration requires water, but not in excess. Optimal moisture for microbial respiration is typically 50–70 % of field capacity (White, 2006; Weil & Brady, 2017). Under waterlogging, oxygen availability drops sharply, and respiration slows or shifts to anaerobic type. Under severe drying, microbial activity also declines.

3. Availability of organic substrate (food) for microorganisms. The more fresh plant residues, root exudates, or organic fertilisers in the soil, the higher the respiratory activity (White, 2006; Eash et al., 2016). This is why soil respiration is usually higher under vegetation than on fallow (White, 2006).

4. Oxygen content in soil air. If oxygen is low (less than 5–10 %), aerobic respiration slows, and microorganisms switch to anaerobic pathways, altering the composition of gases and decomposition products (Weil & Brady, 2017).

4.5. Seasonal and Diurnal Dynamics of Respiration

In temperate climates, soil respiration shows a pronounced seasonal pattern (White, 2006; Weil & Brady, 2017):

• In spring, as the soil warms and microorganisms become active, respiration increases. It is especially intense after spring tillage, which loosens the soil and improves aeration.
• In summer, respiration reaches a maximum, especially in warm, humid weather. At this time, plants are actively growing, roots release many organic substances, and microorganisms actively decompose them.
• In autumn, respiration gradually declines as the soil cools and plants senesce.
• In winter, respiration is minimal, though it does not completely cease even at low temperatures.

Diurnal dynamics are less pronounced than seasonal ones, but they do exist: during the day, at higher temperatures, respiration intensifies; at night, it weakens.

4.6. Effect of Tillage on Respiration

Tillage has a strong effect on respiration (Weil & Brady, 2017; Mukha et al., 2003):

• Ploughing and loosening temporarily sharply increase respiration, as a large amount of oxygen enters the soil and microorganisms become active. However, in the long term, frequent tillage can lead to structure destruction, compaction, and a decline in overall biological activity.
• Minimum and no‑till usually maintain higher organic matter content in the top layer and more stable structure, which promotes uniform respiration throughout the season. However, in cold regions, mulch from crop residues may slow soil warming in spring and temporarily reduce respiration (Weil & Brady, 2017).

4.7. Why Is Soil Respiration an Important Agronomic Indicator?

Soil respiration is not merely an academic interest. This indicator allows us to assess:

• Biological activity of the soil – the higher the respiration, the more active the decomposition of organic matter and nutrient cycling.
• Rate of mineralisation – decomposition of organic matter releases nitrogen, phosphorus, sulphur, and other elements in plant‑available forms (Eash et al., 2016). The higher the respiration, the faster this process proceeds (but organic matter is also lost more quickly – this can be a drawback).
• Effect of agronomic practices – comparing respiration under different tillage, fertiliser, or green manure treatments helps evaluate which are most beneficial for biological activity.
• Aeration status – if respiration sharply drops or RQ becomes noticeably greater than 1, this signals oxygen deficiency and the development of anaerobic processes.

4.8. Link to Other Sections

Soil respiration is closely linked to:

• Gas exchange and diffusion (Sections 2 and 3) – it is diffusion that delivers oxygen to the sites of respiration and removes CO₂. If diffusion is slowed, respiration is limited by oxygen availability.
• Structure and porosity – good aggregation provides macropores for rapid diffusion, supporting high respiration.
• Temperature and moisture – these factors regulate respiration intensity and must be considered when interpreting its indicators.

Thus, soil respiration is an integral indicator that combines physical, biological, and chemical aspects of soil fertility.

Section conclusion. Soil respiration is the total oxygen consumption and CO₂ release by all organisms in the soil. It depends on temperature, moisture, organic matter availability, and aeration. The respiratory quotient helps diagnose the transition from aerobic to anaerobic conditions. Respiration is a vital indicator of soil biological activity; its measurement helps evaluate the effectiveness of agronomic practices and the state of soil fertility. In the next section, we examine the factor that determines how fast gases can move through the soil – gas permeability.

