Water is the dynamic component of soil within the matrix of mineral particles and organic matter. This water is fundamental for all terrestrial life, acting as the solvent for nutrients and the medium for biological activity. Capillary water, held in the small spaces between soil grains, represents the most significant form of moisture for maintaining plant life.
The Physics of Capillary Action
Capillary water is the moisture held within the small pore spaces, or capillaries, of the soil against the downward pull of gravity. This retention mechanism, known as capillary action, is a direct consequence of water molecules interacting with the soil’s solid surfaces.
Two intermolecular forces drive this process: cohesion and adhesion. Adhesion is the attractive force between water molecules and the surfaces of soil particles, causing the water to stick to the solid material. Cohesion is the force of attraction between water molecules themselves, causing them to cling to one another.
Working together, adhesion pulls a thin film of water onto the soil particles, and cohesion then draws more water molecules up and into the narrow pore space. The smaller the diameter of the soil pore, the stronger the combined adhesive and cohesive forces are relative to the force of gravity. This stronger pull allows water to be held higher and more tightly within fine-textured soils like clay, which are characterized by numerous small pores.
The upward movement of water against gravity is noticeable in the capillary fringe, the zone of saturation immediately above the water table, where capillary action draws water into pore spaces otherwise filled with air. This movement continuously redistributes moisture from wetter to drier areas within the soil profile, ensuring a more uniform supply of moisture.
Capillary Water and Other Forms of Soil Moisture
Soil moisture is broadly divided into three categories. Capillary water is distinct because of its position between the two extremes of water availability, retained in the soil’s micropores, which are the small, irregular channels between individual soil particles.
Gravitational water is found in the large soil pores, or macropores. This water is held loosely and drains rapidly downward through the soil profile under the influence of gravity, typically within one to three days after a heavy rain or irrigation event. Because it moves so quickly, gravitational water is generally unavailable for plant uptake, though it is important for recharging underground aquifers.
At the opposite end is hygroscopic water, which is held as a thin film on the surface of soil particles. This water is bound by molecular forces of adhesion that are so strong plants cannot overcome them. Hygroscopic water is present even in dry soil and can only be removed by oven-drying the sample at high temperatures.
Capillary water is held with enough tension to resist gravity but with a force weak enough for plant roots to extract it. It is the primary reservoir of water accessible to plants and is sometimes referred to as the available water. This makes capillary water the most biologically significant form of moisture in the soil.
Essential Role in Plant Hydration
The availability of capillary water is governed by two measurable soil moisture thresholds: field capacity and the permanent wilting point. Field capacity (FC) is the upper limit of water available to plants and represents the amount of water remaining in the soil after gravitational water has drained away. At this point, the water is held at a relatively low tension, around -0.33 bar.
The permanent wilting point (PWP) is the lower boundary of available water, occurring when the soil water tension reaches approximately -15 bars. At this level of tension, the remaining capillary water is held so tightly by the soil particles that plant roots cannot exert enough suction to extract it. When the soil moisture content drops below this threshold, the plant wilts and cannot recover turgor, even when transpiration ceases.
The total amount of water that a plant can use is the difference between the moisture content at field capacity and the moisture content at the permanent wilting point. This range is known as the Total Available Water. Capillary water thus acts as the life-sustaining reserve, providing the moisture necessary for photosynthesis, nutrient transport, and maintaining the structural rigidity of plant cells.