After learning the rules of chemistry, the student
learns to his or her dismay that water often seems to disobey these rules. In fact, the
most abundant substance on the earth's surface has more anomalous properties than any
other common substance. Strange as it is, materials and organisms on our planet behave the
way they do because water behaves the way it does.
- Water is essential, scarce and plentiful. All earthly
organisms need some water. Plants need water, usually in surprisingly large quantities.
One single corn plant may use 1 liter of water per day; a large tree might use 200 or 300
liters per day. Water is known as the universal solvent, and is the solvent in the soil
solution. To a great extent, soil water controls soil aeration and temperature. Water
affects landscape erosion. Water on our planet is unevenly distributed. The wettest place
on earth is a location on the big island of Hawaii, where it rains 460" per year. The
wettest place in the 48 states is the Olympic peninsula of Washington, where it rains
140" per year. In general, the eastern states receive between 30 and 80" per
year. The plains receive between 15 and 30" per year. Rainfall varies greatly among
locations in the west. The state of Nevada averages less than 7" per year. About 84%
of cloud water comes from oceans, but only 75% falls on oceans. Therefore, we have a
hydrologic cycle, with surface water flowing from the continents to the oceans.
- Water is a unique substance. The water molecule would
appear to be small enough to be a gas at room temperature, and yet it is a liquid. Indeed,
most gases are considerably larger than water. Ice floats. This is a strange phenomenon
among chemical substances. Usually the solid phase would be more dense than the liquid. If
winter ice were to sink to the bottom of bodies of water, life on earth would be radically
different. Water has a very high surface tension, and therefore tends to bead up. It also
has a very high specific heat. Water has the highest heat of vaporization of all known
- Hydrogen bonding accounts for water's behavior.
Because both of the hydrogens in a water molecule bond to the same side of the oxygen,
water is polar HOH with a + end on the hydrogen side and a - end on
the oxygen side. Salts readily dissolve in this medium. Hydrogen bonding is a force
between H and either N, O, or F. These three elements are the most electronegative.
Because water is only H's and O's, the stage is set for the ultimate example of hydrogen
bonding. In a pool of water, each H is covalently bonded to an oxygen, but retains a
strong attraction (Hydrogen bond) for the nearest adjacent oxygen. In other words, each
water is attracted to other waters. This phenomenon is called cohesion (the attraction of
water to water). Soil minerals are also a source of oxygens to which water's hydrogens are
attracted. Water, therefore, is strongly attracted to O-rich solids. This attraction of
water to other materials is called adhesion. Adhesion and cohesion are best observed in
the phenomenon of capillarity. You have probably observed water or an aqueous solution
rising against gravity in a capillary tube, perhaps you have experienced this at the
doctor's office as he or she pricks your finger, and then collects a sample of blood in a
small glass tube. In capillarity, water rises until weight of column equals the attractive
force between the water and the glass. For pure water the height of rise is approximately
the following function of tube radius:
h » .15/r (in cm) or h » 15/r (in mm)
Soil and water attract for two reasons. First the soil is porous, and the pores behave
much like capillaries. This is actually a minor consideration because natural drainage of
water through a soil is strong enough to drain pores that are larger than 0.009 mm in
diameter. The more important attraction is between water and solid surfaces. Surface films
of water are always present in soil. The difference in water content between any two soils
hinges on the question, how thick is the film?
- The tendency of water to move or react or do work is
determined by potential. The laws of thermodynamics tell us that spontaneous changes
result in reduced potential energy states. Any given parcel of water has a particular
potential energy. Potential is the work water can do relative to water at zero state. The
zero state is pure water that is unattached to any surface and exists at the reference
elevation in a gravitational field. Negative potential means work must be done to bring
water up to the zero state. Usually, soil water has a negative potential. Suction or
tension are terms used to refer to negative pressure. These terms are used to avoid
negative numbers. A positive tension, means a negative pressure. Many other units are used
to describe water potential. Hydraulic head is the unit used by engineers. Head units are
in length, as the height of a water column. A pump, for example, might deliver 90 feet of
hydraulic head. In science, as opposed to engineering, water potential is the preferred
term. Water potential is expressed in energy units; but the question is, energy per what?
If we express potential as energy per mass, typical units might be Joule/kg. Because it is
more convenient, we often use energy per volume. This is convenient because energy per
volume equals pressure, something we are familiar with. Typical pressure units in use are
pascals, kilopascals, megapascals, and bars.
- Water potential is the sum of four components. The
first component is gravitational. This one is easy to visualize because we have lots of
experience with gravity. The symbol used to depict gravitational potential is yg,
and the value can be + or .
Pressure potential is also easy to visualize. Water will move from a high pressure
environment to a low pressure environment. The symbol used to depict pressure potential is
yp. Ponded or flooded sites are pressurized for example. The pressure component
in soil is either positive or zero.
Matric potential is the most important component in soil, but is more difficult to
visualize. Water will not freely leave soil unless soil is very wet. This is because of
adhesion and cohesion. Imagine placing a clump of dry soil on a table, then dropping a
drop of water onto the clump of dry soil. If you elevate the soil off of the table, will
the water leap out of it, dropping to the table? Of course not. It will stay in the soil,
held my adhesion, or what we often call matric forces. The symbol for matric potential is
ym. These values are negative or zero, but never positive, because this water
is not free to move to the zero state.
Often these three components are sufficient. For instance, to predict hydraulic flow, such
as liquid flow through soil pores, one need consider no other component. However, in
certain circumstances, another component is important. This other component is called
solute potential. The tendency for water to undergo phase changes or to pass through
membranes is controlled by the presence of solutes in the water. This is important in soil
for two reasons: (1) evaporation of soil water is an important phase change, (2) water
flow from soil into cells of organisms, including plant roots, requires transport through
membranes. The symbol for solute potential is ys. As with matric potential,
solute potential is never positive; it can be negative for impure water, or approximately
zero for very pure water. Solute potential is also called osmotic potential, because the
process of passing selectively through a semi-permeable membrane is called osmosis.
- Matric potential is the component of greatest concern.
In wetlands, pressure and gravity are most important. But usually,
total potential (yT) » matric potential (ym)
A normal soil may have ym = -5 bars. It actually ranges from nearly 0 to about
-20 bars. For prospective, 5 bars of pressure is about equal to the pressure of 50 meters
of water. In other words, a force equal to the weight of 50 meters of water would be
required to remove water from a soil in which it was held with a matric potential of -5
bars. This magnitude of matric potential usually eclipses the small effect of the other
components. The matric potential of a soil refers to the potential of the most easily
removed molecule (see Figure 4-2 in the textbook).
Students are encouraged to look up the following
vocabulary words in the textbook glossary or elsewhere and to browse the following
Theory and measurement of water potential; a website from a
company selling instruments that measure water potential.
A website with good diagrams illustrating principles of
osmosis and water potential.