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Soil Water: From Molecular Structure to Behavior

By: Owen W. Duckworth (Department of Soil Science North Carolina State University), Joshua L. Heitman (Department of Soil Science North Carolina State University) & Matthew L. Polizzotto (Department of Soil Science North Carolina State University) © 2014 Nature Education 
Citation: Duckworth, O. W., Heitman, J. L. & Polizzotto, M. L. (2014) Soil Water: From Molecular Structure to Behavior. Nature Education Knowledge 5(8):1
Water is a unique compound that is essential to life on Earth. In the pedosphere, the physical and chemical properties of water regulate the flow of energy and solutes, making soil water a crucial component of terrestrial ecosystems. Many of the familiar properties of water that result in its behavior in soils can be directly related to its molecular structure.
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"Water is the driving force of all nature." - Leonardo da Vinci

Water is a ubiquitous and critically important substance on Earth. Essential to all living things and the health of ecosystems (Moss 2010), water is considered a prerequisite for life on other planets (Marais et al. 2008). The availability of water even partially controls the development and distribution of human settlement (Solomon 2011). As the human population continues to increase, providing humanity with clean water for domestic, agricultural, and industrial uses is considered one of the major societal challenges of the 21st century (National Academy of Engineering 2008).

In soils, water is a major driver of biogeochemical processes (Figure 1). Chemical reactions that control soil formation and weathering reactions occur almost exclusively in liquid water (Lindsay 1979). Physically, water is the diffusive medium that mediates the movement of gases, solutes, and particles in soils. Water regulates the transfer of heat, thereby helping buffer soil temperature (Jury & Horton 2004). Biologically, microbes require water in soil pores to metabolically function (Maier et al. 2008). Additionally, the availability of water is considered to be one of the most important factors for the growth of crops and other plants (Kirkham 2005). In this article, we explore how the molecular structure, chemical properties and physical properties of water control the functioning of soils.

Schematic showing major processes associated with water in soils.
Figure 1: Schematic showing major processes associated with water in soils.
Red arrows indicate predominantly physical processes, whereas black arrows indicate predominantly geochemical processes. For more information about processes 1 and 9, see O'Geen (2013); for processes 3 and 4, see Thompson and Goyne (2012); for processes 5-8, see McNear (2013) and Fortuna (2012).
© 2014 Nature Education Adapted from Lindsay (1979). All rights reserved.

Molecular Structure of Water

The molecular properties of water result in many of its unique and familiar qualities. Individually, water molecules consist of two hydrogen atoms attached by covalent bonds to a tetrahedral oxygen atom (Figure 2A), resulting in a bent molecule with a 104º angle between hydrogen atoms. Because two electron pairs reside on the oxygen atom (with an additional electron pair shared by oxygen and each hydrogen atom), the molecule has a permanent dipole moment, with a positive charge (δ+) residing on the hydrogen atoms and a negative charge (δ-) on the oxygen atom (Pauling 1988).

The hydrogen bonds resulting from electrostatic interactions between the more positive hydrogen atoms and the more negative oxygen atoms in adjacent molecules are relatively strong intermolecular forces. These interactions are responsible for the cooperative nature of water — that is, it is more favorable for water molecules to be surrounded by other water molecules in large groups than to exist as individual molecules or dimers (Franks 2000). In contrast, non-polar molecules that are roughly the same molecular mass as water are typically gases, which only weakly interact with one another, at room temperature (Pauling 1988). Although many models exist to describe the behavior of liquid water (Eisenberg & Kauzmann 1969, Frank 1972, Ives & Lemon 1968), it can be broadly represented as dynamic clusters of tumbling hydrogen-bonded molecules (Figure 2B). Upon freezing, water forms a crystalline solid with molecules arranged in tetrahedra, resulting in an expansion of the solid phase (Figure 2C).

The structure of water.
Figure 2: The structure of water.
Panel (a) A single water molecule. The δ+ and δ- indicate the partial positive and negative charges on the molecule associated with its dipole moment. Panel (b) A cluster of water molecules. Panel (c) Structure of ice.
© 2014 Nature Education Panel (a) adapted from James et al. 2005. Panel (b) adapted from Wales et al. 2013. Panel (c) adapted from Fortes et al. 2004. All rights reserved.

Water has many anomalous physical and chemical properties that result from its molecular structure (Table 1). The polar nature of the molecule helps to explain its high dielectric constant and its ionic dissociation, which result in its ability to separate the charges on ions and dissolve polar solids. The cohesive nature that stems from water molecules' intermolecular attraction results in abnormally high surface tension, heat capacity, heat of vaporization, and boiling point. The ordering of water molecules upon freezing results in a high heat of fusion and reduction of density for the solid phase. As shown in Table 1, these properties are critical to understanding the chemistry and physics of water in soils.

