Water potential in soil — A laboratory demonstration1
L. J. Johnson2
ABSTRACT
The concept of water potential is often difficult
to grasp for students in an introductory soil science course. To assist in understanding the idea,
the student can construct a simple glass-capillary
wedge and use it to physically demonstrate water
potential and its role in retention and movement
of soil water. Mathematical description, using a
simplified version of the capillary rise equation for
a wedge, is emphasized. The concepts of gravitatational water, field capacity, plant available and
unavailable water, water movement down a potential gradient, and removal of soil water by a
plant root may be readily demonstrated. Contrast
between the pore space in the glass capillary
wedge and soil is explained. The students answer
a series of questions by interpreting the physical
phenomena occurring with water in the glass
capillary. Step-by-step explanation and coordination by a laboratory instructor is desirable to maximize the effectiveness of the exercise.
Additional index words: Matric potential, Gravitational potential, Capillary conductivity, Capillary
rise.
T
HE description of water retention and movement
in soil using the potential energy concept was first
proposed at the beginning of this century (Buckingham,
1907). It has, some six to seven decades hence, finally
reached the pages of introductory soil science texts
(Brady, 1974; Foth and Turk, 1972; Hausenbuiller,
1972; Thompson and Troeh, 1973) presenting an intellectual challenge to many undergraduate students,
particularly those with a minimal science background.
The utility of the idea in interpreting water phenomena
in the soil-plant-water system makes it highly desirable
that they understand its essentials. A simple physical
demonstration is often effective in clarifying what at
first seems to be, for beginning students, an obscure and
abstract concept. The laboratory exercise described below attempts to do this for water potential.
'Adapted from: INTRODUCTORY SOIL SCIENCE: A Study
Guide and Laboratory Manual by Leon J. Johnson. Copyright 1979
by Macmillan Publishing Co., Inc. Used by permission of the publisher.
1
Associate professor of soil mineralogy, Dep. of Agronomy, Penn
State Univ., University Park, PA 16802.
JOURNAL OF AGRONOMIC EDUCATION
22
Fig. 1. The glass capillary device.
If the maximum benefit is to be realized, the student
should have been introduced t o the concept of water
potential and its associated terminology prior t o doing
the exercise. Familiarity with total, matric, and gravitational potential, as well as the reference state of water
from which quantitative values of potential are
assigned, is assumed.
A simple wedge-shaped capillary device is constructed
from glass microscope slides and is used, as a surrogate
soil, t o demonstrate retention and movement of water in
terms of potential. With the aid of a little imagination,
the physical and biological categories into which soil
water is often placed are readily illustrated as well as
movement of water down a potential gradient and from
soil into a plant root. Relevance of the capillary rise
equation in interpreting soil water phenomena is also
visually portrayed. Utility of the equation for quantitatively defining the potential (or tension) of water in
pores of known size can be illustrated.
MATERIALS AND METHODS
Only simple equipment is required: 2.54 x 7.62 cm (1 x 3
in.) glass microscope slides, cellophane tape, round or flat
toothpicks, 150 or 400 ml beakers, filter paper circles, and
graph paper (10 divisiondin.).
A capillary is constructed by firmly holding two glass slides
in face-to-face contact, with edges parallel, and taping together one pair of the long edges. It helps to rest the other two
Fig. 2. Water distribution in the glass capillary after
contacting the free water surface.
long edges on a flat surface when applying the tape. If the
slides are now opened as a book and a toothpick is placed in
the upper corner, a wedge-shaped opening is formed by holding the device with the thumb and forefinger. This wedgeshaped opening formed is a capillary with a continuous gradation in pore size, with the smallest pore at the hinged end and
the largest pore farthest from the hinged end (Fig. 1). The
capillary in Fig. 1 is being held with the largest pore facing
180" from the palm of the hand to give a good view of the device. In use it is better held with the open end facing the palm
(Fig. 2).
To insure proper wetting and capillary action it is important
that the glass slide surfaces be thoroughly clean.
