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This article in SSSAJ

  1. Vol. 20 No. 2, p. 284-288
    Received: Feb 21, 1955

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Soil Moisture Availability Related to Power Required to Remove Water1

  1. Sterling A. Taylor and
  2. J. L. Haddock2



A deficit of water in plant root tissues sets up forces across the root surface membrane causing movement of water into the plant. As the removal increases, the forces get stronger; this normally causes water to move into the plant more rapidly. The work per unit time that must be expended to remove a unit volume of water from the soil is here defined as the water extracting power.

Some theoretical predictions are made and partially verified by experiment. They are:

  1. When moisture tension is uniform, the rate of removal of water at any depth is proportional to the root activity at that depth.

  2. The amount of water extracted from any depth interval in the soil is inversely related to the power required to remove water in that interval. For a time after an efficient irrigation, water will be held at low tensions and the rate of water uptake will be more or less proportional to the activity of roots. This results in a more rapid increase of soil moisture tension in the zone where root concentrations are higher, which increases the power needed to remove water. Subsequently, more water is taken up faster from regions where root activities and soil moisture tensions are lower, and the average distance over which water moves and the velocity with which it moves are greater.

  3. In order to attain equal rates of water extraction from all depths of the profile, a soil moisture tension gradient must be established which will be characteristic of the root activity and growth rate for the crop.

  4. When tension gradients have been established in the soil, there occurs a slow adjustment of moisture in the soil to decrease these gradients. This adjustment is proportional to the magnitude of the gradient and the conductivity of the soil for moisture. The slow movement of water through the soil to the plant roots might result in an appreciable removal because the absorbing surfaces of the roots are large.

Root activity is defined as the product of the absorbing surface of uniform roots by a characteristic permeability factor and summed over all roots in any given volume of soil. The age, rate of elongation, and concentration of roots are implicitly included.

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