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Is your irrigation water too salty?

LITTLE ROCK, Ark. -- Irrigation water of poor quality can cause soil-related problems with negative effects on crop performance. The problems are usually related to saline (salty) or alkaline (high pH) conditions in the soil and/or water. Fortunately, studies to date have shown only isolated cases where pesticide levels in irrigation water have been a concern.

Irrigation water that contains salts can cause saline soil conditions when more salt is added than is removed by the natural processes of runoff and deep percolation into the soil. This salt is in addition to what is added to the soil by fertilizers and manures.

The problem occurs when the plant roots take up the salts and move them into the plant. If the salt level is high enough it can cause plant burning and stunting and in some cases stand loss in areas of the field.

Plants are more susceptible to salt damage when they are small and in the seedling stage.

Salt damage is worse on high spots and areas that dry out because the salt accumulates near the surface as the water moves up and evaporates. This wicking effect typically causes salinity damage to be worse on the levees in a rice field. The soil surface in these saline areas often appears to be oily.

Irrigation water that has high chloride salt levels can damage crops.

Researchers breed and screen soybean varieties to determine which ones can exclude chloride from entering the plant. If chloride salt is a problem, select a soybean variety that is a chloride excluder.

Sodium salt is usually associated with particular soils, but irrigation water with excessive sodium can cause the soil to have poor physical conditions that interfere with stand establishment.

Arkansas counties that have experienced the most problems with salty irrigation water are Monroe, Cross, White, Desha, Chicot, Poinsett and Ashley. Producers planning new wells in these counties should check to see if they are in a potentially high salt area. If high salt is suspected in the groundwater, it would probably be better to do a test hole before drilling a well and then finding that the water is too salty to use for crop irrigation.

Salt is usually more of a problem in well water, but in some cases the salt concentration in surface water can be high. This may occur in tail-water recovery systems when the salts that drain off a field increase due to the water evaporating and causing the salt concentration to increase.

Alkaline (high pH) problems are almost always associated with well water and have been noted in about every Arkansas county. The high pH irrigation water contributes to increased soil pH and can lead to zinc and phosphorus deficiencies in the crop. This is part of what is described as "cold water" effect in rice fields.

The colder water temperature of the well water does affect the crop in the top of the field, but the increased soil pH in this area of the field is also part of the problem. It appears that using the Multiple Inlet Irrigation approach to rice irrigation has reduced this effect in some fields.

Soil tests and irrigation water tests are needed to determine if any of these water quality problems are contributing to problems experienced in certain fields. Both of these testing services are available through county Extension offices. There is a charge for the water analysis, but the results include some recommendations. The results are needed before anyone can properly evaluate what may be happening in the field.

If water quality is suspected to be a problem and the irrigation water has not been tested in the last five years, it would be advisable to get an analysis.

This is one of several articles on drainage and irrigation water management. If you have questions or suggestions on topics please contact me: Phil Tacker, 501-671-2267 (office), 501-671-2303 (fax), 501-944-0708 (cell), or (e-mail).

Phil Tacker is an Arkansas Extension ag engineer.

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