Colored liquid may improve visibility of the phenomona
to be observed. When this laboratory exercise was originally
developed, a methylene blue solution was, in fact, used. Although greater contrast does result, the use of water alone has
been found to be satisfactory, although not as colorful. For
photographic purposes, however, such contrast enhancement
is desirable.
The constructed device may now be used to give a physical
demonstration of water potential and the various categories
into which soil water is often classified. Before proceeding the
. students should construct, within a 2.54 x 7.62 cm (1 x 3 in)
area on the graph paper, the curve representing the capillary
rise equation for the wedge shaped device, hd = 15 mm'. In
plotting the points to construct the curve 2.54 cm is used for
the d, pore diameter, scale and 7.62 cm for h, the height of
water rise. This correspondence between the glass slide size
and the area of the graph plotted is deliberate. The near con-
JOHNSON: DEMONSTRATING WATER POTENTIAL
23
CAPILLARY RISE EQUATICQU
hd+ 3 0 m m 2
h - Height of Rise
d- Pore Diameter
h,
-
5
5-
I
2
210
5
~
mm
IO
0 d-0
5
IO
d, mrn
Fig. 3. Comparing the water distribution in the capillary
with the graph of the capillary rise equation.
gruency of the constructed curve and the air-water interface
within the wedge capillary provides an impressive physical
manifestation of the mathematical description of the capillary
rise (Fig. 3). This type of relationship greatly enhantes the potential for learning and remembering.
For optimum educational effect, it is desirable for the instructor to work along with the students step-by-step, assuring
that the significance of each step is understood. Whenever
possible, the relation of a visual demonstration to a real soil
situation should be stressed. The Socratic method of instruction can be effective. Urging the students to answer a series of
questions can stimulate thinking, and when the answers are derived through their own thought sequences (occasionally requiring a little help), the lessons are learned well.
DISCUSSION
In order for the student to mentally transfer the information demonstrated in the exercise to the soil situation, it is necessary that the similarities and differences
between the glass capillary and soil be explained in some
detail. Otherwise, the significance of what is being done
could easily be lost for many students. It should be emphasized that the capillary device is similar to soil in
being a solid (composed predominantly of oxygen and
silicon) containing a range of pore sizes capable of retaining and transmitting water. Thus, the same phenomenon is at work in both cases. In contrast to the capillary
wedge, on the other hand, the pores in soil are irregular-
Fig. 4. Removing water by contracting the entire bottom
wedge of the capillary with a piece of filter paper.
ly disturbed throughout the soil mass. Furthermore, the
capillary device has a much higher proportion of large
pores than the average soil. It is, however, the great
regularity of arrangement and large size of the pores in
the capillary wedge that makes possible the visual manifestation of the water phenomenon that, with soil,
would not be so easily seen.
The relationship of capillarity to matric potential may
first be demonstrated. When the bottom of the vertically held capillary device is placed in contact with a free
water surface in a beaker the water spontaneously
moves up against the pull of gravity, a movement contrary to our normal expectation of water moving
“downhill”. The movement, however, is not contrary if
the concept implied in “downhill” is energy rather than
elevation. Water potential energy decreases as a consequence of the movement. The height of water rise is the
measure of the energy change involved. This capillary
phenomonen can now be related to the matric component of soil water potential. Since matric potential, by
definition, is zero for free water the potential in the
capillary, where it has decreased, must be less than zero
or negative. The finer the capillary the higher the rise
and the more negative is the matric potential. In soil,
capillarity is contained in the microporosity and is an
important mode of soil water retention.
24
JOURNAL OF AGRONOMIC EDUCATION
Fig. 5. Removing water by contacting only a point at the
hinged end of the capillary with a piece of filter paper.
Fig. 6. Removing water by contacting only a point at the
wide end with a piece of filter paper.
Demonstration of the close correspondence between
smaller and smaller causing water content to decrease
with elevation. The curve defined by the air-water interface in the capillary wedge represents the soil water content as a function of elevation for a soil with a rather
uniform distribution of pore sizes.
