This information supplements Exercise 7 in the lab manual, which
introduces the energy status of soil water. To view associated pictures, click on the hyperlinked
text (appears as a different color). When
done viewing the slide, click BACK.
Field Capacity and Methods of Expressing Soil Water Content
In order to develop soil moisture characteristic curves, two variables must be measured:
soil moisture and soil water potential. Gravimetric soil moisture samples are often obtained using
a core sampler sometimes in conjunction with 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 6.1in the lab manual represents the various portions of the soil-plant-air-continuum
(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 (becomes more
negative) approaching 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 in leaf mesophyll cells. Resistance to flow occurs here, too.
Point E represents the stomatal cavity, and the water potential decreases dramatically because
of the change from the liquid to the vapor state. The last dashed line on the right represents
stomatal opening; resistance to flow exists here regardless of stomatal action. The
atmosphere acts as the transpirational sink. 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 low as -31 bars.
The leaf water stress chamber or the Schollander
pressure chamber is shown in this slide. 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 displayed on the gauge is equal to the potential (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 psychrometers.
Here you see the pressure bomb secured to the top of a 70-foot tower.
This setup was used to measure leaf water stress of a red pine plantation at Pack Forest, Warrensburg.
The subject of soil water forms the major part of courses in soil physics and in watershed management,
Last updated May 10, 2013