Field capacity FC

As gravitational water (sometimes referred to as 'excess water') is removed, air returns in its place. On sandy soils this may take a matter of hours after the rain has stopped, but may take far longer on clay where the process may continue for many days. The soil is then said to be at field capacity (FC). More precisely, it is a soil that has been saturated, then allowed to drain freely without evaporation until drainage effectively ceases. In practice it is assessed after two days.

At field capacity the micropores (those less than about 0.05 mm diameter) remain full of water; whereas in the macropores (greater than about 0.05 mm) air replaces the gravitational water, as illustrated in

Gravitational Water

1 Dry soil wetted.

2 Saturated zone extends downward as rain continues.

3 No rain; gravitational water moves downwards E] = Water until water film tension equal to pull of gravity. ¡^ _ Rain

4 Further rain saturates top layers before more soil wetted.

5 Water table forms if obstruction to gravitational water.

6 Drained soil, at FIELD CAPACITY down to drains.

7 At PERMANENT WILTING POINT.

Water table '

Drain

1 Dry soil wetted.

2 Saturated zone extends downward as rain continues.

3 No rain; gravitational water moves downwards E] = Water until water film tension equal to pull of gravity. ¡^ _ Rain

4 Further rain saturates top layers before more soil wetted.

5 Water table forms if obstruction to gravitational water.

6 Drained soil, at FIELD CAPACITY down to drains.

7 At PERMANENT WILTING POINT.

Water table '

Drain

Figure 19.3 Water in the soil

Table 19.1 Soil water holding capacity: the amount of water in a given depth of soil at field capacity can be calculated by simple proportion

Water held in 300 mm soil depth (mm)

Soil texture

at field capacity (FC) i.e. water at permanent wilting holding capacity (WHC) point (PWP)

Available water (AW)

Coarse sand

26

1

25

Fine sand

65

5

60

Coarse sandy loam

42

2

40

Fine sandy loam

65

5

60

Silty loam

65

5

60

Clay loam

65

10

55

Clay

65

15

50

Peat

120

30

90

Figure 19.3. The air expelled has been replaced by 'fresh' air which is higher in oxygen and lower in carbon dioxide content.

The amount of water held at field capacity is known as the waterholding capacity (WHC) or moisture-holding capacity (MHC). Examples are given in Table 19.1 The WHC is expressed in millimetres of water for a given depth of soil. Thus a silty loam soil 300 mm deep holds 65 mm of water when at field capacity. Conversely, if a silty loam had become completely dry to 300 mm depth, it would require 65 mm of rain or irrigation water to return it to field capacity; about an average month's rainfall in many parts of the British Isles. Since 1 mm of water is equivalent to 10m3/ha, a hectare of silty loam would hold 650 m3 water in the top 300mmwhen at field capacity. The principle described enables water-holding capacity or irrigation requirement to be determined for any soil depth. The amount of water required to return a soil to field capacity is called the soil moisture deficit (SMD).

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