Soil Physics

Why Is It Important to Study Processes?

Some soil properties, such as particle size distribution (the ratio of sand, silt, and clay), change slowly over geological epochs. They are often called static. However, the physical state of the soil is something else. It is a dynamic characteristic that can change within a single season or even within a few hours (Shukla, 2023).

Imagine that the chemical composition of two soil plots is absolutely identical. However, on one plot the soil has a good, granular structure (aggregates), while on the other it is compacted and structureless. The physical processes in these two soils will differ radically.

• In well‑structured soil, water will infiltrate quickly, plant roots will easily penetrate deep, and air will circulate freely.
• In compacted soil, water will stagnate on the surface, causing erosion, and roots will suffer from oxygen deficiency.

Consequently, the yield on these two plots will differ, even with identical chemical indicators. This very difference is explained by soil physics (Weil & Brady, 2017).

Thus, soil physics studies not just properties per se, but how these properties govern the movement of water, air, heat, and dissolved substances, creating an environment for plant and microbial life.

In the context of our course, we will consider soil physics as the science of the spatial and temporal organisation of the solid, liquid, and gaseous phases of the soil and of the transport processes that occur within this organisation. This knowledge is the foundation for understanding how to manage fertility, protect soil from degradation, and ensure sustainable agriculture.

In the next chapter, we will move on to considering soil as a multiphase system in order to understand what, in fact, constitutes the “world” in which all these processes take place.

Solid Phase (Skeleton and Matrix)

The solid phase is the foundation of the soil, its “skeleton”. It constitutes, on average, about 50% of the total volume of the soil in the plough layer (Eash et al., 2016). This volume is represented by two main groups of components:

Mineral particles: These are the “building blocks” of the soil, formed by weathering of rocks. They differ in size and form the basis of particle size distribution (texture):

• Sand (2–0.05 mm) – large particles clearly visible to the eye. They create large pores through which water and air pass easily, but they themselves hardly retain moisture and nutrients (Eash et al., 2016; Weil & Brady, 2017).
• Silt (0.05–0.002 mm) – medium‑sized particles. They give the soil a “floury” texture, smoothness to the touch, and have a greater surface area than sand, allowing them to retain water better (Weil & Brady, 2017).
• Clay (<0.002 mm) – the smallest and most important particles. Due to their tiny size, they possess enormous specific surface area (surface per unit mass) (Foth, 1990). It is on the surface of clay particles that key chemical reactions occur; they also retain most of the moisture, but at the same time can swell greatly when wet and shrink when dry.

Organic matter (humus): This is the product of decomposition of plant and animal residues. Although its proportion in mineral soils may be small (1–5%), its role is colossal. Humus is the “glue” that binds sand, silt, and clay into larger structural units – aggregates (Foth, 1990). It has a huge surface area and can hold dozens of times more water per unit mass than the mineral part.

Key idea: Solid particles do not simply lie separately from each other. They form the soil matrix – a spatial lattice in which pores are formed (Huang, 2012). It is the ratio of the solid phase to the pore space that determines how much space remains for water and air.

Liquid Phase (Soil Solution)

The liquid phase is soil moisture, or the soil solution. It fills part of the pores between solid particles. As a rule, its share in the soil volume can range from 0% (in dry desert) to 50% (in waterlogged soil or bog) (Scheffer et al., 2018).

It is important to understand that this is not just “tap water”. It is a complex, concentrated solution of salts, organic acids, and gases (Foth, 1990). It is in the liquid phase that:

• Nutrients dissolve and move to plant roots.
• Most chemical reactions take place.
• Pollutants are transported.

Soil physics considers water not only as a substance but also as a carrier of energy. Water in the soil is under the influence of:

• Matric forces (attraction to the surface of solid particles).
• Osmotic forces (due to salts in the solution).
• Gravitational forces (weight of water) (Foth, 1990; Weil & Brady, 2017).

This energy status determines whether water will move upward, downward, or sideways, and whether the plant can “pull” it out of the soil.

Gaseous Phase (Soil Air)

The remaining part of the pores (not occupied by water) is filled with soil air. On average, in well‑structured moist soil it accounts for about 20–25% of the volume, but this figure changes greatly after rain or during drought (Eash et al., 2016).