Property Molecular Rationale Significance
High dielectric constant Dipole moment allows water to stabilize solutes withboth positive and negative charges Excellent solvent for polar and charged species
Ionic dissociation Water readily splits into protons and hydroxide ions due to the polarity of the molecule Acid-base chemistry of aqueous solutions is facilitated by this property
Expansion upon freezing Ordering molecules in crystalline solid results in more void space than in the liquid phase Ice floats; freezing occurs at the top of a water body
High boiling point Adhesion of molecules hinders transformation to gas phase Water is a liquid at common temperatures
High heat capacity Strong interactions between molecules require a large energetic input to change temperature Temperature is buffered against small changes in thermal energy
High heat of vaporization Adhesion of molecules requires a large thermal input to cause transformation to gas phase Temperature is additionally buffered at environmentally extreme temperatures
High heat of fusion Ordering of molecules upon freezing results in significant release of thermal energy Temperature is additionally buffered at environmentally extreme temperatures
High surface tension Molecules have cooperative interactions that cause cohesion at interfaces Causes formation of drops and capillary behavior
Table 1: Important properties of water and their relationship to its molecular structure. Adapted from Manahan (2004) and Pierzynski et al. (2005).

Chemical Properties of Water and Behavior in Soils

The chemical properties of water govern its behavior in the environment and control many key processes occurring in soils as the aqueous phase interacts with organisms, mineral surfaces, and air spaces. As a result of its nonlinear structure and dipole moment (Figure 2A), water has a high dielectric constant (80.1 at 20°C) (Eisenberg & Kauzmann 1969, Hasted 1972), which is a measure of a substance's ability to minimize the force of attraction between oppositely charged species. Water's dielectric constant, which is significantly higher than that of the solid and gaseous components of soil (dielectric constants of ~2-5 and 1, respectively), is often utilized in electromagnetic measurement approaches to determine soil water content. This unique property of water also makes it a powerful solvent, allowing it to readily dissolve ionic solids. Water acts to dissipate the attractive force of ions by forming solvation spheres (Figure 3) around them (Burgess 1978, Essington 2004, Franks 2000). The polar nature of the water molecules allow them to surround and stabilize the charges of both anions and cations (Pauling 1988), preventing their association.

Solvation spheres.
Figure 3: Solvation spheres.
Water forms solvation spheres around ions in solution, with the negative charge in the oxygen atom aligning with positively charged cations, and the positive charge in the hydrogen atoms aligning with negatively charged anions.
© 2014 Nature Education All rights reserved.

Consider the dissolution of potassium chloride (KCl), a common potassium source in chemical fertilizers (Havlin et al. 2005). When combined with water, the ionic solid dissolves:

KCl(s) + (m+n)H2O(l) ↔ [K(H2O)m]+(aq) + [Cl(H2O)n]-(aq)

where m and n represent the numbers of water molecules within each solvation sphere — numbers that are functions of the charge, size, concentration, and chemical properties of the ions in solution (Burgess 1978). Although KCl is quite soluble and readily dissolves, the extent to which other soil minerals dissolve or precipitate is variable, depending on the specific mineral properties and the soil solution chemistry (Essington 2004). Water's ability to enhance dissolution or prevent precipitation impacts a range of processes and properties in soils, including mineral weathering, soil salinity, and soil fertility (Brantley 2008, Havlin et al. 2005).

Another particularly important chemical property of water that impacts processes occurring within the soil solution is that it is amphoteric, meaning that it can act as either an acid or a base (IUPAC 1997). Due to its polarity, water readily undergoes ionic dissociation into protons and hydroxide ions (Eisenberg & Kauzmann 1969, Pauling 1988):

H2O(l) ↔ H+(aq) + OH-(aq) (1)

Accordingly, when it reacts with a strong base, water acts as an acid, releasing protons:

H2O(l) + NH3 ↔ NH4+(aq) + OH-(aq) (2)

When it reacts with a strong acid, water acts as a base, accepting protons:

H2O(l) + HCl ↔ H3O+(aq) + Cl-(aq) (3)

The amphoteric behavior of water facilitates the acid-base chemistry and dictates the potential pH range of aqueous solutions, thereby imparting soil pH — a "master variable" of soils that influences soil formation, plant growth, and environmental quality (Sparks 2003).