5.1. What Is Gas Permeability?

Gas permeability is the ability of soil to transmit air (or other gases) through itself under a gradient of total pressure (Shukla, 2023; Mukha et al., 2003). Unlike diffusion, which is driven by concentration gradients of individual gases, permeability drives the movement of air as a whole – mass flow (convection).

Imagine the difference: diffusion is like the slow seepage of individual molecules through tortuous pores, while gas permeability is like a stream of air being blown through the soil under a pressure difference. Both processes are important, but they operate differently and depend on different factors.

Quantitatively, gas permeability is described by Darcy's law (Shukla, 2023; Rolston & Moldrup, 2012):

where:

• q₉ – volumetric gas flow per unit area per unit time;
• kₐ – air permeability coefficient (usually in m² or cm²);
• μ₉ – dynamic viscosity of the gas;
• ∇P – gradient of total pressure.

It is important to note: unlike diffusion, where the driving force is the concentration gradient of a single gas, for permeability the driving force is the gradient of total pressure. This is why gas permeability is especially important in situations where pressure differences arise in the soil.

5.2. How Does Gas Permeability Differ from Gas Diffusion?

These two processes are often confused, so it is important to clearly distinguish them (Rolston & Moldrup, 2012; Shukla, 2023):

For the agronomist, the difference is important: diffusion operates continuously and provides the main gas exchange, while permeability becomes critically important during artificial ventilation, sharp barometric pressure changes, or when air is displaced by water during irrigation.

5.3. What Does Gas Permeability Depend On?

Soil air permeability is determined by the same factors as diffusion, but with some specific features (Shukla, 2023; Mukha et al., 2003; Rolston & Moldrup, 2012):

1. Pore size and connectivity. For mass flow of air, large, well‑connected pores are critically important. If pores are small or isolated, permeability drops sharply. This is why well‑structured soils with many macropores have high air permeability, while structureless, dense soils have low permeability (Mukha et al., 2003).

2. Soil moisture. Water in pores sharply reduces permeability: first, it occupies volume that could be air‑filled; second, water films block narrow pore necks, making them impassable for air. At moisture near field capacity, permeability can drop practically to zero (Shukla, 2023).

3. Bulk density (degree of compaction). Compaction reduces total pore volume and especially the volume of large macropores, so permeability decreases (Weil & Brady, 2017). This is clearly seen in soils subjected to heavy machinery traffic.

4. Soil structure. In structured soil, even at relatively high moisture, large inter‑aggregate pores remain through which mass flow of air can occur. In structureless soil, all pores are relatively fine, and permeability is much lower (Mukha et al., 2003).

5.4. Why Is Gas Permeability Important for the Agronomist?

Although the main gas exchange is provided by diffusion, permeability is important for a number of practical aspects:

1. Assessment of natural aeration potential. If air permeability is low, even with a concentration gradient, oxygen cannot rapidly penetrate into the soil depth. This is especially relevant for deep horizons and heavy clay soils (Mukha et al., 2003).

2. Forecasting the consequences of waterlogging. When water fills the pores, permeability drops sharply, and even after excess water drains, recovery of normal aeration may take a long time – especially if the soil is structureless or compacted.

3. Diagnosing compaction. Measurement of air permeability is one of the sensitive methods for assessing the degree of soil compaction. A decrease in permeability can be an early signal that the soil is overcompacted and needs loosening (Weil & Brady, 2017).

4. Design of drainage and reclamation systems. When draining waterlogged soils, it is important to know how fast air can enter the drained horizons. This depends on permeability.

5. Artificial ventilation (e.g., in greenhouses, during remediation). In some cases (e.g., contaminated sites where aerobic degradation of organic pollutants needs to be accelerated), forced air injection into the soil is used. The efficiency of such ventilation directly depends on air permeability (Shukla, 2023).

5.5. How Is Gas Permeability Measured?

For practical purposes, it is important to be able to assess permeability. The main methods (Shukla, 2023; Rolston & Moldrup, 2012) are:

• Laboratory method with constant pressure gradient. A soil sample (cylinder) is placed in an apparatus where air is passed through it at a known rate, and the pressure drop is measured. The permeability coefficient is calculated using Darcy’s law.

