Page 34 - soil-plant-water relationship and water requirement

SOIL-PLANT-WATER RELATIONSHIP AND WATER REQUIREMENT

Certain soil water proportions are set for each soil, determining whether or not water
is available for plant growth. These are called the soil water constants, which are
described below.

• Saturation capacity:
• Field capacity:
• Permanent wilting point:

Figure 3.2-2: Soil Water Constants Diagram

Saturation capacity:

When all of the pores in the soil are filled with water, this is the total water content of
the soil. It is also known as the soil's maximum water retention capacity. At saturation
capacity, the soil moisture tension is almost equal to zero.

Field capacity:

This is the amount of water a well-drained soil contains after gravitational water
movement has materially ceased.

It is taken as the water content after 48 hours the soil has been subjected to heavy
rainfall or irrigation sufficient to cause saturation.

Field capacity can also be determined by finding the moisture content when suction is
1/3 bar for clay and 1/10 bar for sand.

At field capacity, there is still water held loosely between the soil particles by surface
tension. This is known as capillary water, and it is the primary water supply for plant
growth. Plants consume this water until there is no more accessible for crop growth, at
which point withering occurs.

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