Soil Water

1.1. Chemically Bound Water

This is water that is part of the crystal lattice of minerals or chemical compounds. It is immobile, does not dissolve substances, and is absolutely unavailable to plants. This category includes:

• Constitutional (hydrate) water — hydroxyl groups (OH⁻) that are part of iron and aluminum hydroxides, clay minerals, and organic compounds.
• Crystallization water — whole H₂O molecules included in the crystals of certain minerals, such as gypsum (CaSO₄·2H₂O) or mirabilite (Na₂SO₄·10H₂O).

Loss of chemically bound water (dehydration) leads to irreversible changes in minerals. This water is not used by plants, but may be relevant when assessing the total water content of soil by thermal analysis (Mukha et al., 2003).

1.2. Solid Water (Ice)

During the cold season, part of the soil moisture freezes, forming ice in the pores. Solid water is unavailable to plants, but it plays an important role in the soil water regime (Mukha et al., 2003):

• When soil freezes, a vacuum is created in the underlying layers, which promotes the upward movement of moisture from deeper horizons.
• In spring, during thawing, there is a reverse movement of water and dissolved substances.
• Ice serves as a potential source of liquid and vapor water when warmth arrives.

In permafrost regions, ice may persist in the soil year-round, determining the specifics of the water regime and soil formation (Mukha et al., 2003).

1.3. Vapor Water

Water in the form of vapor is constantly present in the soil air, often reaching 100% saturation. Vapor water:

• is slightly available to plants as a direct source of moisture;
• can move through pores in response to vapor pressure gradients (from places with higher vapor pressure to places with lower);
• upon condensation, can transition to the liquid state, especially at night or when the soil cools (Mukha et al., 2003).

Although the amount of vapor water in soil is small (equivalent to a few litres per hectare in the top layer), its movement may be important for the seeds of some plants that can absorb moisture directly from vapor (Weil, 2017).

1.4. Physically Bound (Sorbed) Water

This is water held on the surface of solid soil particles by sorption forces. It constitutes the portion of soil moisture retained against gravity. Depending on the strength of bonding, two forms are distinguished.

1.5. Free Water

This is water that is not bound by sorption forces to the solid phase and moves under the action of capillary or gravitational forces. This water is primarily available to plants.

1.6. Summary Table of Water Forms

For clarity, we summarize the characteristics of different water forms in the table (Mukha et al., 2003; Weil, 2017).

Notes:

• LWC — least water capacity (field capacity);
• CRB — capillary rupture moisture (moisture at which capillary continuity is lost);
• WP — wilting point (permanent wilting of plants);
• MAH — maximum adsorptive water capacity;
• pF — logarithm of the water‑retaining forces expressed in cm of water column.

Brief Summary on Water Forms

We see that water in the soil is represented by a whole spectrum of forms—from tightly bound, absolutely immobile, to free, easily moving. Water availability to plants is determined not so much by the total amount of water, but by the form in which it exists. The transition from one form to another occurs continuously as soil moisture increases or decreases.

Key takeaway: In the same soil sample, all forms of water are present simultaneously, differing in retention strength and mobility. This is a fundamental difference from water in a glass, where all molecules are in the same free state.

In the next section, we will analyze which specific forces hold water in these different forms, and why some forms are easily available to plants while others are not.

2.1. Intermolecular Forces: Cohesion and Adhesion

All properties of water, including its behavior in soil, are determined by the structure of the H₂O molecule. The water molecule has an angular shape (bond angle ≈105°) and is a dipole: the side with hydrogen atoms carries a positive charge, and the side with the oxygen atom carries a negative charge (Weil, 2017). Because of this, hydrogen bonds arise between water molecules—relatively weak but numerous interactions that provide two fundamental forces.

2.2. Capillary Forces

If adhesion and cohesion act at the molecular level, capillary forces manifest at the pore level and arise as the combined result of these two forces at the interface of three phases: solid (particles), liquid (water), and gas (air) (Weil, 2017; White, 2006; Shukla, 2023).

2.3. Osmotic Forces

Water in soil is never chemically pure—it contains dissolved salts, organic acids, and nutrients. These dissolved substances (ions and molecules) attract water molecules, forming hydration shells (Weil, 2017; White, 2006). As a result:

• some water molecules become bound around ions—this is called hydration;
• the energy state of such water is lowered compared to pure water;
• osmotic pressure arises—the force with which water tends to dilute the solution by passing through semipermeable membranes.

In soil, osmotic forces do not directly play a role in retaining water in pores, because soil is not a single semipermeable membrane. However, they are critical for root water uptake: root cells are separated from the soil solution by semipermeable membranes, and if the salt concentration in the soil is higher than in the cells, water will leave the roots rather than enter them. This phenomenon—physiological drought—can occur even at relatively high soil moisture (Weil, 2017; Eash et al., 2016).

In purely physical terms, osmotic forces reduce the vapor pressure over the soil, which can affect evaporation and condensation (White, 2006). However, their main contribution is to lowering the total water potential, which we will discuss in the next section.

2.4. Gravitational Force

Unlike all the forces listed above, gravity is an external force acting on water in the soil just as it does on any other object on Earth. It is directed vertically downward and tends to move water deeper into the profile (Weil, 2017; Shukla, 2023).

Gravitational force:

• removes excess water from large pores (gravitational water);
• provides downward flow of moisture and recharge of groundwater;
• is the driving force of percolation—vertical movement of water through a saturated layer.

However, unlike sorption and capillary forces, gravity acts on all water equally, regardless of its location. Therefore, if it were not for sorption and capillary forces, all water in the soil would very quickly drain downward. It is the equilibrium between the gravitational force, tending to lower the water, and the sorption‑capillary forces, tending to retain it, that determines field capacity—the state at which gravitational drainage practically ceases (Weil, 2017; Mukha et al., 2003).

2.5. Interaction of Forces: From General to Specific

Now we can answer the key question: why does water in soil not behave like water in a glass?

