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Almost all water contains dissolved salts (Cl and Na) and other trace elements which can impact water quality.1 For most irrigation situations the main water quality concern is salinity levels, as high levels can harm plants and reduce yield potential. Salt begins to accumulate as water evaporates from the surface and the crops remove water from the soil.
Symptoms of salt injury are similar to drought as plants will wilt and the leaf area can decrease when low-quality water is available to the plant.2 Symptoms will first appear on older leaves, turning them light green to yellow followed by necrosis (Figure 1). The under canopy of the soybean crop must be evaluated to catch salt injury early on. Leaves may also appear scorched shortly after irrigation is applied. If salinity levels are high enough, a plant may die prematurely.
In fields with furrow irrigation, salt injury may be worse for soybean plants that receive higher rates of low-quality water. These areas may be the first third of the field nearest the water source or any low-lying areas where irrigation water may be stagnate.
Soil testing may not be of much help in working with chloride, as chloride ions move rapidly in the soil when water is added or removed. If excessive salt is suspected in an irrigated field, a water sample should be sent in for analysis. From the analysis, two types of salt problems can be diagnosed, a problem with total salinity (too much Cl) or a problem associated with the combination of total salinity and sodium (Cl and Na), sodicity.
Irrigation, a high water table, manure, fertilizer applications, or the soil parent material may be sources for high Cl or Na levels. Knowing the source of the salt is important to apply an effective management practice(s). When submitting water samples, remember that water from the same source can vary in quality with time. It is good to always test during potential irrigation periods.
Soils with high Cl levels are called saline soils and can reduce plant water uptake. Saline soils normally have a pH value below 8.5 and are low in sodium (Table 1).3 Additional facts about saline soils:
Water that contains high amounts of Na are called sodic soils. High Na levels can lead to the breakdown of soil structure causing the soil to become hard to compact when dry and impervious to water penetration.1 Sodic soils can appear to be dark and smooth.
Leaching is a basic management tool for controlling salinity in soil. To leach the soluble salts, water is applied in excess and then is lost to evaporation. This technique is used to keep the salts in solution until they leach below the plant root zone. This technique may be accomplished during any irrigation and as much as needed. In some areas normal amounts of rainfall may provide enough additional water to allow for salt leaching. Preplant irrigation can also help reduce salt levels as salts can accumulate near the soil surface when fields are fallow.
In fields with saline soils, salt tolerance can be improved when the soybean plant has the ability to regulate the absorbed Cl ion within the plant. Soybean products absorb Cl from the soil at the same rate, but are differentiated into two genotype groups based on their ability to regulate the absorbed Cl (Table 2). The two genotype groups are:
Excluder plants that store Cl in the root while Na will still be translocated to the shoot tips and leaves.
Includer plants take in the chloride and move it to the top of the plant where toxic levels can accumulate and kill the plant.
It is important to note that soybean excluder products planted in areas where both Na and Cl are at the toxic levels will not provide tolerance due to the Na toxicity effect on the plant. Soybean excluders are more common in soybean products planted in the Southern regions and less common in Northern regions.1 Both includer and excluder soybean products are affected by high salt levels, but includers may experience more damage.
Salt tolerance may be a key agronomic characteristic to consider in fields where high levels of Cl or Na limit yield potential. Symptoms and causes of Cl and Na toxicity are frequently confused. Effective management of these problems varies and requires proper diagnosis. Excluder soybean products can be an effective management tool to minimize the impact of Cl toxicity.
Your Asgrow® seed representative can help you identify essential agronomic characteristics needed and match the right products to your fields. Refer to agSeedSelect® for the chloride sensitivity of your Asgrow soybean products. The abbreviations Inc. = Includer and Exc.= Excluder.