Soil water potential is a measurement of the energy status of water within the soil, which determines how readily water moves and how available it is for plants. This concept provides an assessment of the intensity with which water is held by the soil matrix. Understanding this energy state is fundamental because water spontaneously moves from areas of higher energy to areas of lower energy until equilibrium is reached. This movement is the driving force behind water flow in the soil and is a primary factor in plant water uptake.
The Physics of Water Movement in Soil
The movement of water through soil is governed by differences in its potential energy, known as the total soil water potential (\(\Psi_w\)). This potential energy is defined relative to a standard reference state: pure, free water at a specific elevation and atmospheric pressure, which is conventionally assigned a potential value of zero. Water in the soil typically has a lower energy state than this reference, meaning its potential is almost always a negative value.
Since water movement in soil is slow, its kinetic energy is negligible, making potential energy the dominant factor in determining its status and movement. The difference in water potential between two points creates a gradient that dictates the direction and rate of water flow. Water flows from a higher, or less negative, potential toward a lower, or more negative, potential.
Soil water potential is commonly expressed in units of pressure, such as kilopascals (kPa) or bars, because these units represent energy per unit volume. A more negative kPa value indicates that the water is held more tightly by the soil and requires greater energy to be extracted. For example, a saturated soil might have a potential near 0 kPa, while a very dry soil can have a potential far below -1500 kPa.
The Forces That Determine Potential
The total soil water potential (\(\Psi_w\)) is the sum of several component potentials that represent the different forces acting on the water in the soil. Each component reflects a different interaction affecting the energy state of the water.
Matric Potential (\(\Psi_m\))
Matric potential is the most significant component in unsaturated soil. It is caused by the attraction of water molecules to soil particles (adhesion) and to each other (cohesion). This force holds water tightly in small pores and as thin films around particles, reducing its energy and resulting in a negative value. As the soil dries out, the matric potential becomes increasingly negative because the remaining water is held with greater tenacity.
Osmotic Potential (\(\Psi_o\))
Osmotic potential results from the presence of dissolved salts and other solutes in the soil water. These solutes lower the energy of the water, making the osmotic potential negative. This component is particularly relevant in saline soils or when considering water movement across a semipermeable membrane, such as a plant root cell wall.
Pressure Potential (\(\Psi_p\))
Pressure potential is related to the physical pressure exerted on the water. In unsaturated soil, this potential is typically considered zero. It becomes positive below the water table, where the water is under hydrostatic pressure greater than atmospheric pressure.
Gravitational Potential (\(\Psi_g\))
Gravitational potential is due to the water’s position in the Earth’s gravitational field. This potential is calculated relative to an arbitrary reference elevation, increasing as the water’s height above the reference plane increases. While often ignored in small-scale measurements, it becomes important for modeling water movement over deep soil profiles.
Tools for Quantifying Soil Water Status
Measuring soil water potential in the field is necessary for applications like irrigation scheduling and hydrological modeling. The instruments used must quantify the energy status of the water rather than simply the volume present. Different tools are employed based on the range of soil wetness being monitored.
Tensiometers are widely used devices for measuring soil water potential in wet soils. They consist of a porous ceramic cup connected to a pressure gauge or transducer. The ceramic cup equilibrates with the surrounding soil water potential, and the gauge measures the tension exerted on the water. Tensiometers are limited to a range of approximately 0 to -80 kPa, because air can enter the ceramic cup and break the water column in drier soils.
For drier soils, instruments like thermocouple psychrometers are used. These devices measure the relative humidity or vapor pressure of the air in equilibrium with the soil water within a sealed chamber. This measurement is then converted to water potential. Psychrometers are effective across a much wider and more negative range, often from about -98 kPa to -3000 kPa.
Other methods, such as heat dissipation sensors, are also utilized, often employing a ceramic material that changes its thermal properties based on water content.
Water Potential and Plant Uptake
The energy status of soil water directly dictates its availability to plants, as water must move from the soil into the roots. For a plant to absorb water, the water potential in the root must be lower, or more negative, than the soil water potential. This creates the necessary energy gradient for flow, linking the physics of the soil to the biology of the plant.
Two specific thresholds of soil water potential are relevant to agriculture and plant physiology. Monitoring the soil water potential between these two values allows growers to optimize irrigation and prevent crop stress.
Field Capacity
Field Capacity represents the upper limit of useful water storage. This is the potential at which gravitational drainage has effectively ceased, typically occurring around -10 to -33 kPa, depending on the soil texture. Water held at a potential higher than this point will drain away quickly.
Permanent Wilting Point
The Permanent Wilting Point marks the lower limit of water availability. At this potential, the water is held so tightly by the soil matrix that the plant cannot extract it quickly enough to prevent irreversible damage. This point is conventionally set at -1500 kPa, and represents the potential at which plants can no longer sustain their physiological processes.