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2.3.1 Soil moisture content

The soil moisture content indicates the amount of water present in the soil.

It is commonly expressed as the amount of water (in mm of water depth) present in a depth of one metre of soil. For example: when an amount of water (in mm of water depth) of 150 mm is present in a depth of one metre of soil, the soil moisture content is 150 mm/m (see Fig. 36).

Fig. 36. A soil moisture content of 150 mm/m

The soil moisture content can also be expressed in percent of volume. In the example above, 1 m³ of soil (e.g. with a depth of 1 m, and a surface area of 1 m²) contains 0.150 m³ of water (e.g. with a depth of 150 mm = 0.150 m and a surface area of 1 m²). This results in a soil moisture content in volume percent of:

 

Thus, a moisture content of 100 mm/m corresponds to a moisture content of 10 volume percent.

Note: The amount of water stored in the soil is not constant with time, but may vary.

2.3.2 Saturation

During a rain shower or irrigation application, the soil pores will fill with water. If all soil pores are filled with water the soil is said to be saturated. There is no air left in the soil (see Fig. 37a). It is easy to determine in the field if a soil is saturated. If a handful of saturated soil is squeezed, some (muddy) water will run between the fingers.

Plants need air and water in the soil. At saturation, no air is present and the plant will suffer. Many crops cannot withstand saturated soil conditions for a period of more than 2-5 days. Rice is one of the exceptions to this rule. The period of saturation of the topsoil usually does not last long. After the rain or the irrigation has stopped, part of the water present in the larger pores will move downward. This process is called drainage or percolation.

The water drained from the pores is replaced by air. In coarse textured (sandy) soils, drainage is completed within a period of a few hours. In fine textured (clayey) soils, drainage may take some (2-3) days.

2.3.3 Field capacity

After the drainage has stopped, the large soil pores are filled with both air and water while the smaller pores are still full of water. At this stage, the soil is said to be at field capacity. At field capacity, the water and air contents of the soil are considered to be ideal for crop growth (see Fig. 37b).

2.3.4 Permanent wilting point

Little by little, the water stored in the soil is taken up by the plant roots or evaporated from the topsoil into the atmosphere. If no additional water is supplied to the soil, it gradually dries out.

The dryer the soil becomes, the more tightly the remaining water is retained and the more difficult it is for the plant roots to extract it. At a certain stage, the uptake of water is not sufficient to meet the plant’s needs. The plant looses freshness and wilts; the leaves change colour from green to yellow. Finally the plant dies.

The soil water content at the stage where the plant dies, is called permanent wilting point. The soil still contains some water, but it is too difficult for the roots to suck it from the soil (see Fig. 37c).

Fig. 37. Some soil moisture characteristics

2.4 Available water content

The soil can be compared to a water reservoir for the plants. When the soil is saturated, the reservoir is full. However, some water drains rapidly below the rootzone before the plant can use it (see Fig. 38a).

Fig. 38a. Saturation

When this water has drained away, the soil is at field capacity. The plant roots draw water from what remains in the reservoir (see Fig. 38b).

Fig. 38b. Field capacity

When the soil reaches permanent wilting point, the remaining water is no longer available to the plant (see Fig. 38c).

Fig. 38c. Permanent wilting point

The amount of water actually available to the plant is the amount of water stored in the soil at field capacity minus the water that will remain in the soil at permanent wilting point. This is illustrated in Fig. 39.

Fig. 39. The available soil moisture or water content

Available water content = water content at field capacity – water content at permanent wilting point ….. (13)

The available water content depends greatly on the soil texture and structure. A range of values for different types of soil is given in the following table.

Soil	Available water content in mm water depth per m soil depth (mm/m)
sand	25 to 100
loam	100 to 175
clay	175 to 250

The field capacity, permanent wilting point (PWP) and available water content are called the soil moisture characteristics. They are constant for a given soil, but vary widely from one type of soil to another.