3.2 Soil Water Potential
Knowing the soil water content is useful for many applications, but there is another variable which is equally important to understanding soil water processes, and that variable is called soil water potential. Soil water potential is a measure of the potential energy of the soil solution. Potential energy is energy that a substance or object has by virtue of its location or internal condition, and potential energy is always defined relative to some reference condition. For example, a book resting on a table has less potential energy than the same book would have if you held it in the air above the table. If you release the book, it will spontaneously fall back down to the table due to the force of gravity. In this case, the increased potential energy came from raising the book upward against the Earth’s gravitational field, and the book’s spontaneous fall when released illustrates the general tendency of things to move from locations of higher to lower potential energy.
3.2.1 Soil Water Potential Terminology
Thus, soil water potential (ψ) is the potential energy of the soil solution relative to that of water in standard reference state. By convention, that state is defined by:
- pure water
- at atmospheric pressure
- at the same temperature as the soil solution
- at a specified, constant elevation.
Differences in soil water potential cause water in soil to flow from regions of higher water potential to regions of lower water potential. When the water potential is equal throughout a certain region of soil, then that region has reached a condition called equilibrium. At equilibrium, there is no water flow. The concept of equilibrium is important for theoretical purposes and laboratory experiments but rarely, if ever, occurs in the field.
Most scientists consider the total soil water potential to be the simple sum of component potentials from several contributing factors, most notably: gravitational potential, osmotic potential, and pressure potential.
- Gravitational potential (ψg) is the potential energy which is due to the position of substance or object in a gravitational field. Gravitational potential is measured with respect to an arbitrary reference elevation.
- Osmotic potential (ψo) is the decrease in the water potential which is due to the presence of solutes. Osmotic potential affects soil water flow whenever there is a gradient in solute concentration within the soil. Such gradients are especially persistent where there is a membrane, such as the wall of a plant cell, or a barrier, such as the soil surface, that restricts the movement of either water or solutes.
- Pressure potential (ψp) refers to the soil water potential attributable to any other factors besides gravity and solutes. Factors such as capillarity, adsorption, submergence, and air pressure can all influence the pressure potential.
- Matric potential (ψm) is an important subcategory of pressure potential. Matric potential is the decrease in the water