The ability of water to stabilize charged species in solution allows it to support the flow of electrons in soils. As such, water helps mediate oxidation and reduction (redox) reactions within the soil solution. Water itself may participate in these processes (Frank 1972), and it is a product of cellular respiration in soils. In aerobic soils, water is produced from the oxidation of carbon in organic matter (here notated as CH2O) for energy production by microorganisms:

CH2O(s) + O2(g) → CO2(g) + H2O(l) (4)

In the above reaction, the transfer of electrons reduces the oxidation state of oxygen in O2 (0) to that of water (-2). Particularly in anaerobic soils, carbon oxidation may also be coupled to reduction of chemical species other than O2. The specific respiration processes in soils are governed by thermodynamics and reaction kinetics but occur within the soil solution or at the mineral-solution interface (Sposito 2008, Stumm & Morgan 1996). These are important processes that govern microbial community structure, soil mineralogy, soil solution chemistry , and pollutant fate and transport (Lovley 1995, Sparks 2003).

Physical Properties of Water and Behavior in Soils

Liquid water is a key component of the three-phase (solid, liquid, gas) soil system, possibly occupying 50% or more of the total soil volume under saturated conditions (Hillel 1998; Figure 4). Even under relatively dry conditions, water held at large tensions within soil pores occupies approximately 5–10% of the soil volume (Campbell & Norman 1998). Liquid water is held in soil under tension arising from the adhesive and cohesive forces associated with water's molecular structure (Jury & Horton 2004). The capacity of water to be held in soil pores via cohesion and adhesion partially controls water storage and redistribution in the hydrologic cycle (O'Geen 2013).

Saturated and unsaturated soil conditions.
Figure 4: Saturated and unsaturated soil conditions.
Volumetric fractions of solid, water and gas phases in soil for unsaturated (top) and saturated conditions (bottom).
© 2014 Nature Education All rights reserved.

The interaction between water and the soil solid matrix is often visualized with a capillary tube model (Figure 5). Liquid water at the water-gas interface exhibits a meniscus. The inward pull of liquid water molecules from hydrogen bonding (cohesion) is unbalanced at the liquid-gas interface, which is referred to as surface tension. In combination with the polar attraction of water molecules for a wettable soil solid matrix (adhesion to the capillary tube wall), this cohesion creates concave curvature. Water rises in the tube to reach equilibrium between the attractive upward force at the interface and the weight of water pulling downward on the meniscus.

Capillary tube model.
Figure 5: Capillary tube model.
Water in soil pores (left) and the capillary tube model analog (right) defined by tube diameter (R), radius of curvature (r), contact angle (α), and height of rise (h).
© 2014 Nature Education All rights reserved.

The curved interface between liquid and gas can be described by a characteristic radius of curvature (r):

r = R/cos α (5)

where R is the radius of the capillary tube and α is the contact angle between the liquid-solid and liquid-gas interfaces (Figure 5). For hydrophilic surfaces such as those of quartz, which makes up a large proportion of solids in mineral soils, is commonly small such that cos α ≈ 1 (Brutsaert 2005). For non-wettable surfaces, such as minerals coated with hydrophobic organic compounds, α is increased, and r is altered (Bachmann & van Der Ploeg 2002).

The pressure differential (Δp) across the liquid-gas interface can be described as:

Δp = 2σcosα/R (6)

where σ is the surface tension of water. An equilibrium is achieved when the weight of the water column is equal to the pressure differential over the cross-sectional area of the liquid-gas interface (Jury & Horton 2004).

From this equilibrium, the height of rise within a capillary tube (h) is given by:

h = 2σcosα/Rρg (7)

where ρ is the density of water and g is the acceleration of gravity. Based on this relationship, h depends on the properties of water as well as the geometry of the soil solid matrix. Soil pores of relatively small radial dimension, behaving similar to capillary tubes, permit greater height of rise (h ∝ 1/R), which is to say that water in small pores is held at greater tension. Measurements of the amount of water released from soil at varying pressures have been used together with equation 7 to characterize soil pore size distribution. The analog of the capillary tube for soil pores also has been used to understand wetting above the water table (e.g., in the unsaturated zone) and how management practices that alter pore size distribution, such as agricultural tillage, can affect soil's water holding capacity.

