Managing Chloride Toxicity with Soybean Chloride Excluder Products

What is the role of chloride for plant growth?

Chlorine is an essential plant element and is taken up by a plant in the chloride form (Cl^-). It is needed for osmotic regulation, or the movement of water, through plants. It is mobile within a plant and moves from older to newer plant tissue. Deficiency of Cl^- in soybean is rare, but toxicity can occur if the concentration in the leaves becomes too high, potentially leading to yield loss.¹

What is a soybean chloride excluder variety?

Chloride excluder, or salt excluder, is a soybean variety characteristic that is helpful in areas with higher salt or Cl^- levels in the soil. In fields with saline soils, varietal salt tolerance is improved when the soybean plant can regulate the absorbed Cl^- within the plant. All soybean varieties absorb Cl^- through the roots at the same rate. However, soybean varieties can be differentiated into two genotype groups (excluder and includer) based on their ability to regulate the absorbed chloride.² Excluder plants can tolerate high soil Cl^- levels by restricting higher concentrations of Cl^- to the roots. Includer plants accumulate Cl^- throughout the plant and are considered sensitive to high levels of soil Cl^-. Chloride excluder varieties have a dominant gene that segregates and secretes Cl^- out of root cells, preventing the movement of toxic levels of Cl^- from reaching younger plant tissue where injury can occur.³

Why are chloride excluder soybean varieties potentially beneficial under irrigated conditions?

Soybean production under continuous irrigation may be subjected to high levels of salts or Cl^- from ground or surface water. Irrigation water that contains salts can cause saline soil conditions, defined as having an electrical conductivity of >4.0 mmhos/cm (millimhos per centimeter) and pH <8.5.⁴ Salt build up can occur if more salt is left behind after evaporation and transpiration of the irrigation water than is removed by runoff and deep percolation into the soil. The application of fertilizers and manure can also contribute to salt buildup in soil. Chloride toxicity can occur in regions where the water table or irrigation water is high in Cl^-. The problem is more common in southern geographies, including the coastal/tidal region and hard clay pan soils with poor drainage in areas of Kansas, southern Missouri, southern Illinois, and parts of Arkansas and the Mississippi Delta. Chloride excluder soybean varieties are more commonly marketed in southern irrigated areas and less common in Northern areas, such as the Corn Belt where salty irrigation water and saline soils are less of a problem.

What causes chloride toxicity and what are the symptoms produced on soybean plants?

Due to its negative electrical charge, chloride does not adhere to soil particles, so it moves readily with soil water. It is taken up by the plant root system and translocated to the leaves where it accumulates. Toxicity symptoms develop if Cl^- concentrations in the leaves exceed the tolerance level of the plant. Soybean plants with Cl^- toxicity display symptoms that range from faint leaf chlorosis to leaf scorching, to total plant loss. Leaf symptoms usually occur shortly after irrigation water is applied. Leaves can appear scorched along the edges and the scorching is generally worse on the lower leaves of large plants or relatively uniform on small soybean plants (Figure 1). Severe injury can result in plant burning, stunting, and stand losses in field areas.

What effect does salt toxicity have on soybean yield potential?

It can be difficult to predict a soybean yield loss caused by salt toxicity. Both excluder and includer soybean varieties can suffer yield loss in salty soils. However, more yield loss can occur with includer soybean varieties. Salt toxicity may occur in localized field areas, such as potholes or low spots, or be present across most of a field’s acreage. Salt damage can also be worse on elevated areas or areas that dry out because of surface salt accumulation. Excessive levels of both Cl^- and sodium (Na) can be found in soil and irrigation water. It is important to note that Cl^- excluder soybean varieties provide tolerance to Cl^- toxicity but not to Na toxicity. Chloride excluders store Cl^- in the roots while Na is translocated to the shoot tips and leaves where injury can occur.

What should a grower do to manage salt toxicity in soybean under irrigation?

If saline soil conditions are too severe, actions can be taken to prevent damage to the crops grown. Saline soils are easy to reclaim if there is good soil drainage and an adequate supply of low salt irrigation water or precipitation. Extra irrigation water, called the leaching fraction, is applied to leach the salts out of the root zone. This can lower salt levels in the soil and salt stress to the crop. Universities and soil testing laboratories can provide information on how much water to use depending on the salt levels in the soil and the quality of the water being used for leaching.⁴

Proper diagnosis of Cl^- toxicity in a soybean crop is important to successfully apply proper management practices. Soil testing is insufficient in diagnosing Cl^- toxicity but may be helpful in the diagnosis of Na toxicity and other soil problems. Plant tissue testing is the preferred method to confirm Cl^- toxicity. Sampling should be completed during full bloom (R2) to early pod (R3) growth stages. Collect the uppermost fully expanded trifoliate leaves, without the petioles, from 30 random plants and submit them to the lab in a paper bag. The sufficiency range for chloride in soybean is considered to be about 200 to 1400 parts per million (ppm). A yield loss of 5% occurred when concentrations of chloride in the leaves were about 1900 ppm for chloride excluder varieties, and nearly 4000 ppm for chloride includers according to research from Arkansas.¹ Sending a sample of irrigation water for analysis is highly recommended and provides the best indicator of whether or not a problem may develop in a field.⁵

The most practical way to reduce problems from Cl^- toxicity is to select a Cl^- excluder soybean variety. Bayer screens soybean lines for salt tolerance. There is a marker to identify the Cl^- excluder gene, and standard checks are used to confirm that there is a tolerance level to Cl^- toxicity in a specific line. Generally, soybean lines that are positive for the marker (Cl^- tolerant) are scored between 1 and 3 on a 1 to 9 scale. Lines without the marker (Cl^- susceptible) can vary but typically score above 3. Chloride excluder characteristics are typically in the later maturing soybean varieties since the south is where most salt problems occur. Many southern varieties have the excluder gene present, but it is relatively rare in northern soybean varieties. Refer to your seed guide or product characteristics sheet for the Cl^- sensitivity of your soybean product. Your Bayer seed representative can help you identify essential agronomic characteristics needed and match the right products to your fields.

Sources

¹Mueller, N., Lesoing, G. and Sindelar, M. 2020. Soybean micronutrient management in Southeast Nebraska: Chloride. Cropwatch. University of Nebraska-Lincoln. https://cropwatch.unl.edu/2020/soybean-micronutrient-management-southeast-nebraska-chloride/
²Dunn, J. and Widick, D. 2006. Effects of chloride irrigation water on soybean excluder and includer cultivars. Arkansas State University and University of Arkansas. ASA Southern Regional Branch Meeting. https://crops.confex.com/crops/2006srb/techprogram/P14058.HTM
³Temple, L. 2021. Genetic markers improve development of chloride-tolerant soybean varieties. Soybean Research and Information Network. https://soybeanresearchinfo.com/research-highlight/genetic-markers-improve-development-of-chloride-tolerant-soybean-varieties/
⁴Diaz, D.R. and Presley, D. 2017. Management of saline and sodic soils. MF1022. Kansas State University Extension Publication. https://bookstore.ksre.ksu.edu/pubs/management-of-saline-and-sodic-soils_MF1022.pdf.
⁵Roberts, T. Field based determination of chloride tolerance in soybean. Arkansas Soybean Promotion Board. https://www.themiraclebean.com/field-based-determination-of-chloride-tolerance-in-soybean/
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