• Field method using an infiltrometer. A cylinder is driven into the soil, its top is sealed, and air is supplied at a known pressure while measuring the air flow rate. This method provides an estimate of permeability under undisturbed conditions.

• Indirect methods. Since direct measurement is laborious, permeability is often estimated indirectly – from particle‑size distribution, bulk density, air‑filled porosity, and structural state.

In practice, air‑filled porosity (the volume of pores occupied by air) is often used to assess air permeability. It is considered that at air‑filled porosity of about 20 % and above, gas exchange proceeds normally, while at 8–12 % it is strongly retarded (Mukha et al., 2003).

5.6. Gas Permeability and Agronomic Practices

Understanding gas permeability helps explain why certain agronomic practices affect aeration (Weil & Brady, 2017; Mukha et al., 2003):

• Ploughing and loosening temporarily sharply increase permeability, as they create many large pores. However, over time, under the influence of rainfall and compaction, these pores are destroyed, and permeability decreases.
• Minimum tillage preserves natural structure and earthworm channels, maintaining high permeability in the long term (provided there is no compaction).
• Application of organic fertilisers improves structure, promotes aggregate formation, and thus increases permeability.
• Liming of acid soils also improves structure, because calcium ions promote aggregation of clay particles.

It is important to remember: structured soil, even when saturated to capillary capacity, retains some large inter‑aggregate pores through which gas exchange is possible. In structureless soil, all pores become water‑filled upon wetting, and permeability is practically lost (Mukha et al., 2003).

Section conclusion. Gas permeability is the ability of soil to transmit air under a pressure gradient. It differs from diffusion in that it provides mass flow of air as a whole. Permeability depends on pore size and connectivity, moisture, density, and structure. Although diffusion is the main gas‑exchange mechanism, permeability is important for assessing the soil’s ability to rapidly renew air, especially under waterlogging or compaction. Good structure and optimal moisture are key to maintaining high air permeability. In the next section, we discuss what happens when permeability is disrupted – turning to consequences of waterlogging and redox processes.

6.1. Waterlogging – Disruption of Gas Exchange

Waterlogging of soil is a state in which most pores are filled with water and air is displaced. When the soil becomes water‑saturated, the oxygen content in soil air drops sharply, and CO₂ concentration rises. Water in pores not only displaces air but also creates an insurmountable barrier to diffusion: the diffusion coefficient of oxygen through water is about 10 000 times smaller than through air (Weil & Brady, 2017; White, 2006).

As a result, diffusion practically ceases, and gas exchange between soil and atmosphere is disrupted. Under such conditions, soil aeration becomes critically insufficient for normal respiration of roots and aerobic microorganisms.

6.2. What Happens to Plant Roots?

Under oxygen deficiency, plant roots experience oxygen starvation (hypoxia or anoxia). This triggers a chain of negative processes (Weil & Brady, 2017; White, 2006; Mukha et al., 2003):

1. Slowing of root respiration. Respiration is the main energy source for root growth, water and ion uptake. When oxygen is low, respiration slows, and energy supply is impaired.

2. Impairment of water and nutrient uptake. Roots lose the ability to actively absorb ions, as this process requires energy. As a result, even with water present in the soil, plants may show wilting symptoms (due to reduced root membrane permeability to water), and also suffer from nutrient deficiencies (Eash et al., 2016; Weil & Brady, 2017).

3. Shift to anaerobic metabolism. Under oxygen deficiency, roots switch to glycolysis – anaerobic breakdown of carbohydrates, which yields much less energy (only 2 molecules of ATP per glucose molecule instead of 36–38 under aerobic respiration). Ethanol and other toxic products are formed (Weil & Brady, 2017; White, 2006).

4. Slowed growth and root death. Under prolonged waterlogging, roots may completely die, leading to whole‑plant depression (Weil & Brady, 2017).

The sensitivity of plants to waterlogging varies greatly. The most tolerant are hydrophytes (rice, mangroves, sedges), which possess specialised air‑conducting tissues – aerenchyma – enabling oxygen transport from leaves to roots (Weil & Brady, 2017; Eash et al., 2016). Most field crops (wheat, maize, soybean, potato) are very sensitive and can be seriously damaged within a few days of flooding (Weil & Brady, 2017).