In a glass, water is acted upon only by gravity (and to a very small extent by surface tension on the open surface). The water in the glass is in a free state; its molecules do not experience attraction from the walls (apart from trivial wetting of the glass). Therefore, it moves easily and always tends to occupy the lowest level.

In soil, the following forces act simultaneously:

1. Adhesion — attracts water to particle surfaces, especially strongly in thin films.

2. Cohesion — binds water molecules to each other, creating continuous films and menisci.

3. Capillary forces — retain water in pores of small diameter, creating negative pressure.

4. Osmotic forces — lower the energy state of water in the presence of salts.

5. Gravity — pulls water downward, but its effect is weakened or completely compensated by the first four forces.

As a result:

• The same water molecules in different points of the soil experience different total force effects.
• Water has no free surface in large pores until they are completely filled.
• Water can rise upward (capillary rise) and remain stationary in inclined samples because capillary forces act in all directions.
• Some water is "trapped" in fine pores and films and is unavailable to roots, even if its amount is large.

It is precisely this multitude of forces and their spatial heterogeneity that makes the behavior of water in soil unique and requires special concepts for its description—first and foremost, the concept of water potential, to which we turn in the next section.

Brief Summary on Retention Forces

3.1. Why Is the Simple Amount of Water Insufficient?

Imagine two situations.

Situation 1. In a sandy soil, the moisture content is 15% (of dry soil mass). Plants feel excellent; water enters the roots easily.

Situation 2. In a clayey soil, the moisture content is 25%—there is even more water by mass. But the plants are already beginning to wilt: the water is held so strongly that the roots cannot extract it.

Why does this happen?

In sand, water is mainly in large pores and thick films, where the attraction to particle surfaces is weak. Water molecules have high free energy—they can easily leave their place and move into the root.

In clay, water is concentrated in very fine pores and very thin films, where adhesion and capillary retention forces are maximal. Water molecules have low free energy—to extract them, a large amount of work must be expended.

Thus, the decisive factor is not the quantity, but the energy state of the water. It is this state that water potential expresses (Foth, 1990; Weil, 2017).

3.2. What Is Water Potential?

Let us define the concept rigorously.

Water potential (Ψ) is the difference in free energy between water in the soil (at a given point) and pure free water at the same temperature, pressure, and height, but not subject to soil forces (Huang et al., 2012; White, 2006; Shukla, 2023).

Simply put, water potential indicates how much work is required to extract a unit of water from the soil and convert it to the standard state (pure free water at the same height). Or conversely, how much work can be performed by water when moving from soil to the free state (White, 2006; Weil, 2017).

This can be imagined as follows:

• If we take a glass of pure water—that is our reference, our zero point. The potential of such water is taken as zero (Ψ = 0).
• Water in the soil always has lower free energy than water in a glass, because part of its energy has been "spent" on interactions with soil particles and dissolved salts.
• Therefore, the water potential of soil water is almost always negative (Ψ < 0). The drier and the more fine‑textured the soil, the more negative the water potential (Eash et al., 2016; White, 2006).

3.3. Why Is the Potential Negative?

This is important to understand to avoid confusion.

The negative sign is not "bad" or "little"; it is simply a mathematical convention reflecting the fact that the "soil water" system has lower free energy than the reference free water.

Imagine a spring. If a spring is compressed, it possesses potential energy capable of doing work. If the spring has already relaxed, its ability to do work has decreased. Taking the relaxed spring as the zero‑potential reference, we would say that the compressed spring has positive potential (it can do work as it relaxes). In the case of soil water, it is the opposite: free water (reference) has maximum free energy. Water in the soil has already partially lost this energy by interacting with particles. Therefore, its potential is lower than that of the reference—hence the negative sign (White, 2006).

The more strongly water is held by the soil, the more negative its potential becomes. In other words:

• Ψ = 0 — free water (e.g., water in a glass, in a large puddle).
• Ψ = –10 kPa — weakly held water (e.g., water in large pores of sand, easily available to plants).
• Ψ = –100 kPa — moderately held water (typical water in loam at field capacity).
• Ψ = –1500 kPa (≈ –15 bar) — strongly held water. This is the standard limit of availability: at this potential, most agricultural plants can no longer extract water, and permanent wilting occurs (Weil, 2017; White, 2006; Eash et al., 2016).
• Ψ = –10 000 kPa and below — water held so strongly that it is available only to extremely drought‑tolerant organisms (e.g., some lichens) or completely unavailable (Weil, 2017; Mukha et al., 2003).

3.4. Units of Water Potential

Water potential can be expressed in different units—it is important to be able to convert them. In international practice, the following main systems are used (Huang et al., 2012; Weil, 2017; Shukla, 2023; White, 2006):

Useful conversions:

• 1 bar = 100 kPa ≈ 1 atm ≈ 1020 cm H₂O (Weil, 2017)
• 1 kPa ≈ 10 cm H₂O (approximately, for quick estimation)
• pF = log₁₀ (|Ψ| in cm H₂O) (White, 2006; Mukha et al., 2003)

Example: A potential of –1500 kPa can be expressed as:

• –15 bar
• –15 atm (approximately)
• –15 000 cm H₂O (since 1 bar ≈ 1000 cm H₂O)
• pF ≈ 4.2 (since log₁₀ 15 000 ≈ 4.18) (Weil, 2017; White, 2006)

In practice, modern literature and international publications most often use kilopascals (kPa) or megapascals (MPa). In field work, bars (especially in the US) or centimeters of water column are often used for clarity (Eash et al., 2016; Shukla, 2023).

3.5. How Does Water Potential "Work" in Real Soil?

Now we come to the most important part: water potential is the driving force of all processes involving water in soil.

Water always moves from higher potential (less negative) to lower potential (more negative)—just as heat flows from hotter to colder, and electric current flows from higher to lower potential (Foth, 1990; Weil, 2017; Shukla, 2023).

Example 1 (movement of water in soil after rain).

