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Most soybean leaf yellowing occurs due to a nutrient deficiency, which can be prevented by proper fertility management including maintaining the optimal soil pH for plant growth. Any condition that restricts root growth (cool weather, saturated or extremely dry soils, soil compaction, root damage from diseases, insects, or herbicides) can lead to temporary nutrient deficiencies in the plant because the roots cannot reach sufficient levels of nutrients in the soil to sustain normal growth and development. Once environmental conditions improve and root growth resumes, many nutrient deficiency symptoms usually disappear, provided that soil fertility levels are adequate.
Nitrogen (N) deficiency appears as chlorosis (yellowing) of the lower leaves of the canopy as the plant remobilizes N from older leaves to new growth.
Figure 1. Soybean plants with yellow leaves due to a temporary nitrogen deficiency caused by wet soil conditions and a lack of nodule formation.
Nitrogen deficiency can occur in soybeans planted into fields that have been extremely dry or that have been saturated for an extended period of time. These conditions not only slow soybean root development, limiting access to nitrates in the soil, but can also reduce populations of the rhizobia bacteria that live in the soil and in soybean nodules, which provide up to 50% of the N needed by soybeans.1 Soybean leaves can appear light green in color during the time that nodules are forming due to a lack of sufficient N (Figure 1).
Management options: Once the nodules begin producing adequate amounts of N, the normal dark-green color should return. If soybeans fail to nodulate properly, as can occur in fields with insufficient populations of rhizobial bacteria, a rescue N application may help preserve yield potential.
Iron (Fe) is necessary for photosynthesis, nodule formation, and many metabolic processes within the plant. The distinctive symptom of iron deficiency is yellowing of the leaves while the veins remain green (interveinal chlorosis; Figure 2), which is the result of low chlorophyll formation. Iron deficiency symptoms typically appear on the uppermost, youngest leaves between the first and third trifoliate growth stages. This condition is referred to as iron deficiency chlorosis (IDC).
Figure 2. Interveinal chlorosis is a symptom of iron and manganese deficiencies.
Iron deficiency most commonly occurs when iron in the soil is tied up due to high soil pH, and not simply due to low iron levels in the soil. IDC is often associated with shallow depressions in a field or low-lying areas where water and solutes collect over time.
Management options: The most important management option is the use of soybean products with tolerance to IDC. Iron chelate products that carry the ortho-ortho-EDDHA Fe chelate form can be applied in-furrow at seeding in affected areas of the field to improve the plant’s access to iron in the soil.2
For more information refer tp the Agronomic Spotlight - Iron Deficiency Chlorosis in Soybean.
Manganese (Mn) deficiency is common in soybean crops and most often occurs in soils with high pH and on poorly drained soils. Symptoms of Mn deficiency appear as interveinal chlorosis on the newest trifoliates. Unlike N, Mn is immobile in the plant so symptoms will generally appear on the younger leaves, although older leaves may show symptoms as well.
Management options: A foliar application of manganese sulfate may correct an in-crop Mn deficiency. Consider banding manganese sulfate in a 2-by-2 band at planting in deficient areas of the field.
A phenomenon called “yellow flash” can sometimes occur after a post-emergence application of a high rate of glyphosate under dry conditions. Symptoms of yellow flash are yellowing of the newest leaves at the top of the plant while older leaves remain green and can sometimes look similar to Mn deficiency (Figure 3). Yellow flash generally occurs on plants that were already stressed (temperature or drought) when the glyphosate was applied and may be more common at the edges of fields and/or in spray-overlapped areas where application rates were two to three times the intended rate.
Figure 3. "Yellow flash” in soybean following a post-emergent application of glyphosate.
Management options: Affected soybean leaves generally return to their normal color within a week after the application with little, if any, growth reduction. If the soybean plants were not properly growing due to stressful conditions when glyphosate was applied, yellowing may not occur until 10 to 21 days later. If it was dry when yellowing occurred, these leaves may remain yellow until the crop resumes growth after rain or irrigation.
Potassium (K) deficiencies can occur due to restricted root growth or insufficient K levels in the soil. Potassium is mobile in the plant and deficiencies will first appear as yellowing along the edges of the oldest leaves (Figure 4). This condition can intensify and spread up the plant as the deficiency worsens. With severe deficiencies, the leaf edges may become brown and affected plants will appear stunted.
Figure 4. Symptoms of potassium deficiency. Photo courtesy of Daren Mueller, Iowa State University, Bugwood.org.
Management options: If the deficiency is due to inadequate K levels in the soil, a rescue application by broadcasting potassium chloride (potash, 0-0-60) with sufficient irrigation to move the fertilizer into the soil may help protect yield potential.3
Serious infestations of soybean cyst nematodes (SCN) can cause yellowing in soybeans, often in circular patches of the infested areas. In addition, plants will often be stunted. SCN infestations reduce the plant’s ability to access nutrients and water or to tolerate stress, so symptoms of nutrient deficiencies or drought stress may be amplified in SCN-infested fields.
Management options: For additional information refer to the Agronomic Spotlight - Biology and Management of Soybean Cyst Nematode.
Sources: 1 Pedersen, P. 2007. Soybean nutrient requirements. Iowa State University Extension. http://extension.agron.iastate.edu/ 2 Franzen, D.W. 2013. Soybean soil fertility. SF1164. North Dakota State University Extension. www.ag.ndsu.edu/ 3 Mallarino, A.P. 2006. Potassium deficiency symptoms in corn and soybean: What can we do about them? Iowa State University www.ipm.iastate.edu/ Other sources: Staton, M. 2014. Identifying and correcting manganese deficiency in soybeans. Michigan State University Extension. http://msue.anr.msu.edu/; Kaiser, D.E., Lamb, J.A., and Bloom, P.R. 2011. Managing iron deficiency chlorosis in soybean. University of Minnesota Extension. www.extension.umn.edu/; Roozeboom, K. 2012. Possible causes of yellow soybeans. http://www.agprofessional.com/