6.3. What Happens to Microorganisms?

Microorganisms can be divided into three groups with respect to oxygen (Eash et al., 2016; White, 2006):

1. Obligate aerobes – require oxygen for respiration and cannot live without it. These include most decomposer bacteria, nitrifiers. Under waterlogging their activity sharply decreases.

2. Facultative anaerobes – can respire both with and without oxygen, using nitrate, iron and manganese oxides, sulphate, and other compounds as alternative electron acceptors. They become active when O₂ is lacking.

3. Obligate anaerobes – live only in oxygen‑free conditions. They use not oxygen but other oxidised compounds (nitrates, sulphates, CO₂) for respiration. They dominate under prolonged waterlogging.

When oxygen is depleted, a sequential use of different electron acceptors begins – in order of decreasing energy yield of each reaction (Weil & Brady, 2017; White, 2006). This sequence is directly related to the redox potential.

6.4. Redox Potential (Eh)

To quantitatively characterise the redox state of soil, the redox potential (Eh) is used – the electrical potential that develops between a platinum electrode and the soil solution (Eash et al., 2016; Weil & Brady, 2017; White, 2006; Mukha et al., 2003). It is measured in millivolts (mV) with a potentiometer using a platinum electrode and a reference electrode.

What does Eh show? It is a measure of how readily the medium accepts or donates electrons. The higher the Eh, the more oxidising the conditions (plenty of oxygen, electron donation). The lower the Eh, the more reducing the environment, and electron acceptance occurs.

Ranges of Eh values (Weil & Brady, 2017; Mukha et al., 2003; White, 2006):

• 0.4–0.7 V (400–700 mV) – normal oxidising conditions characteristic of well‑aerated soils.
• 0.3–0.4 V (300–400 mV) – oxygen is already scarce; reductive processes begin (first denitrification).
• 0.1–0.3 V (100–300 mV) – manganese (Mn⁴⁺ → Mn²⁺) and iron (Fe³⁺ → Fe²⁺) compounds are reduced.
• below 0 V – strongly reducing conditions; sulphates are reduced to hydrogen sulphide (H₂S) and CO₂ to methane (CH₄).

Important: Eh is strongly pH‑dependent. As pH increases, the Eh value for the same reaction decreases (Weil & Brady, 2017). Therefore, in practice, corrected values or the rH₂ index, which accounts for both Eh and pH, are used (Mukha et al., 2003).

6.5. Sequence of Processes as Eh Decreases

When soil becomes waterlogged, oxygen is rapidly consumed by aerobic organisms. After its depletion, a sequential use of alternative electron acceptors begins (Weil & Brady, 2017; White, 2006):

1. Nitrate reduction (denitrification). At Eh ~0.28–0.22 V (at pH 6.5), microorganisms begin to use nitrates (NO₃⁻) as electron acceptors, reducing them to N₂, N₂O, or NO. This leads to nitrogen losses from the soil as gases – a problem important to the agronomist.

2. Manganese reduction. At Eh ~0.22–0.18 V, Mn⁴⁺ oxides are reduced to Mn²⁺, which becomes mobile and can be toxic to plants at high concentrations (Weil & Brady, 2017).

3. Iron reduction. At Eh ~0.11–0.08 V, Fe³⁺ (in oxides and hydroxides) is reduced to Fe²⁺, which also goes into solution. This changes soil colour – grey or bluish shades appear (Eash et al., 2016; Mukha et al., 2003). Dissolved iron can be toxic, but in some cases its reduction improves phosphorus availability.

4. Sulphate reduction. At Eh ~ −0.15…−0.17 V, sulphates (SO₄²⁻) are reduced to hydrogen sulphide (H₂S), which is toxic to roots and has a characteristic rotten‑egg odour (Weil & Brady, 2017).

5. CO₂ reduction to methane (CH₄). At Eh ~ −0.2…−0.28 V, methanogenic bacteria reduce CO₂ to methane. This is typical for bog soils and rice paddies (Weil & Brady, 2017; White, 2006).

