As this field irrigation scene in Arizona shows (left), water has moved up by capillarity from the irrigation furrow toward the top of the ridge. The photo on the right illustrates some horizontal movement to both sides and away from the irrigation water.

3. Soil water content and water potential

Gravimetric water content: the weight of water in a soil sample per unit weight of dry soil, such as 0.25 kg water per kg of dry soil. (see Box 5.1)

Volumetric water content: the volume of water in a soil sample per unit of total soil volume.

● When considering soil water availability and movement, a general energy expression of soil water status is often used, i.e., the tendency of natural movement for all substances is always from higher energy stage to a lower energy stage. So does for water. Higher energy stage of water often means higher availability for biological use.

When soil water status is expressed in energy forms and compared to pure water under standard temperature and pressure, the difference in energy level between this pure water and that of soil water is called soil water potential.

Yt = Ym + Ys + Yp + Yg

Ym : Matric potential
Ys : Solute potential or osmotic potential
Yp : Pressure potential or submergence
Yg : Gravitational or elevation gradient

● Please refer to for information about methods for measuring soil water.

Relationship between the potential energy of pure water at a standard reference state (pressure, temperature, and elevation) and that of soil water. If the soil water contains salts and other solutes, the mutual attraction between water molecules and these chemicals reduces the potential energy of the water, the degree of the reduction being termed osmotic potential. Similarly, the mutual attraction between soil solids (soil matrix) and soil water molecules also reduces the water’s potential energy. In this case the reduction is called matric potential. Since both of these interactions reduce the water’s potential energy level compared to that of pure water, the changes in energy level (osmotic potential and matric potential) are both considered to be negative. In contrast, differences in energy due to gravity (gravitational potential) are always positive. This is because the reference elevation of the pure water is purposely designated at a site in the soil profile below that of the soil water. A plant root attempting to remove water from a moist soil would have to overcome all three forces simultaneously.