Three physical properties of water – ionic interactions with nutrients and other ions, a strong force of cohesion, and a strong force of adhesion – create the forces that hold water within the plant root zone (Strachan and Jeschke, 2017). Cohesion is the ability of water molecules to stick to other water molecules. Adhesion is the ability of water molecules to stick to other molecules that are not water molecules. These forces interact with soil-bound cations, clay minerals, organic matter, and other solid materials that constitute soil colloids. If these forces were not present, the force of gravity would pull water molecules deeper into the soil profile where they would no longer be available for plant uptake. Soil pores retain plant-available water. Soil texture, organic matter content, and bulk density determine the distribution and size of soil pores (Hillel, 1980).
The soil mineral fraction consists of silicates and aluminum hydroxy silicates, and soil organic matter contains oxygen and nitrogen atoms essential for retaining water. Oxygen and nitrogen atoms in both soil constituents are capable of hydrogen bonding with hydrogen atoms of water molecules. In addition, the chemical structure of aluminum hydroxy silicates in the soil mineral fraction and the molecular structure of organic acids and other materials in soil organic matter create net negative charges that are dispersed among water molecules located next to these soil constituents. Negative charges associated with soil minerals and organic matter also create cation exchange sites. Cations associated with these sites produce positive charges that are dispersed among the water molecules located next to these cations. The combined forces of ionic charge dispersal and hydrogen bonding hold water molecules very tightly within the soil matrix and negate the downward force of gravity (Figure 2).