The composition of soil air differs significantly from that of the atmosphere. This is due to the constant respiration of plant roots and soil microorganisms, which consume oxygen (O₂) and release carbon dioxide (CO₂):

• Oxygen in soil air is lower (can drop to 10–15% versus 21% in the atmosphere).
• Carbon dioxide is tens and hundreds of times higher (reaching 1–5% versus 0.03%) (Foth, 1990).

The presence of oxygen is a critical condition for the life of roots and most beneficial microorganisms. It is the lack of air, not excess water, that is the main cause of plant death in waterlogged soils.

Thermal Energy (Temperature)

Soil temperature is not just a physical characteristic but an active factor regulating all other processes. It belongs to the thermal phase of the system.

• Effect on water: Temperature determines the viscosity of water and the speed of its movement, as well as the intensity of evaporation.
• Effect on air: Gas diffusion in the soil depends on temperature.
• Effect on biology: Temperature is the main regulator of the rate of chemical reactions, enzyme activity, and the growth of microorganisms and roots. Chemical processes and microbial activity are highly dependent on temperature (Foth, 1990).

All Phases Interact!

Phases cannot be considered in isolation. It is their interaction that creates the unique properties of the soil (see Scheme 1).

• Solid phase ↔ Liquid phase: Soil structure (aggregates) determines how quickly water will infiltrate. Water, in turn, causes clay swelling and affects the bonding of particles.
• Liquid phase ↔ Gaseous phase: The more water in the pores, the less air there is. This is called aeration.
• Thermal ↔ Liquid ↔ Gaseous: Warm water evaporates faster, transferring heat in the form of vapour to colder layers (latent heat transfer effect). Cold air can condense moisture (Scheffer et al., 2018).

Scheme 1. Soil as a multiphase system and its interactions.

Figure 1. Interrelation of the four phases in soil and their influence on key processes.

Thus, soil is not just a mixture of earth, water, and air. It is a dynamic, multicomponent system in which a change in one parameter (for example, compaction destroying structure) immediately triggers a chain reaction of changes in water, air, and thermal regimes. Understanding this system is the basis for managing the physical condition of the soil, which we will study further.

From Chemical Composition to Yield: The Key Role of Structure

Imagine a hypothetical situation: two neighbouring fields with soils having identical chemical composition – the same content of nitrogen, phosphorus, potassium, and the same pH. However, the yield on these fields can differ several times. Why? Because the physical state of the soil, namely its structure, differs radically.

Let us examine this cause‑and‑effect chain in more detail:

1. Identical chemical composition → Different structure.

On the first field, the soil has a well‑developed granular (crumb) structure. It resembles cottage cheese or large breadcrumbs. Such structure is formed due to the high content of organic matter, which binds mineral particles into stable aggregates (Weil & Brady, 2017). On the second field, the soil is structureless, pulverised, or, on the contrary, massive and compacted. There are no strong bonds between individual particles and aggregates, or they have stuck together into a continuous mass. Most often, this condition results from intensive tillage, overcompaction by heavy machinery, or loss of humus (Foth, 1990).

2. Different structure → Different water movement.

In well‑structured soil, there are large pores (macropores) between aggregates and small pores (micropores) inside aggregates (Huang, 2012; Weil & Brady, 2017). This two‑tiered system works perfectly:

• Macropores ensure rapid entry of water during rain or irrigation (infiltration) and drainage of excess downwards, preventing stagnation.
• Micropores act as capillaries, holding moisture available to plants, drawing it from large pores.

In structureless soil, the picture is different. If it is pulverised, all pores are roughly equally small, water infiltrates slowly, often forming a crust on the surface. If it is compacted, large pores are destroyed, total pore volume is small, and water either stagnates on the surface or percolates very slowly downward (Weil & Brady, 2017).

3. Different water movement → Different aeration.

Here the third component of our three‑phase system comes into play – air. Since the volume of pores is finite, the more space water occupies, the less remains for air.

• In structured soil, water goes into micropores, and macropores are filled with air. Roots breathe, microorganisms work actively. The soil is said to be well aerated.
• In structureless, especially waterlogged or compacted soil, water displaces air from most pores. Oxygen deficiency (anaerobiosis) occurs. Under such conditions, roots cannot breathe, their growth stops, and they begin to die (Eash et al., 2016; Foth, 1990).

4. Different aeration and moisture supply → Different roots.

The root system is both the “mouth” and the “lungs” of the plant.

