Soil Moisture Classification

In terms of soil-water management, the classification of soil water potentialΨm is based on the availability of water to plants in the soil environment. Terms used to describe this include;

  • Saturation (Gravitational water),
  • Field capacity,
  • Permanent wilting point, and
  • Available water capacity

Water Figure 6

Saturation

At this point soil is said to be at its maximum retentive capacity. That is, all soil pores are filled with water. Saturation usually occurs for short periods of time, either during heavy rainfall events or when soil is being irrigated. TheΨm is close to zero (i.e. ~ 0) and is nearly the same as pure water.

At saturation, soil porosity corresponds to the volumetric moisture content (q - theta). Soil with the greatest pore space would be a well aggregated clay. With respect to soil with a Silty Loam soil texture, q will be close to 50 cm3/cm3 at saturation.

Soil remains at saturation so long as water is infiltrating, otherwise water in large pores will drain freely under the forces of gravity. After two days of free drainage the soil is said to be at Field Capacity.

Field Capacity (F.C.)

Field capacity (F.C.) approximates the amount of water that is held in soil after it has been fully wetted and all gravitational water has been drained away. In practice, field capacity is reached about one to two days after heavy rainfall or irrigation ceases.

In practical terms, field capacity will be reached much faster in a coarser textured soil (e.g. Loamy Sand) than in a fine-textured soil profile (e.g. Heavy Clay).

Field capacity is useful in practical terms because it is where a number of important processes are in transition. At field capacity the soil holds the maximum amount of water that can be stored and can be used by plants.

In addition, there is sufficient air-filled pore space to allow for the aeration of most aerobic microbial activity and plant growth. Water in excess of field capacity drains to quickly for plant use and reduces aeration. From a tillage point of view, soil turns to mud above field capacity and is unworkable.

Field capacity corresponds to a soil water potential (Ψm) of about -10 to -33 J/kg m. In practice a value of -33 J/kg is used. Because forces holding water are surface-attractive forces, the more surface area a soil has the greater is the amount of water adsorbed.

For practical purposes this means clay rich or organic (loamy) soil can hold much more water than sandy soil.

Soil Texture Class qv
Sand 0.07-0.17
Loamy Sand 0.11-0.19
Sandy Loam 0.18-0.28
Loam 0.20-0.30
Silt Loam 0.22-0.36
Silt 0.28-0.36
Silty Clay Loam 0.30-0.37
Silty Clay 0.30-0.42
Clay 0.32-0.40

Water figure 7

As soil water is being used by plants and begins to dry, plants will wilt during the day to conserve water and regain turgor at night when water is not being lost through leaves. Roots in essence catch up with plant demands. However, plants will remain wilted when roots cannot generate water potentials low enough to remove water bound tightly through adhesive forces around soil. Soil is said to be at Permanent Wilting Point.

Permanent Wilting Point (P.W.P.)

Permanent wilting point (PWP) is defined as the minimum soil moisture at which a plant wilts and can no longer recover its turgidity. In the field, soil at Permanent Wilting Point (P.W.P.) will appear dusty and dry.

However, water is still present in micropores and in very thin films around soil particles. The soil water, however, is held tightly by the forces of adhesion (i.e. about 10 molecules thick) and is generally unavailable for use by plant roots and microbes.

Although clay rich soil can hold much more water than sandy soil, it is also capable of retaining water more strongly as well.

Soil Texture Class qv
Sand 0.02-0.07
Loamy Sand 0.03-0.10
Sandy Loam 0.06-0.16
Loam 0.07-0.17
Silt Loam 0.09-0.21
Silt 0.12-0.22
Silty Clay Loam 0.17-0.24
Silty Clay 0.17-0.29
Clay 0.20-0.24

At this stage soil is at Permanent Wilting Point and by convention is taken to be amount of water retained by soil when the soil water potential (Ψm) is -1,500 J/kg.

Water figure 8

Available Water Content (A.W.C.)

The amount of water held between F.C. and P.W.P. is termed the “Available Water Content” (A.W.C.), and it is a measure of the amount of water in the soil that is “potentially” available to plants.

Soil Texture Class qv
Sand 0.05-0.10
Loamy Sand 0.08-0.09
Sandy Loam 0.12-0.14
Loam 0.13-0.13
Silt Loam 0.11-0.15
Silt 0.14-0.14
Silty Clay Loam 0.13-0.13
Silty Clay 0.13-0.13
Clay 0.12-0.16
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