• The topsoil is saturated with water. Its potential is close to zero (Ψ ≈ 0 kPa).
• The lower layer remains dry. Its potential is strongly negative (e.g., Ψ = –100 kPa).
• Water will move downward, from high potential to low potential, until the potentials equalize (or until gravity changes the picture).

Example 2 (capillary rise).

• Groundwater is at depth. The potential of water at the groundwater table is ≈ 0 kPa.
• In higher layers, the soil is dry, with negative potential (e.g., –50 kPa).
• Water will rise upward, from higher potential (0) to lower potential (–50 kPa)—this is what we call capillary rise.

Example 3 (root water uptake).

• Moist soil: water potential in soil Ψ_soil = –100 kPa.
• Root cells: to "pull" water, the root must create an even more negative potential in its vessels (e.g., Ψ_root = –500 kPa).
• Water moves from soil into the root because the potential in the root is lower (more negative) than in the soil. This creates a potential gradient that "pulls" water (Weil, 2017; White, 2006).

3.6. Water Potential as a "Universal Language"

Why is water potential considered the universal language of global science?

Because it allows us to describe all processes involving water—from movement in pores to root uptake and leaf transpiration—in a unified manner.

Here is an example:

We see that this table contains a single scale for all objects (atmosphere, plant, soil). This is the main value of water potential: it allows us to compare and predict water movement in any part of the "soil–plant–atmosphere" system (Weil, 2017; White, 2006).

3.7. Understanding Potential without Complex Mathematics

In this lecture, we avoid complex formulas, but it is important to give a general idea of how water potential is measured.

The most direct method is to measure the relative humidity of air over a soil sample. The drier the soil (and the more salts), the lower the vapor humidity above it. This relationship is described by the Kelvin equation (Huang et al., 2012; White, 2006):

where:

• R — gas constant;
• T — absolute temperature;
• Vₘ — molar volume of water;
• e/e₀ — relative humidity of air over the soil (ratio of actual vapor pressure to saturated).

This formula shows that potential is closely related to water vapor pressure—and it is on this principle that psychrometers and other instruments for measuring Ψ in dry soils work (White, 2006; Shukla, 2023).

For wetter soils (0 to –100 kPa), tensiometers are used (instruments that measure water tension in pores)—we will discuss them in the next part of the lecture (Huang et al., 2012; Weil, 2017; Shukla, 2023).

The main thing to remember:

Water potential is an integral characteristic that takes into account:

• all forces acting on water (adhesion, cohesion, capillarity, osmosis, gravity)—through the corresponding components of potential (discussed in the next section);
• temperature (through the dependence on T in the equation);
• salt composition (through the osmotic component).

Thus, water potential is the single quantity that contains all the information about the "state" of water in the soil. It replaces dozens of particular characteristics and makes accurate prediction of moisture behavior possible.

Brief Summary on Water Potential

• Water potential (Ψ) is the difference in free energy between water in the soil and pure free water (reference).
• It is always negative (except for salt‑free saturated soil, where Ψ ≈ 0).
• The drier and finer the soil, the more negative Ψ—the more strongly water is held.
• Ψ is measured in kPa, bar, atm, cm H₂O (see conversion table).
• Water always moves from higher Ψ (less negative) to lower Ψ (more negative).
• Ψ is a universal measure that allows all processes in the "soil–plant–atmosphere" system to be unified and described quantitatively.

4.1. Gravitational Potential (Ψ_g)

Gravitational potential is the contribution of gravity to the total water potential. It is determined by the height of the water relative to a reference point (usually the soil surface or the groundwater table) (Weil, 2017; White, 2006; Shukla, 2023).

Key properties:

• Ψ_g is always positive (or zero) if the point is above the reference level, and negative if below. This is because energy must be expended to lift water to a height, while water located above has potential energy that can do work when descending.
• It depends only on height and not on soil properties (porosity, texture).
• Ψ_g = ρ_w · g · z, where ρ_w is the density of water, g is the acceleration due to gravity, and z is the height above the reference level (White, 2006; Shukla, 2023).

Significance for water movement: Gravitational potential is the main driving force of downward flow of water in soil (percolation, drainage) and of rise during capillary rise (together with matric potential). It is what causes gravitational water to drain downward after rain.

Important: Gravitational potential acts on all water in the soil equally, regardless of its form. However, in fine pores, its effect is compensated by matric potential, and the water remains in place.

4.2. Matric Potential (Ψ_m)

Matric potential is the most important and complex component for understanding the behavior of water in soil. It reflects the combined action of adhesion, cohesion, and capillary forces that arise at the water–solid phase (soil matrix) and water–air (menisci) interfaces (Weil, 2017; White, 2006; Marshall et al., 1996).

What is the "matrix"? The soil matrix is the assemblage of solid particles (mineral and organic) and the pore space. When water interacts with this matrix, its energy is reduced.

Key properties of matric potential:

• Ψ_m is always negative (or zero at complete saturation), because water, interacting with the solid phase, loses part of its free energy.
• The drier the soil, the more negative *Ψ_m* (because water remains only in the finest pores and films, where attraction forces are maximal) (Weil, 2017; White, 2006).
• Ψ_m depends on soil moisture—this is the main relationship described by the water retention curve (see next section). The lower the moisture, the more strongly water is held, the more negative Ψ_m.
• Ψ_m depends on pore size (capillaries): in fine pores, Ψ_m is more negative than in large ones (White, 2006).

Why is matric potential negative? Imagine we want to extract water from the soil—we must do work to overcome the attraction to particle surfaces and break the capillary menisci. This work is exactly equal to the absolute value of the matric potential. Since the energy of water in the soil is less than that of free water, the sign is negative.

Significance for plants: Matric potential determines how strongly the plant must "pull" water from the soil. The more negative Ψm, the greater the effort (more negative potential in the roots) needed for water to enter the plant. When Ψm falls below –1500 kPa, most plants can no longer create a sufficient gradient—wilting occurs (Weil, 2017; Eash et al., 2016).

