HOW DOES CAPILLARITY AFFECT SOIL WATER HOLDING CAPACITY?

The ability of a soil to hold or maintain water depends first upon its soil structure.Soil structure refers to the grouping of particles of sand, silt, and clay into larger aggregates of various sizes and shapes. The processes of root penetration, wetting and drying cycles, freezing and thawing, and animal activity combined with inorganic and organic cementing agents produce soil structure. Structural aggregates that are resistant to physical stress are important to the maintenance of soil tilth and productivity. Practices such as excessive cultivation or tillage of wet soils disrupt aggregates and accelerate the loss of organic matter, causing decreased aggregate stability.

The movement of air, water, and plant roots through a soil is affected by soil structure. Stable aggregates result in a network of soil pores that allow rapid exchange of air and water with plant roots. Plant growth depends on rapid rates of exchange. Good soil structure can be maintained by practicing beneficial soil management such as crop rotations, organic matter additions, and timely tillage practices. In sandy soils, aggregate stability is often difficult to maintain due to low organic matter, clay content and resistance of sand particles to cementing processes.

One important characteristic of soil is its ability to hold water against the force of gravity and supply a portion of that water to plants. Much of this capacity is related to the number and size of pores and channels distributed throughout a soil. Some water can be held so tightly on polar surfaces in the soil that many atmospheres of pressure are required to force this water out. Plant roots must out-compete the forces that hold water in soil to survive, especially as more and more water is removed from the soil. How water moves largely depends on two characteristics...

Water molecules behave in two ways:

• Cohesion Force: Because of cohesion forces, water molecules are attracted to one another. Cohesion causes water molecules to stick to one another and form water droplets.
• Adhesion Force: This force is responsible for the attraction between water and solid surfaces. For example, a drop of water can stick to a glass surface as the result of adhesion.

How important is capillary action? Think of it this way: all rainfall would drain rapidly from the soil and not be available for long-term use by many organisms nor would it be available for plant root uptake days or weeks after rainfall events. Most of the water available for plants in soil is that water categorized as capillary water. Most water would not even be in the soil in the first place without capillarity.