Lecture 9

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Chapter 4

Soil Water Properties

 

Two soil water properties are of the utmost importance for understanding, describing and managing soil water. These properties are soil water content and soil water potential. Water potential was described in detail in the preceding lecture. Water content simply refers to how much water is present. For each soil, the relation between content and potential is the basis for that soil's ability to provide water to plants.

  1. Water content describes how much water is in soil. This content can be expressed on the basis of soil and water mass or on soil and water volume. The mass method is also called the gravimetric method. Mass or gravimetric water content is designated by the symbol qm. Volumetric water content is designated by the symbol qv.


  2. Mass water content is a ratio of water mass to dry soil mass. Normally it is best visualized as grams of water per gram of dry soil. Mass water content is best expressed as a fraction (and often incorrectly expressed as a %). The following example shows how to calculate qm.

        Example: a moist soil weighs 120 g
            when dried the soil weighs 100 g
            the water mass was 20 g
            therefore,
    qm = 20 g water/100 g soil = 0.20

    Rarely, as in an organic soil,
    qm can be greater than 1.


  3. Volumetric water content is the volume of water per volume of soil.
    Volumetric water content can be expressed as fraction, but can correctly be expressed as a percent. As the example below illustrates,
    qv can be calculated from qm by multiplying qm by bulk density and by the density of water.

    ·Example:
            mass of water     x     mass of soil     x    volume of water     =    
    qv
              mass of soil             volume of soil            mass of water

                                                      
    rB                            1 cc/1 g

    Volumetric water content is generally more useful than
    qm.


  4. Depth of water in the field is another useful quantity. Remember that:
    qv = volume of water
            volume of soil

    Note that volume of water = area x depth of water
    and, volume of soil = area x depth of soil

    Because the area is the same quantity for water and soil it cancels; therefore
    qv = depth of water
            depth of soil

    Or, one can rewrite the equation such as: depth of soil x
    qv = depth of water

    From this equation one can solve problems such as in the following example.

    Example: how much water is in 50 cm of soil when
    qv = 0.15?
                50 cm x .15 = 7.5 cm


  5. Characteristic curves relate y to q. See Figure 4-8 in the textbook for an example of a characteristic curve. Each soil has a unique characteristic curve. From the relation described by the curve, one can determine:
            water content at the permanent wilting point (PWP)
            water content at field capacity (FC)
            plant available water (PAW)


  6. Plant available water is water that the soil will relinquish to the plant. Each soil has a particular qv at its FC. Field capacity is the amount of water a soil can hold against the pull of gravity. As a working definition, it can be considered as the amount of water in a soil about 24 hours after a soaking rain. It occurs at a potential of -0.33 bars for clays and loams and -0.10 bars for sand. Typical qv values for soils at field capacity are:
                Sandy ~ 0.15
                Loamy ~ 0.30
                Clay ~ 0.40

    Likewise, each soil has a particular
    qv at the PWP. A large array of plants can pull no harder on soil water than -15 bars. For this reason, -15 bars is considered to be the PWP. Of course some plants can pull harder than others, so -15 bars is the PWP by definition, not because any one specific plant will wilt at that point. Typical qv values for soils at the PWP are:
                Sandy ~ 0.07
                Loamy ~ 0.14
                Clay ~ 0.21

    Water held more loosely than FC drains away by gravity. Water held more tightly than PWP is generally unavailable to plants. Water between PWP and FC is plant available water PAW. Not all PAW is used by plants; some evaporates.


  7. Each soil has a unique available water capacity. Available water capacity (AWC) is the maximum PAW a soil could possibly provide. AWC tells the size of the container, PAW tells how full it is. AWC = qv at FC - qv at PWP. Typical AWC values for various soils are as follows:
                For sand » 0.15 - 0.07 = 0.08
                For loam » 0.30 - 0.14 = 0.16
                For clay » 0.40 - 0.21 = 0.19

    Technically, the term is unitless, but can be thought of 0.08 inches of water per inch of soil, or 0.08 meters of water per meter of soil, etc. AWC is reported in soil surveys as inches per inch.

    Soils vary greatly in AWC. For example, within one Texas county:
                clay AWC values range from about 0.06 to 0.20 in/in
                loams range from about 0.03 to 0.20
                sands range from about 0.02 to 0.10.



Students are encouraged to look up the following vocabulary words in the textbook glossary or elsewhere and to browse the following web site.



Vocabulary

Field capacity
Permanent wilting point
Available water capacity
Plant available water
Water retention curve



Web site

Sciences of Soils is an online journal; this article from the journal discusses rigorously the concept of field capacity. URL: www.hintze-online.com/sos/1997/Articles/Art2

 

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