This sequence is called the redox series (Weil & Brady, 2017). It is very important because it determines which elements will be lost, accumulated, or change their availability when the soil is waterlogged.

6.6. What Other Processes Occur Under Waterlogging?

In addition to the above reductive reactions, waterlogging causes:

• Accumulation of toxic organic acids (acetic, butyric, lactic) and alcohols. They are formed during incomplete oxidation of organic matter under anaerobic conditions and can inhibit roots, especially seedlings (White, 2006).

• Ethylene (C₂H₄) release. This gas, even at very low concentrations, inhibits root growth and causes various disorders (leaf epinasty, stem thickening). Ethylene is formed during anaerobic decomposition of organic matter at the interface between aerobic and anaerobic zones (White, 2006; Weil & Brady, 2017).

• pH increase. Reduction of iron and manganese consumes hydrogen ions (H⁺), so pH in the reduction zone may rise (Weil & Brady, 2017; White, 2006).

• Change in soil colour. Oxidised iron compounds give red, yellow, brown tones; reduced ones give grey, bluish, greenish tones. From soil colour, an experienced pedologist can judge aeration conditions (Eash et al., 2016; Mukha et al., 2003).

6.7. What Are Hydromorphic and Hydric Soils?

Soils that are regularly or permanently waterlogged are called hydromorphic. They have characteristic features:

• Gley horizon – with bluish, grey, greenish tones due to reduced iron and manganese compounds (Eash et al., 2016; Mukha et al., 2003).
• Accumulation of organic matter (decomposition is slowed under oxygen deficiency, leading to peat horizons).
• Specific microflora – anaerobes and facultative anaerobes dominate.

Soils that are hydromorphic permanently (bog, peat soils) are classified as hydric (Weil & Brady, 2017). They have special properties: low redox potential, high water‑holding capacity, but poor aeration. However, even such soils can be productive – for example, rice paddies where water is artificially maintained to suppress weeds (Weil & Brady, 2017).

6.8. Practical Conclusions for the Agronomist

1. Waterlogging is not just “too much water”. It is a shutdown of gas exchange, a sharp drop in Eh, and a shift to anaerobic processes. The consequences for plants and soil fertility can be catastrophic.

2. Symptoms of waterlogging: plant stress, leaf yellowing, root death, grey or bluish spots in the soil, hydrogen sulphide odour, gas bubbles in the waterlogged layer.

3. Remedial measures: drainage (removal of excess water), raised beds or ridges to improve water runoff, loosening to restore structure and porosity, addition of organic matter to improve structure (Eash et al., 2016; Weil & Brady, 2017).

4. Crop selection. In areas with periodic waterlogging, tolerant crops should be chosen (e.g., some rice varieties, oats, perennial grasses). Sensitive crops (potato, tomato, legumes) require particularly careful water management.

Section conclusion. Soil waterlogging is a critical disruption of gas exchange that triggers a chain of negative biological and chemical processes. The drop in redox potential (Eh) leads to sequential reduction of nitrates, manganese and iron oxides, sulphates, and CO₂, with formation of gaseous products (N₂O, H₂S, CH₄) and mobile metal forms. Plants suffer oxygen starvation, growth is inhibited, and ultimately yield declines. In the next section, we conclude our lecture by examining the connection of the gas phase with chemical processes occurring in the soil.

7.1. Why Does the Gas Phase Determine Soil Chemistry?

The soil gas phase is not just “air in the pores”. It is a crucial regulator of chemical processes occurring in the soil. Oxygen acts as the primary oxidant, and its presence or absence determines the forms in which many chemical elements exist, how available they are to plants, and whether toxic compounds are formed (Weil & Brady, 2017; Mukha et al., 2003).

In essence, aeration determines the redox state of the soil, and the redox state determines the chemical fate of elements. Therefore, understanding the gas regime is key to understanding soil chemistry and fertility.

