Introduction to Soil Chemistry

74 soil basics iv
Soil
particle
–3100 kPa
Hygroscopic
coefficient
–1500 kPa
Permanent
wilting point
Plant available
water
–100 kPa
Loosely held
water
–30 kPa Maximum
water holding
capacity
Water removed
by gravity
0 kPa
Saturated
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 -3100pKa and is called hygroscopic water. A soil sample, heated
in an oven for 24h 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 (-1bar =
-100kPa).
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 understand
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