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Estimating Soil Moisture by Feel and Appearance

Irrigation Water Management (IWM) is applying water according to crop needs in an amount that can be stored in the plant root zone of the soil.

The "feel and appearance method" is one of several irrigation scheduling methods used in IWM. It is a way of monitoring soil moisture to determine when to irrigate and how much water to apply. Applying too much water causes excessive runoff and/or deep percolation. As a result, valuable water is lost along with nutrients and chemicals, which may leach into the ground water.

The feel and appearance of soil vary with texture and moisture content. Soil moisture conditions can be estimated, with experience, to an accuracy of about 5 percent. Soil moisture is typically sampled in one-foot increments to the root depth of the crop at three or more sites per field. It is best to vary the number of sample sites and depths according to crop, field size, soil texture, and soil stratification. For each sample the "feel and appearance method" involves:

  1. Obtaining a soil sample at the selected depth using a probe, auger, or shovel;
  2. Squeezing the soil sample firmly in your hand several times to form an irregularly shaped "ball";
  3. Squeezing the soil sample out of your hand between thumb and forefinger to form a ribbon;
  4. Observing soil texture, ability to ribbon, firmness and surface roughness of ball, water glistening, loose soil particles, soil/water staining on fingers, and soil color. [Note: A very weak ball will disintegrate with one bounce of the hand. A weak ball disintegrates with two to three bounces];
  5. Comparing observations with photographs and/or charts to estimate percent water available and the inches depleted below field capacity.

 

Example:
Sample Depth Zone USDA Texture Available Water Capacity (AWC) for Zone Soil Moisture Depletion (SMD) Percent Depletion
6 inches 0 to 12 inches sandy loam 1.4 inches 1.0 inches 70
18 inches 12 to 24 inches sandy loam 1.4 inches .8 inches 55
30 inches 24 to 36 inches loam 2.0 inches .8 inches 40
42 inches 36 to 48 inches loam 2.0 inches .5 inches 25

Result:
AWC total for all sample depths = 6.8 inches.
SMD total for all sample depths = 3.1 inches.
A 3.1 inch net irrigation will refill the root zone.

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