This places the soil water under tension, much like people in a chain when one member tries to pull away from the others, or a block of steel is placed under tension if pulled on at both ends. Therefore, water in soil can be held under tension because of the adsorption of water to the soil particles. The decrease in water potential caused by the adsorption of water to the soil surfaces is called the Matric Potential component of the soil water potential. Matric potential is always negative or zero since the adsorption of water onto soil surfaces can only lower the potential energy relative to reference water. Solutes that reduce the potential energy of water are called osmotically active solutes.
Inorganic salts are all osmotically active and many large organic molecules are osmotically active.
The reduction in potential energy from dissolved solutes arises partly from the hydration of the solute or the forming of chemical bonds between the solute and water molecules. Soil water contains many dissolved salts as well as organic molecules released by plant roots and microorganisms. Soil water is not pure water but rather a solution and the presence of osmotically active solutes reduces the soil water potential. The reduction in soil water potential caused by the presence of dissolved solutes is called Osmotic or Solute Potential component of the soil water potential. Osmotic potential is always negative or zero because dissolved solutes can only lower the potential energy of water.