7.2. Oxygen as the Primary Oxidant

In a well‑aerated soil, oxygen is the main electron acceptor during oxidation of organic matter and many mineral compounds. It participates in:

• Oxidation of organic matter to CO₂ and water (mineralisation) – this process releases nutrients bound in organic forms (Eash et al., 2016).
• Oxidation of reduced forms of elements – for example, conversion of Fe²⁺ to Fe³⁺, Mn²⁺ to Mn⁴⁺, leading to their precipitation as immobile oxides and hydroxides.
• Nitrification – oxidation of ammonium nitrogen (NH₄⁺) to nitrate (NO₃⁻), available to plants (White, 2006; Weil & Brady, 2017).

In the absence of oxygen (under anaerobic conditions), these processes cease or are replaced by reductive reactions that radically alter the chemical character of the soil.

7.3. Redox‑Sensitive Elements

Redox potential (Eh) and pH together determine the forms in which many elements exist in soil. For some, this dependence is particularly important (Weil & Brady, 2017; White, 2006):

7.4. Gaseous Products of Chemical Transformations

Many chemical and biochemical processes in soil are accompanied by gas release. The most important are:

1. Carbon dioxide (CO₂) – released during root and microbial respiration, as well as during chemical decomposition of carbonates by acids (Mukha et al., 2003).

2. Methane (CH₄) – formed under strongly reducing conditions (wetlands, rice paddies). It is a greenhouse gas (Weil & Brady, 2017; Eash et al., 2016).

3. Nitrous oxide (N₂O) and molecular nitrogen (N₂) – formed during denitrification. N₂O is a strong greenhouse gas, while N₂ is inert and lost irreversibly (Weil & Brady, 2017; White, 2006).

4. Hydrogen sulphide (H₂S) – formed by sulphate reduction under anaerobic conditions. Toxic to roots (Weil & Brady, 2017).

5. Ammonia (NH₃) – may be released during decomposition of organic matter or upon application of ammonium fertilisers in alkaline conditions (Eash et al., 2016).

6. Ethylene (C₂H₄) – formed under anaerobic conditions, inhibits root growth even at low concentrations (White, 2006; Weil & Brady, 2017).

7.5. Link between Eh and pH: How Aeration Affects Acidity

As noted, Eh and pH are closely related (Weil & Brady, 2017; White, 2006; Mukha et al., 2003):

• Reduction reactions, especially those of iron and manganese, consume H⁺ ions, so pH in the reduction zone often rises (Weil & Brady, 2017).
• Nitrification, by contrast, is an acid‑forming process: oxidation of NH₄⁺ to NO₃⁻ releases H⁺ ions, acidifying the soil (Eash et al., 2016).
• Therefore, in well‑aerated soils natural acidification often occurs, while in waterlogged soils pH may increase.

This is important for the agronomist: on waterlogged soils, pH adjustment may be required; on well‑aerated soils, acidity control is needed when applying ammonium fertilisers.

7.6. Aeration and Nutrient Availability

The link between aeration and nutrient availability can be summarised as follows (Eash et al., 2016; Weil & Brady, 2017; White, 2006):

7.7. Practical Conclusions

1. Maintaining aeration means maintaining the right chemistry. For nutrients to be available to plants and toxic compounds not to accumulate, oxidising conditions must be maintained in the root zone.

2. Waterlogging changes not only physics but also chemistry. It does not simply “flood” the roots with water – it triggers reductive processes that can make the soil toxic or deficient in nutrients.

3. Drainage is a key measure. In regions with excess moisture, drainage is not just an agronomic practice but a way to manage soil chemical processes (Weil & Brady, 2017).

4. Fertiliser application should take aeration into account. For example, applying nitrate fertilisers to waterlogged soil is pointless – they are quickly reduced to gaseous forms and volatilise. On waterlogged areas, ammonium forms are preferable (Eash et al., 2016).

5. Aeration affects crop choice. Sensitive crops require good aeration; tolerant ones can grow even under temporary waterlogging.

Section conclusion. The soil gas phase is not just air – it is an active regulator of chemical processes. Oxygen, as the primary oxidant, determines the forms in which elements exist: oxidised (usually immobile, but often available) or reduced (often mobile, but sometimes toxic). Aeration governs the redox potential, which in turn determines the fate of nitrogen, iron, manganese, sulphur, carbon, and many other elements. Therefore, managing the soil gas regime means managing its chemical fertility.