Agriculture Reference
In-Depth Information
(
h
ter
g
from equation B6.1.1). When the water is adsorbed to a very clean
glass surface, the angle of wetting
(alpha) is assumed to be zero, so we have
2
P
h
g
(6.1)
r
where
h
the height of the water column,
r
the radius of the capillary tube,
and
(gamma)
the surface tension of water (force per unit length). Not all soils
can be assumed to have a zero angle of wetting: where
is significantly greater
than 0, we have nonwetting soils, as explained in box 6.2.
Summary
. The combined effect of adsorption and capillarity on the free en-
ergy of soil water is expressed through the
matric potential
,
m
. Over the normal
range of potentials in nonswelling soils (0 to
1500 kPa), the matric potential
m
can be related directly to the pore radius through equation 6.1. Table 6.1
shows the radius of the largest pores that would just hold water at
m
values cor-
responding to the soil wetness states in table B6.1.1.
Osmotic Forces
. Dissolved solutes (ions and molecules) also reduce the free
energy of water through the action of an osmotic force. The reduction in free en-
ergy is measured by the
osmotic potential
,
s
. Osmotic potential is numerically
equal (but opposite in sign) to the osmotic pressure of the soil solution. The os-
Box 6.2
Nonrepellent and Water-repellent Surfaces in Soil
is measured from the liquid-solid
interface to the liquid-air interface. At most soil mineral surfaces, the angle is close
to 0 and the water is said to “wet” the surface (fig. B6.2.1a). However, if these
surfaces become coated with hydrophobic organic compounds, the water molecules
are more strongly attracted to each other than to the mineral surfaces. The water
forms distinct droplets and the contact angle increases markedly (fig. B6.2.1b).
The surface is said to “repel” the water, and the whole soil may become
water
repellent
. Such an effect occurs in sandy surface soils that experience severe drying.
Water subsequently applied to the surface neither wets the surface, nor infiltrates—
rather, water runs off. Many soil surfaces may exhibit some degree of water
repellency if they remain air-dry for a prolonged period, but the effect gradually
disappears as the soil wets up again.
The angle of wetting or
contact angle
Figure B6.2.1
(a) An air-water-solid interface with a small contact angle: water “wets” the surface.
(b) An air-water-solid interface with a large contact angle: water is “repelled” by the
surface (White 1997). Reproduced with permission of Blackwell Science Ltd.
Air
Air
Water
α
Water
α
Solid
Solid