• In structured soil, roots easily penetrate deep through large pores without encountering mechanical resistance. They branch actively, exploring a large volume of soil and extracting water and nutrients. Root hairs closely contact soil aggregates (Weil & Brady, 2017).
• In compacted soil, roots cannot break through the dense mass. Their growth is restricted to the surface layer. They become thin, weak, often distorted. The contact area with soil is small, so the plant cannot obtain the required amount of resources even if they are present in the soil (Shukla, 2023).

5. Different roots → Different yield.

This is the final and most obvious result. A plant with a strong, healthy root system, supplied with water and oxygen, can produce a high yield. A plant suffocating in waterlogged or “entombed” in dense soil will be stressed, often diseased, and its productivity will be minimal.

Graphic Illustration of the Principle

Below is a diagram schematically summarising this logical chain:

Figure 2. Influence of the physical state of the soil (structure) on root growth and yield under identical chemical composition.

Conclusion: Physics Creates the Conditions

From this chain, the main idea becomes clear: physical properties of the soil are not just “characteristics” but the “infrastructure” of fertility. Chemistry and biology can supply the plant with nutrients, but without the correct physical “packaging”, this nutrition will not be delivered. Soil physics creates the conditions for:

• Efficient use of chemical fertilisers: if water does not deliver ions to the roots, fertilisers remain useless ballast or even pollutants in the soil.
• Active biological life: aeration and optimal moisture are key factors for the work of decomposers, which break down organic matter and convert it into available forms.
• Realisation of the genetic potential of plants: only under favourable physical conditions can the plant fully use its hereditary capabilities to form yield.

Thus, the agronomist's work begins not with chemical analysis, but with an assessment of the physical condition of the soil. It is the understanding of processes occurring in the solid, liquid, and gaseous phases that allows making the right decisions: when and how to till the soil, how to irrigate, which crops to choose.

In the next part, we will move on to a specific list of key processes studied by soil physics in our course.

1. Water Movement in Soil (Soil Hydrology)

This is perhaps the most fundamental and thoroughly studied process in soil physics. It includes several key stages:

• Infiltration – the process of water entering the soil through its surface (Radcliffe & Šimůnek, 2012; Weil & Brady, 2017). Infiltration determines what portion of atmospheric precipitation or irrigation water will enter the soil and what will run off the surface, causing erosion.
• Water transport in unsaturated soil (capillary movement) – the movement of water through pores under the action of capillary forces (matric potential) (Foth, 1990; Weil & Brady, 2017). This process ensures the supply of moisture to plant roots from wetter soil layers.
• Drainage (percolation) and perched water table – downward movement of water under the influence of gravity (gravitational potential) (Radcliffe & Šimůnek, 2012). The speed of this process determines how quickly the soil will get rid of excess moisture. If a water‑restricting layer (e.g., a clay horizon) is encountered, a perched water table may form – temporary or permanent saturation of the upper soil layers, which is extremely unfavourable for most crops.
• Capillary rise – upward movement of water from the groundwater table into the root zone. This process is especially important in arid regions and on floodplain soils.

Why is this important for the agronomist? Water movement is the “circulatory system” of the soil. It determines plant water supply, as well as the transport of dissolved nutrients and pollutants. By managing this process (e.g., through drainage or irrigation), we directly affect yield.

2. Air Movement in Soil (Gas Exchange, Aeration)

Air movement in soil is the process of gas exchange between the soil and the atmosphere. It occurs mainly through diffusion – the movement of gas molecules from areas of high concentration to areas of low concentration (Radcliffe & Šimůnek, 2012; Scheffer et al., 2018).

• Soil respiration: The main driving force of gas exchange is the consumption of oxygen (O₂) by roots and microorganisms and the release of carbon dioxide (CO₂). As a result, in soil air the content of (O₂) is reduced and (CO₂) is increased compared to the atmosphere (Foth, 1990).
• Role of macropores: It is large pores (macropores) that serve as the main pathways for gas exchange. If macropores are filled with water, gas diffusion slows sharply, and oxygen deficiency occurs.

Why is this important for the agronomist? Aeration is the “breathing” of the soil. Without oxygen supply, normal development of roots and aerobic microorganisms that convert organic matter into plant‑available forms is impossible. Compaction and waterlogging are the main enemies of good aeration.