4.3. Osmotic Potential (Ψ_o)

Osmotic potential (also called solute or solution potential) is due to the presence in the soil water of dissolved salts, ions, and organic molecules. These particles attract water molecules, forming hydration shells, and thus lower the free energy of water (Weil, 2017; White, 2006; Foth, 1990).

Key properties:

• Ψ_o is always negative (or zero for pure water without salts).
• The magnitude of Ψ_o is proportional to the concentration of dissolved substances (the more salts, the more negative the potential) (White, 2006; Shukla, 2023).
• Ψ_o does not depend on the properties of the solid phase, but only on the composition of the soil solution.

Important difference from matric potential: Osmotic potential affects water movement only in the presence of semipermeable membranes (e.g., root cell walls). In open pores, without membranes, osmotic forces do not create a directed flow of water in the soil—they merely lower the total potential, but water does not move by itself toward the salty area because there is no barrier. However, if a membrane exists (e.g., a root hair), water will move through it toward the higher salt concentration (osmosis), trying to dilute the solution (Weil, 2017; White, 2006).

Significance for plants: In saline soils, Ψ_o can be very negative (e.g., –1000 kPa and below). This means that even if the soil is wet, the total potential (matric + osmotic) is so low that roots cannot "outpull" the water—physiological drought occurs (plants wilt even when the soil is wet) (Weil, 2017; Eash et al., 2016).

Approximate relationship: Ψ_o (in kPa) ≈ –36 × EC, where EC is the electrical conductivity of the soil solution in dS/m (Shukla, 2023). This empirical relationship is useful for quick estimation.

4.4. Pressure Potential (Hydrostatic) (Ψ_p)

Pressure potential (or hydrostatic potential) occurs only in saturated (water‑filled) soil, when all pores are filled with water and the water is under positive hydrostatic pressure (Weil, 2017; White, 2006; Shukla, 2023).

Key properties:

• Ψ_p is always positive (or zero at the level of the free water surface—the water table).
• It equals ρ_w · g · h, where h is the depth below the groundwater table (i.e., the height of the water column above the point) (White, 2006).
• In unsaturated soil, *Ψ_p = 0* (pressure equals atmospheric) (Weil, 2017; White, 2006).

Important relation: Matric potential (Ψ_m) and pressure potential (Ψ_p) cannot exist simultaneously at the same point (except in transition zones). In saturated soil, Ψ_m = 0 (water is not held by the matrix because all pores are filled), and Ψ_p > 0. In unsaturated soil, Ψ_p = 0 and Ψ_m < 0. They are like two sides of the same coin: in the saturated state, water experiences pressure; in the unsaturated state, tension (Weil, 2017; White, 2006).

Significance: Pressure potential is important for describing water movement in the saturated zone (groundwater, aquifers). In agronomy, it is important for assessing waterlogging, flooding, and drainage.

4.5. Other Components (Briefly)

In some special cases, other components are distinguished:

• Pneumatic potential — arises when air pressure in the soil changes (e.g., during air injection or sudden atmospheric pressure changes). Under normal conditions, it is negligible (White, 2006).
• Overburden potential — taken into account for swelling soils, where the weight of overlying layers creates additional pressure on the water. It is more often used in engineering geology than in agronomy (Shukla, 2023; Weil, 2017).

For agronomic practice, it is generally sufficient to consider three main components: Ψ_m, Ψ_o, and Ψ_g, and also Ψ_p when groundwater is present.

4.6. How Components Sum to Total Potential

The total water potential Ψ is the algebraic sum of all acting components. This means we simply add the values (taking signs into account) at each specific point.

Examples:

1. Wet unsaturated soil without salts, at the surface:

• Ψ_g = 0 (reference height = surface)
• Ψ_m = –30 kPa (typical for field capacity of loam)
• Ψ_o ≈ 0 (no salts)
• Ψ_p = 0 (unsaturated)
• Ψ = –30 kPa — water is available to plants.

2. Dry soil with slight salinity, on a slope (1 m above the reference level):

• Ψ_g = +10 kPa (approximately 1 m H₂O = 9.8 kPa)
• Ψ_m = –1000 kPa (strongly held)
• Ψ_o = –200 kPa (salts)
• Ψ_p = 0
• Ψ = 10 – 1000 – 200 = –1190 kPa — water is practically unavailable.

3. Saturated zone below the groundwater table (2 m below the water table):

• Ψ_g = 0 (if reference is the water table) or negative (if reference is the surface)
• Ψ_m = 0
• Ψ_p = +20 kPa (2 m H₂O)
• Ψ = +20 kPa — water is under positive pressure.

4.7. Why Components Are Important for Practice

Understanding the components allows:

• Diagnose causes of plant stress: if plants wilt at high moisture, the osmotic component (salinity) is likely large.
• Predict water movement: if there is a gradient in Ψ_m or Ψ_g in the profile, one can calculate the direction and rate of flow (see Darcy's law in later lectures).
• Assess irrigation efficiency: when irrigating, one must consider not only the amount of water, but also its potential (saline water may be poorly available even at high moisture).
• Plan reclamation: for example, leaching of saline soils lowers Ψ_o, increasing water availability.

Summary Table of Water Potential Components

Brief Summary on Components

• Total water potential Ψ is composed of several components, each corresponding to a specific type of force.
• Matric potential is the most important for agronomy; it reflects the soil's retention capacity and determines water availability.
• Osmotic potential becomes critical under saline conditions.
• Gravitational potential and pressure potential are important for describing water fluxes in the profile and the saturated zone.
• Knowing the components, we can quantitatively predict whether water will move, where, and with what intensity, and whether the plant can use it.

5.1. Soil‑Hydrological Constants: Key Moisture Points

Soil‑hydrological constants are quantitative indicators of moisture corresponding to specific values of water potential and characterizing transitional states of water in the soil (Mukha et al., 2003; Weil, 2017). They can be thought of as "reference points" on the continuous moisture scale, each with a clear physical meaning.

