Most of the issues about soil water relate to its energy state and its movement (e.g. Evapotranspiration and Deep Drainage). Classical physics recognises kinetic (i.e. movement) and potential (i.e. position) energy. In soil, water does not move rapidly so kinetic energy is negligible. Therefore, water moves constantly in direction of potential energy (i.e. wet to dry soil), where the gradient of potential energy with distance is the moving force causing flow.

An indication of the tendency of soil water to move is expressed by the soil water potential (Ψ). Ψ is defined as the work water can do as it moves from its present state to the reference state. The reference state is the energy of a pool of pure water at an elevation defined to be zero.

In soil, the reference state is the energy level of water in the soil at saturation. That is, when all pores are filled with water. At this point soil water potential (Ψ) is nominally zero (~0). In most cases, however, soil water potential (Ψ)is less than zero. This is indicated by giving soil water potential (Ψ) a negative sign (-ve).

In practical terms, and as the soil dries out soil water potential (Ψ)decreases and becomes increasingly more -ve. So that when soil water potential (Ψ) is “high” it means Ψ is less –ve and is therefore very close to 0. When soil water potential (Ψ)is high it means soil water is
held loosely,
highly available and
ready to move somewhere else.

There are three important factors affecting total soil water potential (Y t). This includes soil water potentials of
Ψ_g Gravitational
Ψ_o Osmotic
Ψ_m Matric.

The general relationship between total soil water potential (Ψ_t) and the various factors is expressed as

Ψ_t = Ψ_g + Ψ_o + Ψ_m

The force of gravity acts on soil water as it does on all other bodies. In a soil profile the gravitational potential (Ψ_g) of water near the soil surface is always higher thanΨg in the subsoil. As a result of heavy precipitation or irrigation, therefore, the difference inΨg causes downward flow of water deeper into the soil profile.

The osmotic potential (Ψ_m) is attributable to the attraction between a water molecule and various ions (e.g. cations) and solutes (e.g. soluble salts) in the soil solution. The presence of large amounts of soluble salts results in osmotic potentials (Ψ_m) that reduce soil water potential. This makes it difficult for plants to remove soil water even though water may be present. This is known as physiological drought and is why plants wilt and appear stunted in saline soil profiles.

Finally, adhesion (attraction) of water to the soil matrix, provides a matric force (i.e. adsorption and capillarity) which reduces energy of water particles near surfaces. Effects of surface adsorption on ability of water to do work For example, water adsorbed to soil or held in capillary pores by H bonding. In saturated soil, water free to flow,Ψ_mis not a factor and value is 0.

From the above discussion, it is evident that these soil water potentials do not act in the same way; in that their separate gradients may not be equally effective in causing water flow. Nevertheless, the advantage in usingΨ_t is that it provides a unified measure by which the state of water in soil can be expressed.

More importantly, whilst these forces and pressures are significant, in specific field situations the matric potential (Ψ_m) is the most important in all unsaturated soil because the interaction between soil and water is ubiquitous. The movement and availability of water to move throughout the soil profile is therefore primarily determined by Ψ_m.