3. Heat Movement in Soil (Thermal Regime)

The thermal regime of the soil is determined by the processes of input, transfer, and dissipation of thermal energy. The main mechanisms:

• Thermal conductivity – the transfer of heat from warmer parts of the soil to colder parts through direct contact of solid particles, water, and air (Scheffer et al., 2018).
• Convection – transport of heat together with moving water or air.
• Latent heat exchange – transfer of heat in the form of latent heat of evaporation and condensation. Evaporation of water from the soil surface cools it, while condensation of water vapour in colder layers warms it (Foth, 1990).

Why is this important for the agronomist? Temperature is the “pulse” of life in the soil.

• It determines the rate of seed germination, root growth, and microbial activity (Weil & Brady, 2017).
• The rate of chemical reactions and availability of nutrients depend on it.
• Knowledge of the thermal regime is necessary for choosing optimal sowing dates, using mulching, and other agronomic practices.

4. Mechanical State of Soil (Stresses and Deformations)

This process describes the behaviour of soil under the action of external forces. It is closely related to soil strength and its ability to resist loads.

• Compression and compaction: The process of reducing pore volume under load (weight of tractors, agricultural machinery, static pressure) (Scheffer et al., 2018; Shukla, 2023). Compaction is one of the main anthropogenic problems in modern agriculture.
• Shear (deformation) and plasticity: The ability of soil to change its shape without rupturing continuity (e.g., formation of ruts or plough pans). This property depends on moisture: wet clay is plastic, dry clay is brittle.
• Shrinkage and swelling: Especially characteristic of soils rich in clays (especially montmorillonite type). When wetted, such soils swell, closing pores, and when drying, they shrink and crack (Weil & Brady, 2017).

Why is this important for the agronomist? Mechanical state directly relates to the “physical health” of the soil. Compaction worsens all other processes – reduces infiltration, aeration, hinders root growth. Understanding the mechanical behaviour of soil allows choosing the right machinery, determining optimal timings for field work (especially when the soil is neither too wet nor too dry), and avoiding irreversible degradation of structure.

5. Transport of Dissolved Substances (Mass Transport, Convection and Dispersion)

This process is the “bridge” between soil physics and soil chemistry. Together with moving water, dissolved substances are also transported:

• Nutrients (ions (NO₃^-), (K⁺), (Ca²⁺), etc.).
• Metabolic products of plants and microorganisms.
• Pollutants (pesticides, heavy metals, nitrates).

The main mechanisms of mass transport are:

• Convection (advection): Passive transport of substances along with the water flow.
• Diffusion: Movement of ions and molecules from areas of high concentration to areas of low concentration.
• Hydrodynamic dispersion: “Smearing” of the concentration front due to differences in water velocities in different pores (Radcliffe & Šimůnek, 2012).

Why is this important for the agronomist? This process determines whether nutrients will reach the roots or be leached beyond the root zone, and how quickly pollutants can reach groundwater. Understanding mass transport is the key to efficient and environmentally safe fertiliser application.

Interrelation of Processes: A Comprehensive Picture

All these processes are closely intertwined. For clarity, let us present their interrelation in the form of a diagram:

Figure 3. Interrelation of physical processes in soil and their influence on fertility.

As can be seen from the diagram, all processes are united through the common medium – the pore space and soil structure. A change in one process (e.g., compaction that destroys structure) triggers a cascade of changes in all the others. That is why soil physics cannot be studied as a set of separate topics – it is a unified, holistic science about the life of the soil as a physical body.

In the next, concluding part of the lecture, we will briefly consider how soil physics interacts with related disciplines to complete the overall picture.

5.1 Physics Creates Conditions for Chemical Processes (Agrochemistry)

Chemical reactions in the soil, such as mineral dissolution, ion exchange, oxidation‑reduction, and formation of complex compounds, occur in an aqueous medium – the soil solution (Weil & Brady, 2017). Therefore:

• Water movement determines the transport of reagents to the reaction site and removal of products. Without water movement, chemical reactions quickly fade due to local depletion or accumulation of substances.
• Water regime (moisture, capillary rise, drainage) directly affects salt solubility, nutrient availability, and the rate of chemical weathering (Foth, 1990).
• Structure and porosity determine the contact area of the solid phase with the solution, and thus the rate of exchange processes. Well‑aggregated soil has a developed internal surface where chemical reactions actively occur.

