The movement, retention, and loss of water in the soil, plants, and atmosphere are primarily governed by energy dynamics. In the soil system, water has two forms of energy: kinetic (which is negligible due to slow movement) and potential energy, which drives water movement. Potential energy depends on position and guides the direction of flow—from higher to lower energy levels—until equilibrium is achieved.
Soil water’s energy status is defined by free energy, which decreases due to soil matrix, solute concentration, and gravitational effects. As a result, water naturally moves from regions of higher free energy (wet soil) to regions of lower free energy (dry soil). Elevation also plays a significant role; water at a higher point has more potential energy and flows downward within the soil profile.
Factors Affecting the Potential Energy of Water
Matric Force
Water is attracted to soil solids through adhesion, creating a matric force that causes adsorption and capillarity. This significantly reduces the energy state of water around soil particles.
Osmotic Force
Osmotic force arises from the attraction of water to solutes and ions in the soil. This lowers the water’s energy state in the soil solution.
Gravity Force
Gravitational force pulls water downward. Water located at a higher elevation in the soil profile possesses more energy than that at lower elevation, which causes downward flow.
Soil Water Potential (SWP)
Soil Water Potential (SWP) refers to the energy status of soil water relative to pure, free water in a hypothetical reference state (zero elevation, atmospheric pressure, and uniform temperature). Since soil water has lower free energy than pure water, energy must be expended to move it—this required energy is defined as its potential.
Components of SWP:
- Gravitational Potential (Ψg) – Related to elevation.
- Matric Potential (Ψm) – Caused by soil adsorption and capillarity.
- Osmotic Potential (Ψo) – Results from dissolved solutes.
- Submergence Potential (Ψs) – Present in saturated conditions below the water table.
Importance:
Soil water moves from regions of high to low potential. SWP also helps estimate how much energy a plant must exert to absorb water from the soil.
Water Movement Based on Soil Moisture Levels
In wet soil, water fills large pores and is loosely bound, allowing high mobility due to high free energy. In contrast, dry soil retains water tightly in small pores and thin films, restricting its movement and lowering energy.
When wet and dry soils are adjacent, water flows from wet (high energy) to dry (low energy) zones until equilibrium. This process is entirely driven by the difference in soil water potential.
Gravitational Potential (Ψg)
Gravitational potential is positive and depends on the height of the water above a reference point. It primarily influences water in macropores, especially under saturated conditions, and causes downward movement. Mathematically, it’s expressed as:
Ψg = gh, where
g = acceleration due to gravity
h = height of water above the reference point
Matric Potential (Ψm)
Matric potential arises from adhesion and capillarity. Water held by the soil matrix loses energy due to wetting, making Ψm always negative (except near zero in saturated soils).
In wet soils, water is in large pores and mobile (less negative Ψm).
In dry soils, it is tightly bound in fine pores (more negative Ψm).
Water flows from moist (high Ψm) to dry areas (low Ψm), especially important during unsaturated flow, which provides critical water to plant roots.
Osmotic Potential (Ψo)
Osmotic potential is the energy loss due to dissolved salts or solutes in the soil water, always resulting in a negative value. As solute concentration increases, Ψo becomes more negative.
Water flows from low solute areas (less negative) to high solute areas (more negative). While Ψo has little effect on bulk water movement in soil (due to absence of membranes), it’s critical for plant uptake since root membranes allow osmosis.
Key Issues:
- Salinity stress causes physiological drought.
- Saline irrigation water must be managed properly to reduce osmotic stress on crops.
The total water potential is the sum of Ψm, Ψo, and Ψg.
Submergence Potential (Ψs)
Submergence potential occurs under saturated soil conditions, particularly in aquifers, due to the weight of overlying water. It represents hydrostatic pressure and is only relevant below the water table.
Quantitative Expression of Soil Water Potential
Soil water potential can be measured in three primary ways:
- Energy per unit mass: Expressed in erg/gm or joule/kg.
- Energy per unit volume: Measured in dynes/cm² or newtons/m², representing pressure.
- Energy per unit weight (Hydraulic head): Expressed in terms of height of a liquid column. For instance, 1 atm pressure equals a water column of 1033 cm or mercury column of 76 cm.
Total potential head is usually described in centimeters of water, which allows for practical field measurements.
Conclusion
Understanding the concepts of soil water energy, including soil water potential and its components—gravitational, matric, osmotic, and submergence potentials—is fundamental for effective water management in agriculture. These principles also serve as essential building blocks for students pursuing careers in agricultural sciences and veterinary research in Nepal. Mastery of these topics empowers learners to better understand soil-plant-water relationships, irrigation strategies, and plant water uptake under both optimal and stressful conditions. Want to learn more about soil water energy and soil moisture characteristics curve, Enroll in the full course here and start learning : https://pedigogy.com/courses/learn-soil-physics-genesis-and-classification-with-rahul/
