Knowing the available soil water for use by the developing plant is information that can be used to make important management decisions throughout the growing season. If favorable moisture is available, then plant populations can be adjusted and needed nutrients applied to increase production; that statement based on environmental conditions remaining favorable for plant growth through the remainder of the growing season.
However, if the soil moisture level declines to the point that the plant is still wilted early in the morning then no additional investment is justified and plant development will be unfavorably impacted. Apply irrigation water, if available, to keep the plants from reaching this point of permanent wilt.
Where large acreages are irrigated, several days may be required to cover the entire acreage; consequently, irrigation must be started soon enough to arrive at the last portion of the field before its available water has been exhausted.
Most producers that apply water obtain information as to the rate at which moisture is being used by crops from different soil depths by using soil moisture probes. This provides a basis for determining when and how much water to apply. For most crops, irrigation should be started when about 50 percent of the available moisture in the soil root zone is depleted.
An alternative to the soil moisture probes is a method of estimating the available soil moisture by feel. To do this take a small amount of soil; squeeze it in the hand so as to form a ball, then refer to the chart below for a description of the feel.
| Degree of Soil Moisture |
Percent Useful Soil Moisture Remaining |
Feel or Appearance of Soils | |||
|---|---|---|---|---|---|
| Coarse (Sand) |
Light (Sandy Loams) |
Medium (Silt and Clay Loams) |
Heavy (Clay) |
||
Dry |
0 |
Dry, loose, single-grained, flows through fingers. |
Dry, loose, flows through fingers. |
Powdery, dry, sometimes slightly crusted but easily breaks down into powdery condition. |
Hard, baked, cracked; sometimes has loose crumbs on surface. |
Low |
50% or less |
Still appears to be dry; will not form a ball with pressure.* |
Still appears to be dry; will not form a ball.* |
Somewhat crumbly, but will hold together from pressure.* |
Somewhat pliable; will ball under pressure.* |
Fair |
50 to 75% |
Still appears to be dry; will not form a ball with pressure.* |
Tends to ball under pressure but seldom will hold together. |
Forms a ball, somewhat plastic; will sometimes slick slightly with pressure. |
Forms a ball; will ribbon out between thumb and forefinger. |
Excellent |
75% to
|
Tends to stick together slightly; sometimes forms a very weak ball under pressure. |
Forms weak ball, breaks easily, will not slick. |
Forms a ball and is very pliable; slicks readily if relatively high in clay. |
Easily ribbons out between fingers, has a slick feeling. |
Ideal |
At field
|
Upon squeezing, no free water appears on soil but wet outline of ball is left on hand. |
Upon squeezing, no free water appears on soil but wet outline of ball is left on hand. |
Upon squeezing, no free water appears on soil but wet outline of ball is left on hand. |
Upon squeezing, no free water appears on soil but wet outline of ball is left on hand. |
Too Wet |
Above field
|
Free water appears when soil is bounced in hand. |
Free water will be released with kneading. |
Can squeeze out free water. |
Puddles and free water forms on surface. |
*Ball is formed by squeezing a handful of soil very firmly with fingers. |
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By knowing the texture of the soil and the depth of topsoil, a producer can estimate the amount of water available to produce a crop. If the soil texture is sand, then it can hold between 0.5 and 1.0 inch of water per foot of soil depth. If a producer had four foot of topsoil, there is 2 to 4 inches of water available for the crops use. If the soil texture is sandy loam, there is 1.0 to 1.5 inches of available water per foot of topsoil. If the soil texture is silt or a clay loam, there is 1.5 to 2.0 inches of available water per foot of topsoil. If the soil texture is clay, there is 2.0 to 2.5 inches of available water per foot of topsoil.
Plant, water and soil interrelationship is a complex study. This is only a small reflection of the information available. Additional information is available from a number of reliable sources. My preference is unbiased research based information that is available from most university systems.