Thus, agrochemistry studies what substances and in what quantities are present in the soil, but it is physics that determines how effectively these substances will move to the roots and be taken up by plants. In our lectures, we will not delve into chemical mechanisms, but we will always remember that the physical state of the environment is a limiting factor for the action of fertilisers and amendments.

5.2 Physics Creates Conditions for Biological Activity (Soil Biology and Microbiology)

The vital activity of soil organisms – from bacteria and fungi to earthworms and plant roots – is possible only when three key conditions are met:

1. Sufficient moisture (liquid phase) for metabolic processes and transport of nutrients.

2. Available oxygen (gaseous phase) for aerobic respiration of most beneficial microorganisms and roots themselves (Foth, 1990).

3. Optimal temperature (thermal energy) to maintain enzymatic activity.

All these conditions are created and regulated by physical processes. For example:

• Aeration (gas exchange) determines whether aerobic processes (organic matter decomposition, nitrification) or anaerobic processes (denitrification, production of toxic substances) will predominate in the soil. This affects not only plant nutrition but also greenhouse gas emissions.
• Soil structure provides diverse niches for microorganisms (micropores for bacteria, macropores for protozoa and fungi) (Huang, 2012).
• Water and thermal regimes determine the seasonal dynamics of biological activity, e.g., enhanced mineralisation of organic matter in spring and its slowing down in hot dry summers.

Therefore, soil biology studies the inhabitants of the soil and their functions, but it is physics that creates their habitat. In our course, we will not discuss biological processes in detail, but we will consider that any change in physical properties immediately affects the biota.

5.3 Physics as the Foundation of Fertility (Crop Science and Crop Production)

Ultimately, all physical processes are aimed at providing plants with the factors of life: water, air, heat, and available nutrients. This is why soil physics underlies crop science and crop production.

• Tillage: The aim of all agronomic practices (ploughing, cultivation, chiselling, minimum tillage) is to change physical properties (structure, density, porosity) in the desired direction: create a loose plough layer, break up plough pans, improve infiltration, enhance aeration. Crop science is, in essence, the management of the physical condition of the soil.
• Reclamation: Drainage and irrigation are direct interventions in the water regime; liming and gypsum application are often aimed not only at chemistry but also at improving structure (clay flocculation).
• Soil erosion control: Erosion is a physical process (removal of particles by water or wind), and its control is based on physical principles (preserving structure, increasing surface roughness, creating plant cover).

In our course, we will not analyse specific tillage practices – that is the subject of a separate discipline. We will focus on understanding why and how these practices work at the physical level.

Brief Summary: The Place of Physics among Soil Sciences

The interaction of soil physics with other disciplines can be schematically represented as follows:

Figure 4. Integrative role of soil physics in the system of agronomic disciplines.

Thus, soil physics is an integrative discipline. It does not just give a set of properties – it explains how the habitat of plants and microorganisms is organised, how fertility is realised. Without knowledge of physical foundations, it is impossible to apply fertilisers correctly, design crop rotations, or choose tillage and reclamation systems. In this course, we will give you this foundation.

Conclusion to the First Lecture

We have completed the introductory lecture. We learned:

1. What soil physics studies – the physical processes occurring in the soil environment.

2. What soil is as a system – a three‑phase body (solid, liquid, gaseous phases) with active participation of thermal energy.

3. Why physical properties are important – they determine water movement, aeration, root growth, and ultimately yield, even with identical chemistry.

4. What processes does physics study – water movement, air movement, heat movement, mechanical processes, and substance transport.

5. How physics relates to other sciences – it is the infrastructure for agrochemistry, biology, and crop science.

These basic ideas will serve as our support when studying specific sections: soil moisture, structure, density, thermal and air regimes, and transport processes. In the following lectures, we will delve into details, always keeping in mind the systemic approach we have established today.

Self‑check questions:

1. Name the three main phases of soil. What role does thermal energy play in their interaction?

2. Give an example when two soil plots with identical chemical composition produce different yields due to differences in physical properties.

3. Why is aeration (gas exchange) a critical factor for root nutrition?

4. List the main physical processes that the agronomist regulates through tillage and reclamation.

In the next lecture, we will begin a detailed study of the solid phase of the soil – its particle size distribution and mineralogy.