Soil
Water is classified according to how "tightly" it is being held in
the Soil.
Free water or gravitational water will drain from a soil until the soil water potential reaches -1/3 bar. This is called field capacity. Gravitational water is not considered available to plants because it is in the soil only a short time and reduces oxygen levels to the point where the plant will not be absorbing water anyway.
As the soil continues to dry--or water is used by plants--more and more energy is needed by the plants to remove the water. Eventually a point is reached where the plant can no longer remove water. This is called the wilt point and occurs at -15 bars water potential for most plants. From -1/3 to -15 bars is the zone of available water. (See Diagram)
If the soil dries to an air dry state, the potential is -31 bars. (This assumes that the air has 100% relative humidity.) Plants cannot exert enough tension to pull water away from the soil. Tension is used to express water potential with positive numbers. So a tension of +15 bars equals a potential of -15 bars.
Additional drying requires putting the soil in an oven to drive off the tightly held water. Water is held in the soil like a series of beads, the farther the beads are from the soil particle, the weaker they are held by cohesion forces ( see water film diagram)
The investigation in the laboratory will be using the soil placed in the drying cans last week. The sample in the lab was somewhere between field capacity and the wilt point (that is, between -1/3 and -15 bars).
You let the sample air dry, and later it was placed in the oven and dried, then weighed to obtain the oven dry weight. When working with determining % water by weight or volume, we always use the oven dry weight as the standard value with which to compare. In this investigation, we also determine the % water near the field capacity, using the mud ball technique. Be sure to look at how to do this technique here » MUD BALL TECHNIQUE.
Sample Water Problems are available in Chapter 5.
Measuring soil water potential in the field can be done with a vacuum gauge tension meter (Tensiometer). A tension is built up between the soil and the gauge. The instrument only works in the range of 0 to -.8 bars. However, this is OK for determining when to irrigate soils, since irrigation should begin when the soil approaches -0.6 bars.
Bouyoucos moisture blocks also measure soil water in the lower tension ranges. They work by relating the electrical conductance of the soil to the amount of water present. Electrical conductance increases as the amount of soil water increases. See Moisture Blocks.
Measurement of water potential at lower potentials is made with the pressure chamber (see pressure chamber) in the range of -1 to 0 bars. Soil is placed on a membrane in the chamber which looks like a pressure cooker. As pressure is applied, water is forced out of the soil at the various potentials. This procedure is used to obtain the water potential values for the various soil types.
This diagram shows the amount of available water with the range of soil
textures. The y axis is a scale in inches of water per foot of soil, the
x axis is in soil texture. The area between the two lines is plant
available water.
Notice that sandy and clay textures have about the same amount of available water, yet sand has considerably less total water. Note also the maximum available water-holding capacity occurs in the silt loam soil. The relationships of soil water potential to the rate of plant growth will follow this curve .
For this area on a fairway water was standing for several weeks and the water potential was 0. The turf was unable to maintain growth under those conditions. See wet fairway
Once gravitational water has been removed and the soil is at field capacity,
plant growth will be at a maximum. As water potential decreases (becomes more
negative) plant growth will decrease until the wilting point of that plant
species is reached (near - 15 bar).
See the plant water
potentials at the plants and soil water
drawing.
Water will move through soils from areas of wet soil to areas of dry soil due to capillarity. Thus, water moves from areas of high soil potential (small negative number) to areas of low potential (large negative number). This movement is slow and becomes slower as the soil becomes drier. Wet soils have high HC while dry soils have low HC. Air replaces water in soil channels and blocks the flow of water.
As water moves from the soil (-.3 to -15 bar), into the roots (-3 to -20), through the stem, into the leaves (-15 to -30)and through the leaf stomata to the air (-500), it moves from a high water potential (small negative number) to a low water potential (large negative number). The water potential in the air is related to its relative humidity and is always less (more negative) than the water potential in the soil. Plants can extract only the soil water that is in contact with their roots.
If a soil has pores that are connected, then water can move downward due to
gravity which is a faster rate of transmission than water moving by
capillarity. The larger the pores the faster the rate of transmission.
See How plants get water from soil for more information on getting water into plants from NDSU Extension Service, North Dakota State University
Additional Web sites with information on preferential go to Preferential Flow Information
For information on water movement through soils go to these web sites: How water moves through soil
Soil Water Chapter 4
Soil Water Chapters
© Terence H. Cooper & Regents of the University of Minnesota, 2005 The University of Minnesota is an equal opportunity educator and employer.