Water movement down a potential gradient may be
readily illustrated using the capillary wedge and a piece
of filter paper. The filter paper is placed in contact with
the filled capillary: 1) along the entire bottom edge (Fig.
4), 2) just at a point at the narrow or hinged end (Fig. 5 ) ,
and 3) at a point at the wide end (Fig. 6). Observation is
made of the direction of water movement in each case.
In 1) and 2) a similar movement of water into the filter
paper occurs; initially from the widest pore and progressively toward decreasing pore size. All the water is not
removed when movement ceases. By contrast, in 3),
only a small amount of water is removed from the larger
pores in near contact with the filter paper. The continuous water film in the smaller pores “snaps off’’ and
remains in the capillary. A number of lessons can be
learned here. Why did water move out of the capillary
into the filter paper? How can the lack of removal of all
water from the capillary be explained? What property
of the filter paper determined which pore sizes in the
capillary were emptied and which remained filled? Why
was water removed from the widest pores first? The
filter paper contacted only a point at the hinged end of
the air-water interface in the capillary and the previous-
ly prepared plot of the capillary rise equation (Fig. 3)
provides an excellent opportunity to relate the mathematical description to the physical manifestation of the
phenomenon. This educational opportunity should now
be “nailed..to the wall!” Water potential (or tension), as
indicated by the height of rise, is now visually seen as
being a direct function of the size of the pore containing
the water. Distribution of the water within the wedge
can be explained in terms of total, matric, and gravitational potentials. This explanation can then be used to
interpret the distribution of water in soil above a water
table. When water movement ceases in either the
capillary device in contact with a free water surface or in
an initially saturated soil above a water table, the total
water potential is zero. Since matric and gravitational
potentials are the only two components involved in
these two situations, their sum is equal to zero. With
gravitational potential being the height of rise, h, above
the free water surface or water table, matric potential is
then - h.
The bottom of the capillary wedge, where the pore
space is completely filled with water, is analogous to the
capillary fringe directly above a water table in a soil. As
elevation increases above a free water surface (water
table) the sizes of the pores retaining water become
25
MEYERS: TEACHING SOIL FERTILITY
the capillary in 2), whereas in 1), it was in complete contact across the bottom (Fig. 4). How then, can the similarity in the amount and sequence of removal be explained? In particular, how was the water transported
across the bottom of the device in 2)?. Capillary conductivity or movement of water down a potential gradient should be emphasized at this opportunity. This
movement is contrasted with the lack of water movement across the bottom in 3).
A further use for the device is to simulate the concepts
of saturation, gravitational water, field capacity,
permanent wilting point, and plant available water, and
to demonstrate removal of soil water by a plant root
(filter paper). A visual demonstration of this type can
assist the understanding and remembering of these
ideas.
Immersing the device in water represents saturation.
Lifting the capillary out of the water and permitting
drainage demonstrates the loss of gravitational water
and the entry of air into the soil macropores. Upon
termination of drainage, the water status is analogous to
the concept of field capacity. If a piece of filter paper is
used to depict a plant root then on contacting the soil
(device at field capacity) with the surrogate root water is
absorbed by a plant. All the water is, however, not removed and when absorption essentially stops the soil is
at a water content representing the permanent wilting
point. The water remaining consists of some capillary
water (the finest pores) and all the hygroscopic moisture
(thin water films). In the absorption of water by the
plant root the transport of water from within the soil toward the plant root by capillary conductivity (down a
potential gradient) should be emphasized. This action
vividly demonstrates one of the means by which a plant
can obtain water from soil with which it is not in contact.
CONCLUSIONS
A simple tool such as that described above is often
one of the most effective and efficient ways of introducing unfamiliar concepts to beginning students. No
doubt, with the exercise of a little more imagination,
application of the device could be further developed.
Opportunities for devising instructional aids of this type
are limited only by our creativity. Other examples of
this type are the illustration of ion exchange reactions
(Himes, 1976) and the use of sponges in demonstrating
container soil water distribution (Spomer, 1974).