1. Unique Physical and Chemical Properties of Water

  2. Types of Water

  3. Soil Moisture Tension

  4. Soil Moisture Constants

  5. Soil Moisture Calculations

  1. Unique Physical and Chemical Properties of Water

    1. Chemical Properties

      Water's chemical description is H2O. As fig 1 shows, there is one atom of oxygen bound to two atoms of hydrogen. The hydrogen atoms are "attached" to one side of the oxygen atom, resulting in a water molecule having a positive charge on the side where the hydrogen atoms are and a negative charge on the other side, where the oxygen atom is. Since opposite electrical charges attract, water molecules tend to attract each other. As fig 2 shows, the side with the hydrogen atoms (positive charge) attracts the oxygen side (negative charge) of a different water molecule. This is called cohesion. The attraction of a water molecule to another surface is called adhesion.

      fig. 1

      water molecule 1

      fig. 2
      water molecule 2

      - Water in a pure state has a neutral pH. As a result, pure water is neither acidic nor basic. Water changes its pH when substances are dissolved in it.

    2. Physical Properties

      1. Specific heat

        Specific heat is the amount of energy required to change the temperature of a substance. Water has a high specific heat. Therefore, it can absorb large amounts heat energy before it begins to get hot. It also means that water releases heat energy slowly when situations cause it to cool. Water's high specific heat allows for the moderation of the Earth's climate and helps organisms regulate their body temperature more effectively. One calorie of heat is required to raise 1 gram of water 1 degree centigrade.

      2. Conductivity of heat

        Water conducts heat more easily than any liquid except mercury. This fact causes large bodies of liquid water like lakes and oceans to have essentially a uniform vertical temperature profile.

        Water exists as solid ice below 0 degrees Celsius. It exists in liquid form between 0 - 100 degrees celsius.
      3. Solvency
        Liquid water is able to dissolve a large number of different chemical compounds. This enables water to carry solvent nutrients in runoff, infiltration, groundwater flow and living organisms.
      4. Surface tension

        Water has a very high surface tension. In other words, water is sticky and elastic, and tends to clump together in drops rather than spread out in a thin film.

      Water is the only substance on Earth that exists in all three physical states of matter: solid, liquid, and gas. Incorporated in the changes of state are massive amounts of heat exchange.

  2. Forces on Soil Water

    1. Adhesion: The attraction o f soil water to soil particles.

    2. Cohesion: The attraction of water molecules to other water molecules.

    3. Capillarity: A capilary is a very thin tube in which a liquid can move against the force of gravity as shown in fig 3 below. The narrower the tube the higher the liquid rises due to the forces of adhesion and cohesion.


      fig 3

  3. Types of Water associated with the Soil

    Three basic types or forms of soil water.

    All these forms start as free water that is added to the soil by rain or snow. Their final forms depend on the moisture conditions of the soil.

    Each type is controlled by a different force and behaves differently in the soil.

    1. Physical Classification

      1. Gravitational water --- -1/3 bar
      2. Capillary water --- -1/3 to -31 bars
      3. Hygroscopic water --- -10,000 bars

      1. Gravitational water: free water that moves through the soil due to the force of gravity.

        1. Gravitational water is found in the macropores. It moves rapidly out of well drained soil and is not considered to be available to plants.

        2. It can cause upland plants to wilt and die because gravitational water occupies air space, which is necessary to supply oxygen to the roots.

        3. Drains out of the soil in 2-3 days

      2. Capillary water: Water in the micropores, the soil solution.

        1. Most, but not all, of this water is available for plant growth
        2. Capillary water is held in the soil.against the pull of gravity
          Forces Acting on Capillary Water
      3. micropores exert more force on water than do macropores
        Capillary water is held by cohesion (attraction of water molecules to each other) and adhesion (attraction of water molecule to the soil particle).

        The amount of water held is a function of the pore size (cross-sectional diameter) and pore space (total volume of all pores)

        This means that the tension (measured in bars) is increasing as the soil dries out.

      3. Hygroscopic water: This water forms very thin films around soil particles and is not available to the plant. The water is held so tightly by the soil that it can not be taken up by roots.

        • not held in the pores, but on the particle surface. This means clay will contain much more of this type of water than sands because of surface area differences.

        • Hygroscopic water is held very tightly, by forces of adhesion. this water is not available to the plant.

        • Gravity is always acting to pull water down through the soil profile. However, the force of gravity is counteracted by forces of attraction between water molecules and soil particles and by the attraction of water molecules to each other.

  4. Soil Moisture Tension or Soil Water Potential

    When working with water we are interested in the force with which water is held in the soil.

    • bar is the term used to measure this force.

    • one bar equals one atmosphere of pressure

    bar - pressure exerted by a column of water 1023 cm in height. approx. = to 1 atmosphere = 14.7 lb in2 or 760 mm of mercury.

  5. Water Tension and Water Content

    As water content decreases tension on the water becomes greater or soil water potential becomes less.

    Tension (suction) is measured in bars. 1 bar = 14.7 lbs. per sq. in. If water tension = 1 bar, a plant root must exert a pull (suction) greater than 14.1 psi to get water from the soil.

    Soil Water Potential
    Moisture Condition
    0 bar saturation
    -1/3 bar (4.1 psi) field capacity
    -15 bar (220 psi) wilting point
    -31 bar air dry

    1. from 0 to -1/3 bar all gravitational water drains

    2. from -1/3 to -31 bars capillary water

    3. from -31 bar on we have hygroscopic water

    These terms are of most importance in irrigation. We try to irrigate soil so that the bar pressure stays between -1/3 and -1 bar, in most cases.

  6. Soil Moisture Constants

    These are the terms most commonly used when working with soil water. Terms we wil use when making soil moisture calculations.

    1. Saturation - all soil pores are filled with water. This condition occurs right after a rain. - this represents 0 bars.

    2. Field capacity - moisture content of the soil after gravity has removed all the water it can. Usually occurs 1-3 days after a rain. - this would be -1/3 bar.

    3. Wilting point - soil moisture percentage at which plants cannot obtain enough moisture to continue growing. - this is -15 bars.

    4. Hygroscopic water - when the soil is about air dry - Water held at water potential less than than -31 bar. This water is not available to plants.

    5. Oven dry - soil that has been dried in a oven at 105 degrees C for 12 hours. All soil moisture has been removed. This point is not important for plant growth but is important for calculations since soil moisture percentage is always based on oven dry weight..
    6. Plant available water is that held in the soil at a water potential between -1/3 and -15 bar.