The prevalence of water within the soil system also drives terrestrial temperature dynamics (Campbell & Norman 1998). When liquid water enters the soil matrix, it displaces the soil gas phase (Figure 4). Water requires a relatively large energy input or loss (heat capacity, 4.18 kJ kg-1 K-1) to change its temperature because of strong interactions (hydrogen bonding) between dipolar water molecules. Alternately, weak intermolecular interactions in soil gases allow soil temperatures to change readily with a small energy input or loss. Furthermore, because soil water can exist in liquid, gas (vapor) and solid (ice) phases, latent heat loss or gain from soil associated with phase change also impacts the thermal regime (Campbell & Norman 1998). In moist environments with ample soil water, water vapor loss from soil through evaporation requires a large energy input (heat of vaporization, 2449 kJ kg-1) without accompanying temperature change because of the large energy input required to disrupt the hydrogen bonds between liquid water molecules (Figure 2B). Similarly, a large latent heat loss is required for soil freezing due to the high heat of fusion (334 kJ kg-1) associated with the more rigid, low-density structure of ice (Figure 2C). Thermal properties of water within soil mediate environmental conditions for the biological community and regulate the thermodynamics of biogeochemical reactions.

Summary and Conclusions

The structure of water can directly explain a unique set of inter- and intra-molecular forces, most of which stem from water's dipole moment, that result in a range of familiar properties. These properties are critical to the weathering processes that form soils. They regulate flows of matter and energy that allow soil to be a vibrant medium supporting a host of living organisms. Soil water is thus a critical resource that lays the foundation for ecosystem health and function. As the human population continues to increase, understanding the behavior and importance of water in soils will become ever more critical for effectively utilizing soil to address society's growing food, energy, and water needs (Falkenmark & Rockström 2006, Sposito 2013).


adhesive: the property of a substance that strongly interacts with the surface of other substances

amphoteric: the ability to act as an acid or a base

boiling point: the temperature at which a liquid changes to a gas

cohesive: the property of a substance that strongly interacts with other molecules of the same substance

cooperative: the property of being more energetically favorable for water molecules to be surrounded by other water molecules in large groups than to exist as individual molecules or dimers

covalent bond: a chemical bond associated with the sharing of electrons between atoms

dielectric constant: permittivity of a substance expressed as a ratio to the permittivity of free space; a high dielectric constant allows a medium to stabilize charged species

dimer: a structure composed of two associated identical subunits

dipole: the separation of positive and negative charge on a molecule

dissolution: a phase change resulting in a solid converting to a dissolved species

heat capacity: the quantity of heat required to change the temperature of a substance by a given amount

heat of fusion: the quantity of heat that must be absorbed or released in order for a substance to transition between liquid and solid phases

heat of vaporization: the quantity of heat that must be absorbed or released in order for a substance to transition between gas and liquid phases

hydrogen bond: intermolecular electrostatic attractive interaction between polar molecules in which hydrogen interacts with the negative pole of a molecule; usually this pole is an atom of oxygen, nitrogen, or a halogen

ionic dissociation: a reaction of molecules of the same substance that produces ions. For water, this is the reaction of water to produce protons and hydroxide ions.

latent heat: heat that is absorbed or released without change in temperature; heat associated with the transition between solid, liquid, and gas phases

pedosphere: the outer layer of the Earth's surface that is composed of soil

redox: the class of chemical reactions that involve the transfer of electrons (oxidation and reduction)

soil pH: a measure of the acidity of soils that is defined as the negative logarithm of proton activity (pH = - log[H+])

solvation spheres: water molecules that are in direct contact with a dissolved species; also known as solvation shells, hydration shells or hydration spheres

surface tension: a property associated with cohesion of similar molecules at a liquid-gas interface that provides a tendency toward inward contraction of the liquid

tetrahedra: a central atom bound to four atoms that are located at the apices of a pyramid composed of equilateral triangles

unsaturated zone: the area above the water table that contains both water and air in the pore space

water table: surface where the water pressure in pores equals atmospheric pressure; below the water table, pores are filled with water

References and Recommended Reading

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Eisenberg, D. & Kauzmann, W. The Structure and Properties of Water. 296 Oxford University Press, 1969.

Essington, M. E. Soil and Water Chemistry: An Integrated Approach. CRC Press, 2004.

Falkenmark, M. & Rockström, J. The New Blue and Green Water Paradigm: Breaking New Ground for Water Resources Planning and Management. Journal of Water Resources Planning and Management 132, 129-132 (2006).

Fortes, A. D. et al. No evidence for large-scale proton ordering in Antarctic ice from powder neutron diffraction. The Journal of Chemical Physics 120, 11376-11379 (2004).

Fortuna, A. The soil biota. Nature Education Knowledge 3, 1 (2012).

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