Knowledge of soils, plants is crucial to efficient irrigation

Scheduling irrigation to use available water as efficiently as possible requires knowledge of crop moisture needs and soil properties.

A little information on plant physiology wouldn’t hurt either, says Texas Extension Agricultural Engineer Dana Porter.

Porter, speaking at the recent West Texas Agricultural Chemical Association annual conference in Lubbock, said farmers should understand soil and crop water needs, whether they use center pivot or subsurface drop irrigation systems.

“We need to know how much water we have in the soil,” Porter said. “Is it saturated, at field capacity or at a wilting point? Different soil types hold different amounts of water. Sandy soils hold less, and loamy and clay soils hold more.”

She said knowing soil permeability helps schedule irrigation. “Determine how much water can stay in the soil at various depths — 1 foot, 2 feet, and 3 feet. Know how much water is available at different depths.

“Also, determine at what point soil begins to lose water and then how much to irrigate to bring the field to capacity.”

She said farmers may want to irrigate to slightly less than capacity to allow for rainfall. She recommended using a management allowable depletion (MAD) rate to determine irrigation timing and amount.

“For most crops, 50 percent to 60 percent depletion rate is OK. For drought sensitive crops, consider 30 percent to 50 percent. Limit depletion to 50 percent to avoid drought stress.”

Porter said understanding a crop’s root zone also helps schedule irrigation. She said the root zone is “the effective area for available soil moisture, accessible nutrients, etc. Shallow rooted plants are more susceptible to drought stress.

“Root zone is the potential depth set by the plant, but is limited by soil factors. Soil profiles should be moist, not saturated. Plow pans, caliche layers, dry layers, saturated layers, salt accumulation and other factors limit effective root zones.”

Effective root zone for corn and cotton is 2.6 to 5.6 feet, for alfalfa and sorghum — 3.3 to 6.6 feet. Porter said most of the water extracted by a crop comes from the top 1 to 3 feet of the root zone. “Irrigation management is more critical for shallow-rooted crops.”

Porter also recommended farmers understand crop moisture needs. Corn needs from 28 inches to 32 inches a year with a water use efficiency rating of 250 to 450 pounds per acre inch. Cotton water use ranges from 13 inches to 27 inches with a 50 to 100 pounds per acre inch water use efficiency rating. Grain sorghum water demand is 13 to 24 inches and 200 to 500 pounds per acre inch water use efficiency.

Wheat demand is 13 inches to 28 inches and 2.4 to 6.4 bushels per acre inch. Alfalfa uses from 36 inches to 40 inches for a 280 to 400 pounds per acre inch efficiency rating.

“Water demand is crop specific and varies with weather and growth stage,” Porter said. “The plant needs enough water to prevent stress, but not enough to cause waterlog. Roots grow in moist soil and need both water and oxygen.”

She said farmers may find more information about specific crop water needs from, a Web site that provides daily weather updates, wind speed, temperatures, rainfall amounts and other information.

She said cotton water demand peaks in July and early August in the High Plains. Corn high moisture demand peaks in July. Seasons may vary, however.

Porter said the Southwest has seen high adoption of irrigation and farmers are learning how to make the most of new technology. Knowing when specific plants need the most water and understanding the limits of specific soil types, she said, will make that technology more efficient.

She cites other Web sites that offer information on moisture management and irrigation timing, including:;websoilsurvey,; and

email: [email protected]

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