This information supplements Exercise 6 in the lab manual, which introduces the energy
status of soil water. The text is now separate from the slides. To view the associated
slides, click on the text that appears as a different color. When done viewing the slide,
click BACK on the netscape tool menu. Now you can print this text without the slides.
Figure 6.1 (lab manual) represents a body of water in a beaker. The pressure on either
side of the air-water interface is equal. The water molecule shown at the surface
illustrates the unequal distribution of forces surrounding the molecule. The lateral
forces of attraction between water molecules is greater than the attraction of water
molecules to air. Surface tension is the result. The water molecule depicted lower down in
the liquid shows the equal attractive forces in all directions, and the water molecule is
free to move about in the liquid or even escape through the air-water interface and become
a water vapor molecule. Thus, if the water is pure and exists under standard conditions of
temperature and pressure, little work needs to be done to move or change the water
molecule. Suppose that we could somehow impart surface changes to the bottom of the beaker
(as if it was a clay particle). The water molecules located sufficiently close to the
beaker bottom would be influenced by the surface charges there, and the strength of that
attractive force increases as the water molecule gets closer to the bottom.
This increasing force of attraction to the bottom of the beaker proportionally decreases
the freedom of the water molecule to move - its free energy! Thus, more work needs to be
done to move these molecules. The distance to which the surface charges attract water
molecules is called the Adsorption Force Field (AFF). Notice that in the first drawing,
the AFF has no influence on the air-water interface; but if we look at the drawing on the
right, you'll notice the water in the beaker has been reduced to a level where the
interface is now under the influence of Adsorption Force Field, as well as all the
molecules in the liquid. Water molecules are now not as free to escape into vapor or to
move in the liquid; their free energy has been reduced and more work has to be done to
move these molecules. In the first case, the water molecules near the surface had 0 water
potential, but in the second case (or right drawing) they had somewhat less than 0
potential, or negative potential, with the negative value increasing as the interface
becomes progressively closer to the active interface on the beaker bottom. This analogy
may now be carried to the idea of thick and thin films on surfaces of soil particles.
Water potential would be less in thin films than in thick films, and the water in the
latter would be easier to move.
At this point you may wish to review Parts II and III in the lab manual exercise 6 (Field
Capacity and Methods of Expressing Soil Water Content, respectively).
In order to develop the soil moisture characteristic curves, two variables must be
measured: soil moisture and soil water potential. Gravimetric soil moisture samples are
obtained using a core
sampler. Those cores are obtained using a drilling device.
Individual cores are
obtained from each desired depth in the profile. Another instrument used to measure soil
moisture content is the neutron
probe. The unit is mounted on a cart for convenience and safety. A less accurate
determination of soil moisture can be made using a gypsum block.
Figure 6.2 on page 4 in the lab manual represents the various portions of the SPAC on the
horizontal axis, such as soil, roots, stem, leaf, and atmosphere. The vertical axis shows
decreasing water potential along various segments of the continuum, and the interfaces
between them. Therefore, B would be the root surface. Curve 1 represents high moisture
content and high water potential in soil A, and also at the root surface, B. The potential
decreases or becomes more negative as you approach the root surface because of the drying
effect of water absorption by the roots. Some resistance to flow occurs at the root
surface. The water potential decreases slowly within the vascular system of the root and
stem until point D is reached. This is the end of the vascular system mesophyll cells of
the leaf. Resistance to flow occurs here, too. Point E represents the stomatal cavity, and
the water potential decreases rather sharply because of the change from the liquid to the
vapor state. The last dashed line on the right represents the stomatal opening, and
resistance to flow exists here regardless of stomatal action. The atmosphere acts as the
transpirational sink and water potential continues to decrease sharply, but eventually
becomes more dependent on atmospheric conditions rather than on plant physiological
conditions. Thus, in Curve 1, no part of the soil-plant system is near wilting point, and
the plant would be considered as being under low stress.
Curve 2 illustrates increased atmospheric stress, but at the same soil water potential as
Curve 1. Decreased potential throughout the remainder of the continuum can be seen, with
the leaf mesophyll nearing wilting point. Curve 3 illustrates the situation at a lower
soil water potential. All segments of the continuum have lower potential than before, but
atmospheric stress has not been increased from Curve 2, and so only in the mesophyll has
the wilting point been reached. Finally, in Curve 4, atmospheric stress has increased,
which increases plant transpiration, lowering water potentials everywhere. Curve 4 shows
that wilting point has been reached in all portions of the plant and injury may occur.
Though not illustrated in the figure, if soil moisture content was further reduced,
decreasing soil water potential, the potentials all along the continuum would likewise
decrease.
One of the interesting things about the water relations of trees, is that trees can
tolerate much higher stress conditions than other kinds of plants such as vegetable and
field crops. Needle water potential of Douglas-fir has been measured as high as -31 bars.
You see the leaf water stress chamber or the Schollander pressure
chamber. Briefly, a fascicle or petioled leaf is placed upside down in the aluminum
chamber with about a quarter-inch of the end extending upward from the hole in the cap.
When the cap is tightened, a rubber collar seals the needle or petiole on the outside.
Nitrogen gas is slowly admitted into the chamber until water is seen in the end of the
vascular bundles of the fascicle or petiole. The pressure is read on the gauge, and this
is equal to the negative pressure with which the water is held in the leaf or the needle.
The system works on the basis that water cannot be pushed from the plant tissue until the
gas pressure slightly exceeds the tension (or negative pressure) on the tissue water. Leaf
water stress or potential can then be related to soil water potential. The latter is
obtained by measurement of soil moisture content and then related to water potential
measurements with thermocouple psychronometers.
Here you see the pressure bomb secured to the top of a 70-foot tower. This
setup is used to get leaf water stress of a red pine plantation at Pack Forest,
Warrensburg. The steel tower in the red pine stand is shown, with the neutron probe tube at
the base of the tower.
Needless to say, much more can be said about soil water, which is important to many
scientific disciplines. The subject forms the major part of courses in soil physics
watershed management, and hydrology. Additional waters will be covered in the lectures to
give you a more complete story on soil water. For further discussion, see Soil and
Water by Daniel Hillel, Academic Press, and an excellent text by Paul J. Kramer
entitled Plant and Soil Water, a Modern Synthesis published by McGraw-Hill in 1969.
You should now read through Part V, the Field Soil Moisture Regime, and then proceed to
the investigations under Part VI.