Most important constants (from wettest to driest):

1. Full water capacity (FWC) — maximum saturation.

2. Least water capacity (LWC) / Field capacity (FC) — upper limit of available water.

3. Capillary rupture moisture (CRM) — boundary of capillary water mobility.

4. Wilting point (WP) — lower limit of available water.

5. Maximum hygroscopicity (MH) — water held by sorption from vapor.

Let's examine each in detail.

5.2. How Texture and Structure Affect Water Retention

The water retention capacity of soil is determined by two main factors:

1. Particle‑size (textural) composition — particle size.

• Sandy soils (large particles): few fine pores, water is held weakly (low Ψ_m at the same moisture). FC is low (10–15%), WP is low (3–5%), but the available water (FC – WP) is relatively small (≈8–10%) (Weil, 2017; Eash et al., 2016).
• Loamy soils (intermediate size): optimal combination of pores of different sizes. FC is high (25–35%), WP is moderate (10–15%), and available water is maximal (≈15–20%) (Weil, 2017).
• Clay soils (fine particles): many fine pores, water is held very strongly. FC is high (35–45%), but WP is also high (20–25%), so available water is less than in loams (≈15–20%, but water is difficult to access at low potentials) (Weil, 2017; Eash et al., 2016).

2. Soil structure — aggregation.

• Well‑structured soil (with agronomically valuable aggregates) has more macropores (between aggregates) and more mesopores (within aggregates). This increases total porosity and improves the water‑air ratio at FC (Weil, 2017).
• Structureless, compacted soil has fewer large pores and more fine pores. At the same FC, air permeability is poorer, and water availability to roots decreases because of the high proportion of difficult‑to‑access fine pores (Weil, 2017).

Key takeaway: The best retention capacity from the perspective of plant availability is found in loamy structured soils—they have high FC and moderate WP, giving the maximum reserve of available water.

5.3. Estimating Productive Moisture Reserves

In agronomic practice, it is important to be able to quantitatively estimate the reserves of available (productive) water in the soil. The following indicators are used (Mukha et al., 2003).

Total water reserve (TWR) — the total amount of water in the entire studied soil layer (e.g., the root zone 0–100 cm). Calculated as the sum of water reserves for each horizon (taking into account its thickness, bulk density, and moisture).

Reserve of difficult‑to‑access water (RDAW) — the amount of water corresponding to the wilting point (WP) for each horizon. This is the water not used by plants.

Reserve of productive water (RPW) — the difference between the total water reserve and the reserve of difficult‑to‑access water:

Alternatively, it is the amount of water in the soil above the wilting point.

Formula for calculating water reserve in a layer (Mukha et al., 2003):

where:

• W — water reserve in the layer, m³/ha or mm of water layer;
• a — soil moisture, % of dry soil mass;
• dᵥ — soil bulk density, g/cm³;
• h — layer thickness, cm.

Conversion to mm of water layer: 1 m³/ha = 0.1 mm of water layer (Mukha et al., 2003). This is convenient for comparison with precipitation or irrigation rates.

Assessment of productive water reserves (adapted from Mukha et al., 2003):

5.4. Example Calculation of Productive Water Reserve

Problem. Estimate the productive water reserve in the 0–50 cm layer. Known:

• Layer 0–20 cm: bulk density 1.2 g/cm³, moisture 25%; WP for this layer is 12%.
• Layer 20–50 cm: bulk density 1.4 g/cm³, moisture 22%; WP is 10%.

Solution:

1. Total water reserve:

• Layer 0–20 cm: W₁ = 25 · 1.2 · 20 = 600 m³/ha = 60 mm.
• Layer 20–50 cm: W₂ = 22 · 1.4 · 30 = 924 m³/ha = 92.4 mm.
• TWR = 600 + 924 = 1524 m³/ha = 152.4 mm.

2. Reserve of difficult‑to‑access water (RDAW, at WP):

• Layer 0–20 cm: W_WP₁ = 12 · 1.2 · 20 = 288 m³/ha = 28.8 mm.
• Layer 20–50 cm: W_WP₂ = 10 · 1.4 · 30 = 420 m³/ha = 42 mm.
• RDAW = 288 + 420 = 708 m³/ha = 70.8 mm.

3. Reserve of productive water (RPW):

• RPW = 152.4 – 70.8 = 81.6 mm.

Assessment: According to the table for the 0–50 cm layer (intermediate between 0–20 and 0–100 cm), a reserve of about 81 mm corresponds to good water supply.

Brief Summary on Water Retention Capacity

• Water retention capacity is a complex property described by a system of soil‑hydrological constants.
• Least water capacity (field capacity) and wilting point are the two key constants determining the reserve of productive (available) water.
• The difference between FC and WP is available water that can be used by plants.
• The largest reserves of available water are typical of loamy structured soils.
• Calculation of productive water reserves allows quantitative assessment of crop water supply and planning of reclamation measures.
• Maximum hygroscopicity (MH) is the "dead reserve" of water, never considered as available.

6.1. What Is a Water Retention Curve?

The water retention curve (soil water retention curve, SWC) is a graphical representation of the functional relationship between soil water content (usually volumetric water content θᵥ, in %) and matric potential (Ψₘ), most often expressed on a logarithmic scale (Weil, 2017; White, 2006; Huang et al., 2012; Eash et al., 2016; Shukla, 2023).

Typically, on the graph:

• X‑axis (horizontal) — volumetric water content θᵥ (in % or m³/m³), increasing left to right.
• Y‑axis (vertical) — matric potential Ψₘ (in kPa, bar, or cm H₂O), often on a logarithmic scale (i.e., pF), because the potential range is enormous—from 0 to hundreds of thousands of kPa. The logarithmic scale allows compact representation of the entire range from saturation to air‑dry state (Weil, 2017; White, 2006).

