Soil water, or soil moisture, is the water held within the pore spaces between solid soil particles. This water is a fundamental component of the terrestrial ecosystem, serving as the primary source of hydration and dissolved nutrients for plant life. The maximum amount of water a soil can hold is determined by its total porosity. The different categories of soil water are defined by the physical forces that act upon the water molecules, determining how tightly they are held and whether they are accessible to plant roots.
Water Held by Molecular Attraction
The most firmly bound type of soil water is known as hygroscopic water, which is held against the surface of soil particles through strong molecular forces called adhesion. This water exists as an ultra-thin film, often only a few molecules thick, or as water vapor surrounding the solid surfaces. The adhesive force is so powerful that it effectively locks the water molecules onto the soil structure.
This water is considered biologically unavailable to plants because the force required for a root to extract it far exceeds the plant’s osmotic capability. It is held at extremely high tension, often greater than 31 bars. Hygroscopic water remains in the soil even after air-drying and can only be removed by oven-drying at high temperatures. The moisture content at which a plant can no longer absorb water and permanently wilts is closely related to the presence of this strongly bound water.
Plant Available Water
The most ecologically significant form of soil water is capillary water, as this is the primary reservoir used by plants for growth and survival. This water is retained in the smaller pore spaces, known as micropores, against the downward pull of gravity. Capillary water is held in place by a combination of surface tension and cohesive forces between water molecules, a phenomenon called capillary action.
Cohesion and adhesion combine to create a concave meniscus that draws the water upward and holds it within the narrow soil pores. The amount of water remaining after gravitational drainage has ceased is called “Field Capacity,” and capillary water exists between this point and the permanent wilting point. This water is moderately mobile and can move laterally or slightly upward through capillary rise, which helps replenish moisture near the root zone. Because the soil holds it with a tension much lower than the plant’s root suction, typically between 0.33 and 31 bars, it is readily accessible for uptake.
Water That Drains Freely
Gravitational water is the third category, defined by the force of gravity, which acts as the dominant influence on its movement. This water is present in the largest soil channels and spaces, called macropores, immediately after a heavy rainfall or irrigation event. When macropores are filled with water, the soil is said to be saturated.
The force of gravity quickly pulls this water downward through the soil profile, typically draining it within a few hours to the deeper subsoil or the water table. While technically available to plants, its tenure in the root zone is so brief that it is considered inefficient for sustained use. This rapid movement is primarily associated with drainage, infiltration rates, and the potential for nutrient leaching. Once the gravitational water has drained away, the soil is considered to be at field capacity.