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Irrigation management with saline water

Robert Fears for Progressive Forage Published on 29 April 2019
Irrigation

Salinity is an irrigation water quality problem in many areas of the U.S., including the Arkansas River Valley of Colorado, the Salt River Valley of Arizona and the Rio Grande valleys of New Mexico and West Texas.

This problem has existed in these areas for 75 to 100 years, and through research and experience, management practices have been developed to facilitate effective use of saline water. A review of these practices is of value since saline water will continue to be a problem for irrigated crop production.

Increasing salinity causes

“Salinity problems result from high water tables that bring salts to the soil surface, or in arid regions, salts accumulate due to irrigation combined with high evaporation rates. The salinity problem is increasing because farmers switched from flood irrigation to sprinklers for improved watering efficiency. Sprinklers apply significantly less water than flood irrigation,” says Don Miller, product development manager for Alforex Seeds.

“The increased amounts of water used in flood irrigation tend to leach salts down below the root zone. It is thought, however, that use of any type of irrigation will eventually lead to increased soil salinity.”

“The term ‘salinity’ describes the concentration of ionic salts such as calcium, magnesium, sodium, potassium, chloride, bicarbonate, carbonate and sulfate. Certain amounts of these salts can promote plant growth, while excessive amounts are detrimental to plants and soil,” says Thomas Marek, of Texas A&M AgriLife Research. “High salinity in water and soil is a problem because it competes with plants for water. Salts can have toxic effects on plants and burn their roots and foliage. Excessive amounts of some minerals interfere with plant absorption of other needed micronutrients. High concentration of sodium in soil can cause dispersion of soil aggregates, resulting in damaged soil structure and interference with soil permeability.”

“Soil and water tests determine whether the presence of salts and their concentrations will negatively affect production of certain crops,” Marek says. “In laboratory analyses, salt concentrations are reported as electrical conductivity (EC) or as total dissolved solids (TDS). EC of a water sample is proportional to the concentration of dissolved salts and is expressed in millimhos per centimeter (mmhos/cm), micromhos per centimeter (µmhos/cm) or deciSiemens per meter (dS/m). TDS is a measure of mass concentration of dissolved salts in water and is usually reported as milligrams per liter (mg/l) or parts per million (ppm).”

Irrigation management

“An obvious and simple method to minimize salinity effects is through reduction of irrigation applications, which will result in less salt accumulation in the field,” says Dana Porter, Texas A&M AgriLife Extension. “This is accomplished either by switching to dryland farming, maximizing precipitation effectiveness, adopting highly efficient irrigation and tillage practices, or using a higher quality irrigation water source, if available. Some salts are added through fertilizers and as components or contaminants of soil amendments, so soil fertility testing is suggested for refinement of the nutrient management program.”

“Irrigation leaching of salts has been a common practice for many years,” says Porter. “This is accomplished by occasional excessive irrigation applications, which dissolve, dilute and move salts lower into the soil profile. Amount of required irrigation depends upon salt concentrations in the crop root zones. This practice is more challenging where either quantity or quality of irrigation water is limited.”

“Excessive deep percolation losses of water are usually discouraged because of irrigation efficiency reduction and potential for contribution to groundwater contamination. However, occasional applications of relatively large amounts of irrigation water may be necessary for leaching of salts,” Porter says. “Managing irrigation amounts and timing to support a large root zone facilitates better root uptake of nutrients and offers improved protection from short-term drought conditions.”

In contrast, light frequent irrigation applications often result in a small wetted zone, resulting in limited dilution and leaching of salts. Small irrigation amounts combined with evaporation from the soil surface increase possibilities of salt accumulation near the soil surface, contributing to crop germination problems and seedling damage. In arid and semiarid conditions, a smaller wetted zone generally results in a smaller root zone, which makes plants more vulnerable to salt damage and drought stress injury.

Crop selection

Crop selection offers a way to manage irrigation water salinity because some forage species and varieties are more tolerant to salts than others. Barley, rye and bermudagrass are relatively salt-tolerant, while wheat, oats and sorghum are moderately tolerant. Forages that are moderately sensitive to salt include corn, some varieties of alfalfa and many clovers. Additional alfalfa varieties have been developed that are more salt-tolerant than some of the older varieties. Barley and some of the other salt-tolerant crops are more salt-sensitive at emergence and early growth stages than in later stages of growth.

What to look for

Salt patterns in the seedbed and root zone are affected by wetting patterns, evaporation and water uptake by roots. In row crops, salt accumulation is often detectable by visible white residues along the side of the furrow, in the bottom of a dry furrow or on the top of the row. In broadcast forage crops, look for residue at the outer edge of the irrigation-wetting zone.

Negative effects of salt on plant growth are sometimes avoided by planting seed outside the areas of high salt accumulation. Row placement and water movement within the soil profile can affect the amount of water available to plant seedlings as well as the amount of water available for salt dilution.

Use organic matter

Soil organic matter also helps mitigate salt effects on plants. Organic matter can contribute to a higher cation exchange capacity (CEC), which lowers the exchangeable sodium percentage and then, in turn, helps mitigate sodium’s negative effects.

Soil structure and permeability are improved by organic matter, resulting in ready movement of water through the soil and maintenance of soil water-holding capacity. Mulch, such as dead plant material, can also limit evaporation from the soil surface and reduces the need for irrigation.

The salinity problem seems to be increasing, and forage growers must continue to hone their skills in managing it to remain profitable.  end mark

PHOTO: Occasional applications of relatively large amounts of irrigation water may be necessary to leach salts deeper into the soil profile. Photo by Lynn Jaynes.

Robert Fears is a freelance writer based in Georgetown, Texas. Email Robert Fears

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