The curve is determined experimentally for each genetic horizon or layer of soil. A soil sample is saturated with water, and then various matric potentials are imposed sequentially (e.g., using tensiometers, vacuum tables, pressure plate apparatus), and the corresponding moisture is measured. The obtained points are connected by a smooth curve (Weil, 2017; White, 2006; Shukla, 2023).

6.2. What Does the Water Retention Curve Show?

The water retention curve provides three types of critical information:

6.3. Influence of Texture and Structure on the Shape of the Curve

The shape of the retention curve depends strongly on particle‑size distribution (texture) and soil structure (Weil, 2017; White, 2006; Eash et al., 2016; Shukla, 2023).

6.4. The pF‑Scale: Logarithmic Representation of Potential

In Russian‑language and European soil literature, the pF concept is widely used (Mukha et al., 2003; White, 2006). It is the logarithm (base 10) of the absolute value of the matric potential expressed in centimeters of water column:

Conversion examples:

Advantages of the pF‑scale:

• Compactly represents the huge range of potential changes (from 0 to 7 and above).
• Convenient for plotting—logarithmic scale on the Y‑axis makes the curve more illustrative.
• Old and many modern works on soil physics use pF‑values (Mukha et al., 2003; White, 2006).

Limitation: pF is not an SI unit. International publications increasingly use kPa or MPa, but understanding pF is necessary for reading specialized literature.

6.5. Influence of the Slope: Differential Water Capacity

The slope of the retention curve at each point (dθ/dΨₘ or dθ/d(pF)) has physical meaning—it is the differential water capacity (or specific water retention capacity) (White, 2006; Huang et al., 2012). It shows how much water is released per unit decrease in potential.

• Steep slope (large value of dθ/dΨ) means that a small increase in retention force leads to a significant decrease in moisture (characteristic of sandy soils at high potentials).
• Gentle slope (small value) means that even a large change in potential gives only a small change in moisture (characteristic of clay soils at low potentials).

Practical significance: Differential water capacity is important for calculating the rate of water movement in unsaturated soil (see Darcy's law, lectures 5–6). It also allows estimation of how quickly the soil releases water to plants when potential in the root zone decreases.

6.6. Hysteresis: Why the Curve Differs between Wetting and Drying

One of the most important features of the retention curve is hysteresis (Weil, 2017; White, 2006; Marshall et al., 1996; Shukla, 2023). This phenomenon consists in that the curve obtained during drying (decreasing moisture) does not coincide with the curve obtained during wetting (increasing moisture): at the same matric potential, the moisture on the drying curve is higher than on the wetting curve (Weil, 2017; White, 2006).

Causes of hysteresis:

1. Ink‑bottle effect — pores have irregular shapes: wide cavities connected by narrow necks. During drying, water is held in the wide part until the potential becomes low enough to break the meniscus through the narrow neck. During wetting, conversely, wide cavities fill only after water passes through narrow necks, requiring a higher (less negative) potential. As a result, at the same potential, the drying curve has higher moisture (Weil, 2017; Marshall et al., 1996).

2. Different contact angles for advancing and receding water—it is larger during wetting (hydrophobic effects), which reduces capillary pressure (Marshall et al., 1996; Weil, 2017).

3. Air entrapment during wetting—some pores remain filled with air, so the moisture at saturation on the wetting curve is slightly lower than on the drying curve (Weil, 2017).

4. Swelling and shrinkage of clay soils—during drying, pore volume decreases, and during wetting it increases, also changing the shape of the curve (Weil, 2017; Marshall et al., 1996).

Practical significance of hysteresis:

• At the same moisture, water in a drying soil (after rain) is held less strongly (less negative potential) than in a wetting soil (during capillary rise or light irrigation). This means that in a soil that is drying, water is more available than in one that is wetting to the same moisture.
• For accurate water balance calculations, one must know which branch (drying or wetting) the process is following at the moment.
• In agronomic practice, the drying curve is usually used, because in the field the soil most often dries after rain or irrigation (Weil, 2017; White, 2006).

6.7. How Is the Retention Curve Used in Practice?

1. Assessment of available water reserves for irrigation planning.

Knowing the curve for the root zone, one can determine Ψₘ from measured moisture and vice versa. This allows accurate calculation of how much water is still available to plants and when irrigation is needed (Eash et al., 2016; Shukla, 2023).

2. Diagnosis of soil fertility.

The shape of the curve (especially steepness and position relative to axes) provides information about structure quality, degree of compaction, salinity, and organic matter content (Weil, 2017).

3. Reclamation and assessment of soil suitability for irrigation.

Comparison of curves for different horizons allows assessment of how water will be distributed in the profile, whether capillary rise is possible, and whether waterlogging will occur (White, 2006).

4. Modeling of water regimes.

The retention curve is a mandatory input parameter for all mathematical models of water and solute movement in soil (e.g., HYDRUS, SWAP). Without it, calculations are impossible (Huang et al., 2012; Shukla, 2023).

5. Drought risk assessment.

Knowing the curve and current moisture, one can determine how many days (at average evapotranspiration) the soil can supply plants with water before reaching WP (Eash et al., 2016).

6.8. Example: Three Curves—Three Soils

Let's consider three typical retention curves (after Weil, 2017; White, 2006; Shukla, 2023, see Fig. 5.12 in Weil, 2017).

Conclusion: Loam gives the largest reserve of available water, and this water is distributed over a wide range of potentials, providing gradual depletion. Sand has little available water, and it is quickly exhausted. Clay has a lot of water, but a significant portion is difficult to access.

Brief Summary on the Water Retention Curve

• The water retention curve is a fundamental soil characteristic relating moisture and matric potential.
• All major soil‑hydrological constants (FC, WP, MH, etc.) are easily identified on it.
• The shape of the curve is determined by texture and structure: sand—steep, clay—flat, loam—optimal.
• The pF‑scale (logarithm of potential in cm H₂O) is a convenient way to compactly represent a wide range of potentials.
• Hysteresis—the difference between drying and wetting curves—is due to complex pore geometry, contact angle, and swelling.
• The retention curve is an essential tool for water balance calculations, irrigation planning, fertility assessment, and modeling.

