Geoscience Reference
In-Depth Information
taken at small depth intervals just above a constant water table. Such samples may
be obtained from a pit dug to the depth of a water table that occurs at a relatively
shallow depth close to the soil surface. The fl ow of water from the undisturbed soil
eventually reestablishes a water table at the bottom of the pit. After waiting several
more hours, there is an equilibrium between the height of the capillary rise of water
within the soil pores and the distance above the free water level in the pit.
When the lower end of a vertical capillary tube made of
hydrophobic
walls is
submerged into water to a suffi cient depth, the water moves upward into it to a level
below that of the horizontal free water surface and manifests a curved upper surface
in the form of a spherical cap, i.e., convex. Indeed, a capillary depression is created
because water molecules along the spherical cap surface are attracted to higher
numbers of neighboring water molecules than those on the free water surface. The
resulting surface pressure of the spherical cap water surface illustrated by an arrow
labeled
p
s
in Fig.
8.6
. is larger than that of the reference pressure
p
r
of the horizontal
plane of free water. This bigger surface pressure does not allow the water to enter
into the height of water level in hydrophilic tube. The difference of surface pressure
p
s
− p
r
is the pressure by which the water level is pressed down, when compared to
horizontal plane of water level. Humins may have hydrophobic properties and if
they form a fi lm partly covering mineral particles, the soil could be in some instances
partly hydrophobic, especially if it is dried out. When it rains after a dry period, we
can observe that the fi rst raindrops form small spheres that roll across on the dry soil
surface. This effect of hydrophobicity of an excessively dry soil surface rich in
humic substances usually disappears soon after it continues to rain to moisten the
topsoil.
Up to now we only considered capillary forces being responsible for keeping
water within a soil. However, there are many other forces continually acting upon
soil water that should not be ignored. Hence, we next speak and make a simple
experiment about adsorptive forces. Taking a clod of loam soil, we crumble it, lay it
out on a plate in a thin layer of about 1 cm, and keep it at room temperature until it
looks completely dry. At that time we take a 10-g sample of the apparently dry soil,
place it in a hot oven for several hours, and weigh it again. We set the temperature
of the oven at 105 °C - high enough to remove water molecules adsorbed on soil
particle surfaces yet not suffi ciently hot to destroy OH
−
bonds within soil constitu-
ents. To our surprise when we remove the sample from the oven, its weight is less
than 10 g. The difference in its weight before and after being in the oven is the mass
of water removed from the soil. Let us suppose that the weight of the sample after
drying was 9.5 g. We released 0.5 g of water from the apparently dry soil that was
bound mainly by adsorptive forces and not by capillarity. The water content of our
apparently dry loam used in our illustrative example was 0.5/9.5 = 0.053 or 5.3 % by
weight.
Genuine adsorption of water readily occurs in completely dry soils when they are
in contact with humid air from which water molecules are attracted to the solid soil
particle surfaces. When the air humidity is about 20-30 %, a continuous layer hav-
ing an average thickness of one water molecule is completed. Although such very
dry air occurs mainly in desert regions under natural conditions, it could be purposely
