Page 33 - soil-plant-water relationship and water requirement

SOIL-PLANT-WATER RELATIONSHIP AND WATER REQUIREMENT

Figure 3.2-1: Soil Water Diagram

Gravitational water:

In a soil, which is completely saturated, large pores are filled with what is called
gravitational water because it can drain out under gravity. It evaporates so quickly that
it is unavailable to the crops. The time it takes for water to drain out varies from one
day to four days in sandy soils and four days in clay soils.

Capillary water:

Capillary water is the water that remains in the soil after gravity water has drained away.
Surface tension holds this water in the soil. Plant roots collect capillary water over time
and thus serve as the primary source of water for plant growth.

Hygroscopic water:

Hygroscopic water is the water that an oven-dried sample of soil absorbs when exposed
to damp air. It's held in place as a very thin coating on the surface of the soil particles,
and it's under a lot of negative (gauge) pressure. Plants do not have access to this water.

When compared to coarse-textured soils (clay and clay-loam soils), fine-textured soils
(clay and clay-loam soils) have a higher porosity (sand). As a result, they are able to hold
more water than sandy soils. Plants, on the other hand, do not have access to a major
percentage of the water stored in fine-textured soils. This is due to the fact that fine-
textured soils have smaller pores that hold water more securely.

Plants must overcome the pressures that keep water in soil pores in order to absorb it.

24