7.1. Availability as a Function of Water Potential

Plants absorb water passively (mostly)—water enters the roots along a gradient of water potential from soil to root (Weil, 2017; White, 2006). For this, root cells must create a more negative water potential in their vessels than the water in the soil. The lower (more negative) the potential of soil water, the greater the "effort" (i.e., more negative potential in roots) that the plant must exert.

Thus, availability is determined not so much by the moisture itself, but by the potential difference Ψsoil – Ψroot that the plant can generate (Eash et al., 2016; Weil, 2017).

Key boundaries on the potential scale (after Weil, 2017; White, 2006; Eash et al., 2016):

• Ψ_soil > –30 kPa (around FC) — water is easily available, uptake is active.
• Ψ_soil ≈ –100…–300 kPa — availability begins to decrease, but many crops can still absorb water at sufficient rates.
• Ψ_soil ≈ –1500 kPa — standard availability limit. Most agricultural crops (maize, wheat, soybean, etc.) cannot create a more negative potential in roots, so permanent wilting occurs (Weil, 2017; Eash et al., 2016). Some drought‑tolerant plants (xerophytes, e.g., some species of wormwood, saxaul) can extract water at potentials down to –3000…–8000 kPa (Weil, 2017; White, 2006).
• At Ψ_soil < –3100 kPa (MH), water is held almost exclusively by sorption forces and is unavailable to all higher plants.

Important: Wilting point (WP) is a conventional value adopted for ease of comparison. In reality, this limit differs among plant species and also depends on evaporation rate (on a hot, windy day, wilting may occur at a higher potential) (Weil, 2017; White, 2006). Therefore, in agronomy the "irrigation threshold" is often used—the moisture at which irrigation should be started to prevent yield reduction. It is usually above WP (e.g., at Ψ ≈ –500…–800 kPa) (Eash et al., 2016; White, 2006).

7.2. Water Availability and Soil Textural‑Structural Features

As we saw from the retention curves, different soils at the same potential contain different amounts of water. This directly affects the reserve of available water and the rate of its supply to roots.

Conclusion (Weil, 2017; Eash et al., 2016): Loamy structured soils provide the best availability of water, because they combine a sufficiently large reserve and good hydraulic conductivity. Clays often suffer from poor water movement to roots even at high moisture, creating "physiological dryness" (plants experience stress even at high moisture). Sands, on the other hand, release water quickly, but their reserve is too small.

Influence of structure: Well‑aggregated soil has mesopores inside aggregates that hold available water, and macropores between aggregates that provide drainage and aeration. Upon compaction, macropores are destroyed, the proportion of fine pores increases, and the retention curve shifts: FC may decrease slightly, but WP increases (due to the increased number of fine pores), and available water decreases, while water becomes more difficult to access (Weil, 2017). This is one of the mechanisms of the negative impact of soil compaction on plants.

7.3. The Second Key Factor: Rate of Water Movement to Roots

Even if water is present in the soil in an available form (potential above WP), it must reach the root at a sufficient rate to meet transpiration demand. This aspect is often overlooked, but it is critical (Weil, 2017; White, 2006; Eash et al., 2016).

The hydraulic conductivity (K) of unsaturated soil decreases sharply with decreasing moisture (i.e., with decreasing potential) (White, 2006; Weil, 2017). In sandy soils, at potentials above –100 kPa, conductivity is still high, but as moisture approaches WP, K becomes very small. In clay soils, K at saturation is low, but as potential decreases, it decreases less dramatically; however, the absolute values are always lower than in sands.

As a result:

• In sandy soil at moisture near field capacity, water easily reaches roots. But as soon as the available reserve is reduced by half, conductivity may drop so much that roots cannot take it up, and plants experience stress even at moisture above WP (Weil, 2017).
• In clayey soil, water moves slowly always, so plants may experience water deficit even at relatively high moisture due to the slow replenishment of water around roots.

Practical conclusion: To assess availability, it is important to know not only the amount of water, but also the rate at which it can move to roots. In agronomy, the concept of "critical moisture" is often used—the moisture at which hydraulic conductivity drops so much that water supply cannot keep up with demand. It is usually higher than WP and depends on the crop and weather conditions (Eash et al., 2016; White, 2006).

7.4. Osmotic Potential and Salinity: When Water Is Present but "Unavailable"

As discussed in Section 4, osmotic potential (Ψₒ) lowers the total water potential of the soil solution. In saline soils, Ψₒ can be very negative (e.g., –1000 kPa and below). This means that the total water potential Ψ = Ψₘ + Ψₒ becomes significantly more negative than the matric potential alone (Weil, 2017; White, 2006; Shukla, 2023).

For the plant, this manifests as:

• The root must create a potential more negative than the sum of the matric and osmotic potentials of the soil to draw in water.
• If there are many salts in the soil, roots must expend extra energy on osmotic regulation (accumulating salts in cells) to lower their internal potential.
• When the osmotic potential of the soil becomes too low, even with wet soil (high Ψₘ), the total Ψ may be lower than the root can generate. Then physiological drought occurs—the plant wilts despite the presence of water in the soil (Weil, 2017; Eash et al., 2016).

Assessing the impact of salinity:

• At soil solution electrical conductivity > 4 dS/m (≈ –144 kPa osmotic potential), sensitive crops already begin to suffer.
• At 8–10 dS/m (≈ –300…–360 kPa), many moderately tolerant crops are affected.
• At > 16 dS/m (≈ –576 kPa), only salt‑tolerant plants (halophytes) can develop normally (Shukla, 2023; Weil, 2017).

Practical conclusion: When assessing water availability in saline areas, the osmotic component must be taken into account. Irrigation should not only wet the soil but also provide a leaching regime to remove salts from the root zone (Eash et al., 2016; Weil, 2017).

