Soil Water: From Molecular Structure to Behavior

"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 21^st 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.

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).

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).

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.

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) + O_2(g) → CO_2(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).

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).

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.

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.

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).