2.4: Soil Water Potential

The amount of water in the soil is not the only concern in irrigation management. Plants must be able to extract water from the soil. Soil water potential (Ψ_t) is an indicator or measure of the energy status of soil water relative to that of water at a standard reference (Hillel, 1980). This energy is due to the position of the water relative to the reference and the internal state of the water and is often expressed as energy per unit of volume (pressure) or energy per unit of weight (head). Common units of pressure and head, and their equivalents are shown in Table 2.1. The standard reference is often denoted at a high energy level and assigned a value of zero. Thus, soil water potential and its components can all have negative values. A high energy level of water potential will have a smaller negative value (lower magnitude) than a water potential at a lower energy level. For example, in a wet soil, matric potential (discussed below), Ψ_m, will have a small negative value, say Ψ_m = – 0.3 bar, while in a dry soil Ψ_m may be –15 bars. The three major components of soil water potential are gravitational potential (ψ_g), matric potential (ψ_m), and osmotic potential (ψ_o). The soil water potential (ψ_t) then is:

\(Ψ_t=Ψ_g+Ψ_m+Ψ_o\) (2.7)

Table 2. 1 Common units of pressure and head and their equivalents.

Unit	Pressure Equivalent	Water Head Equivalent
1 atmosphere (atm)	101.3 kPa (kilopascals)	1034 cm H2O
	101.3 bar	34 ft H2O
	101.3 cb (centibar)	76 cm Hg
	14.7 psi (lb/in^2)	29.9 in Hg
1 psi (lb/in^2)	6.89 kPa	2.31 ft H2

Equation 2.7 ignores the impact of overburden pressure on soil water potential. The gravitational potential is due to the force of gravity pulling downward on the water in the soil. Matric potential is a result of the forces the soil particles place on the water by adhesion and surface tension at the soil-air interface. These combined forces cause capillarity, which is sometimes referred to as soil water tension. Soil water tension is expressed as a positive value. Osmotic potential is caused by dissolved solids (salts) in the soil water. The osmotic potential affects the availability and movement of water in soils when a semipermeable membrane (like plant roots) is present. This topic is discussed in more detail in Chapter 7.

Where rainfall is significant and irrigation water is nearly free of salts, the concentration of salts in the soil is generally low, so the osmotic potential is near zero. The osmotic potential does not influence the flow of water through the soil profile. It does, however, have an effect on water uptake by plants and on evaporation. During evaporation, water changes from liquid to vapor at the soil-air interface near the soil surface but salts are left behind in the soil. The higher the salt content of soil water (lower osmotic potential) the lower the rate of evaporation. Water uptake through plant roots is also influenced by the osmotic potential; the higher the salt concentration in the soil solution the more work a plant has to do to absorb water from the soil. Thus, where soil salinity is appreciable, osmotic potential must be considered for evaluating plant water uptake or where water vapor flow is important.

The component of soil water potential that dominates the release of water from soil to plants when salts are minimal is the matric potential. Several forces are involved in the retention of water by the soil matrix. The most strongly held water is adsorbed around soil particles by electrical forces. This water is typically held too tightly for plants to extract. Water is also held in the pores between soil particles by a combination of attractive (surface tension) and adhesive forces. The strength of the attractive force depends on the sizes of the soil pores. Large pores will freely give up pore water to plants due to the much higher matric potential in the soil or to drainage due to the gravitational potential (Martin et al., 2017). For a given amount of water in a particular soil, there will be a corresponding matric water potential. Here we will express the magnitude of the matric potential as soil water tension thus in the positive realm. The curve representing the relationship between the water tension within the soil and its volumetric water content is referred to as the soil water release or soil water retention curve. The soil water release curves in Figure 2.6 show that water is released (volumetric water content is reduced) by the soil as the tension increases.

Soil water release curves are often used to define the amount of water available to plants. Two terms are used to define the upper and lower limits of water availability. The upper limit, field capacity (θ_fc), is defined as the soil water content where the drainage rate, caused by gravity, becomes negligible. Thus, the soil is holding all of the water it can without any significant loss due to drainage. The permanent wilting point (θ_wp), the lower limit, is the water content below which plants can no longer extract water from the soil. At this point (WP) and at higher tension values, plants will wilt permanently and will not recover if the water stress is relieved. Neither of these two limits are exact. The WP has traditionally been defined as the water content corresponding to 15 bars of soil water tension or 1,500 cb. This is a reasonable working definition because the water content varies only slightly over a wide range of soil water tension near 15 bars. For example, if the plants permanently wilt at 20 bars of tension, the water content is not much different than at 15 bars and the error in the estimate of water available to plants is small. Example values of θ_fc and θ_wp are given in Figure 2.6 for several soil types.

Figure 2.6. Example soil water release curves for three soil textures showing the values of θ_s, θ_fc, and θ_wp for each soil.

This figure shows the soil water release curve for hypothetical fine sand, fine sandy loam, and silt loam soils. The vertical axis is volumetric water content (unitless), the horizontal axis is the soil water tension in units of centibars (cb). The saturated volumetric water contents (corresponding to soil water tension of 1 cb) are 0.36, 0.42, and 0.53, respectively. The water contents at the permanent wilting point (1500 cb) are 0.06, 0.10, and 01.6, respectively.

Field capacity is often considered to be the water content at a matric potential of minus one-third bar or a tension of 33 cb. This is not a good definition for all soils. This tension value for FC is fairly good for some fine-textured soils but is too large a tension for medium and coarse-textured soils. The field capacity values shown in Figure 2.6 are more representative than a strict one-third bar definition. Methods will be presented later where the actual field conditions will be used to estimate field capacity for irrigation management.

Users of soil water measurements must keep in mind that there is a difference between volumetric water content at field capacity (θ_fc) and volumetric water content at saturation (θ_s). If the voids are completely filled with water and air is absent, the soil is said to be saturated. The volumetric water content equals porosity at saturation, i.e., θ_s = φ. As gravity causes drainage to occur, air enters the soil and soil water content reaches θ_fc as drainage from gravity ceases. Thus, θ_fc is less than θ_s.

The relationships among the soil water that is free to drain due to gravity, the soil water available for plant water use, and the soil water that is not available to be extracted by plant roots is illustrated in Figure 2.7 on a soil water release curve. The water that is free to drain by gravity is between θ_s and θ_fc. Available water is that water between θ_fc and θ_wp, and unavailable water is that water between θ_wp and 0.

Figure 2.7. Graphical representation of free-draining water, water available for plant uptake, and unavailable water on a soil water release curve.

The approximate fractions of free/gravitional water, plant available water, and plant unvailable water are shown by varying degrees of shading in the soil water release curve for a hypothetical silt loam soil.