7.5. The Concept of the "Least Limiting Water Range" (LLWR)

The traditional definition of available water (FC–WP) does not take into account two important limitations: oxygen deficiency under excessive wetness and high mechanical resistance of soil upon severe drying. Therefore, modern agrophysics uses a more comprehensive approach—the Least Limiting Water Range (LLWR) (Weil, 2017).

Idea of LLWR:

The moisture range in which three conditions for normal root growth are simultaneously met:

1. Sufficient aeration (air content in pores > 10% by volume) — limits the upper boundary of the range (too wet).

2. Sufficient water (potential above WP, i.e., > –1500 kPa) — limits the lower boundary traditionally.

3. Mechanical resistance of soil does not exceed ~2000 kPa (i.e., roots can penetrate without excessive effort) — upon severe drying, soil becomes hard and roots cannot grow (Weil, 2017).

How this works in practice:

• In loose, well‑structured soil, LLWR almost coincides with the FC–WP range, because aeration is sufficient even at FC, and mechanical resistance becomes critical only at moisture near WP (Weil, 2017).
• In compacted soil, LLWR is significantly narrower: the upper boundary (aeration) shifts downward (at lower moisture, air is displaced), and the lower boundary (mechanical resistance) shifts upward (soil becomes too hard already at moisture above WP). As a result, the range of moistures favorable for roots is greatly narrowed (Weil, 2017).

Illustration (Weil, 2017, Fig. 5.39):

• Non‑compacted loam: LLWR ≈ 15–30% moisture.
• Compacted loam: LLWR ≈ 20–25% — range narrows, roots can grow only in a narrow moisture interval.

Agronomic significance: Considering LLWR allows more accurate prediction of when soil becomes unfavorable for root growth and planning measures to combat compaction (tillage, organic matter addition) and regulate water regime.

7.6. Water Availability in Different Parts of the Profile and the Role of the Root System

Water availability is determined not only by the properties of a single layer, but by the entire soil profile and the depth of root penetration (Weil, 2017; Eash et al., 2016; White, 2006).

• The available water reserve in the profile is calculated as the sum of available water in each horizon penetrated by roots (see Section 5.3). The deeper the roots, the larger the volume of soil they can "explore," and the more available water can be used (Weil, 2017; Eash et al., 2016).
• In dry conditions, plants with deep root systems (e.g., alfalfa, sunflower) can extract moisture from deeper horizons where moisture is still high, while the surface layer has already dried out (Weil, 2017; Eash et al., 2016).
• Stratification (layering) of the profile can strongly affect availability. For example, if a thin loamy layer lies over sand, water from the loam will not drain into the sand (capillary barrier), and available water in the upper layer is preserved longer (Weil, 2017; White, 2006). Conversely, if sand lies over loam, water quickly drains downward and the upper layer dries out.

Practical conclusion: When assessing water supply, it is necessary to consider not only the properties of each horizon, but also the thickness of the root zone and possible barriers to water and root movement.

7.7. Practical Recommendations for the Agronomist

1. For irrigation decisions, it is better to rely not on fixed moisture, but on water potential (measured with tensiometers) or on the productive water reserve as a fraction of FC (e.g., irrigate when it drops to 60–70% of FC for most crops) (Eash et al., 2016; Weil, 2017).

2. Consider texture: on sands, irrigations should be more frequent but with smaller doses; on clays, less frequent but with larger doses (taking slow infiltration into account) (Eash et al., 2016).

3. Pay attention to compaction: it narrows LLWR, impairs aeration at high moisture, and increases mechanical resistance upon drying. Regular tillage and organic matter addition improve structure (Weil, 2017).

4. In saline areas, conduct leaching irrigations and monitor osmotic potential (by electrical conductivity). For calculating leaching requirements, use knowledge of the retention curve and salts (Weil, 2017; Shukla, 2023).

5. To forecast drought development, use moisture data and the retention curve to calculate how many days the current available water reserve will last under average daily evapotranspiration (Eash et al., 2016).

7.8. Summary Table: Availability and Factors

Brief Summary on Water Availability

• Water availability to plants is determined not by its quantity, but by the energy state (water potential) and the rate of supply (hydraulic conductivity).
• The lower limit of availability is the wilting point at Ψ = –1500 kPa (conventionally for most crops).
• The upper limit is the moisture at which aeration is impaired (≈ FC, but in compacted soils the critical moisture may be lower).
• The LLWR concept (least limiting water range) provides a more realistic assessment of root conditions than the simple FC–WP difference.
• Salinity and compaction significantly reduce availability, requiring special measures (leaching, tillage, organic matter).
• For practical irrigation and reclamation management, it is necessary to use the water retention curve and measure water potential or productive water reserve.

Conclusion to the Lecture

We have traveled from the question "why does water in soil not behave like water in a glass" to understanding that water availability to plants is a complex function of energy state, pore‑space structure, and movement rate. We have become acquainted with the forms of water, retention forces, water potential and its components, soil‑hydrological constants, the water retention curve, and finally how all this knowledge is applied to assess plant water supply.

Main takeaways from the lecture:

1. Water in soil is held by a complex of forces (adhesion, cohesion, capillarity, osmosis, gravity), making its behavior fundamentally different from that of free water.

2. Water potential is a universal language allowing quantitative description of the energy state of water and prediction of its movement.

3. The main contribution to water retention in unsaturated soil comes from matric potential, related to pore size and thickness of water films.

4. The water retention curve graphically relates moisture and matric potential, enabling determination of all key constants (FC, WP, MH) and estimation of pore space.

5. Water availability to plants is determined not so much by its quantity as by potential and hydraulic conductivity; salinity and compaction significantly limit it.

6. For practical farming, this knowledge should be used for optimizing irrigation, combating compaction, and reclaiming saline soils.

These principles will serve as the foundation for subsequent lectures on water movement in soil, water regimes, irrigation, and reclamation. Understanding the physics of soil moisture is the key to sustainable water resource management in agriculture.