Agriculture Reference
In-Depth Information
Plant available
water
Soil
particle
Figure 4.2.
Potential of “layers” of water surrounding a soil particle.
plants but can be lost by evaporation. The layer closest to the soil solid is held
at more than
3100 pKa and is called
hygroscopic water
. A soil sample, heated
in an oven for 24 h at 105°C and then left exposed to the air will adsorb water
until a layer of hygroscopic water has been formed, illustrating the strong
attraction of water for soil surfaces.
The unit kPa is in common use today and is part of the International System
of Units (SI). However, it is also common to encounter the terms
bars
and
atmospheres
(atm) when reading about soil water. One bar is approximately
equal to one atmosphere pressure, which is abbreviated atm (
-
-
1 bar
=
-
100 kPa).
The movement of water through soil is controlled by the diameter of soil
pores and the surface tension of water. Water drains from larger pores and
moves down through soil and into the groundwater. In small pores the surface
tension of water is strong enough to prevent movement of water into or out
of pores. However, because of surface tension, pores will also draw water up
from a free water surface until the surface tension is balanced by the pull of
gravity. The smaller the pore, the higher the water will be raised. If water,
moving down through soil, reaches a compacted zone, for example, having few
and extremely small pores, it will move laterally along the dense layer. Thus,
water can move down, up, and sideways in soil depending on the soil's pores.
Pores can control water movement in other ways related to size. To under-
stand this control, soil pores can be grouped or classified simply as large, those
that allow water to drain or move and small, and those that do not. Water in
