Yellowing of soybean leaves can be caused by several factors including soil pH, nutrient deficiencies, restricted roots, diseases, herbicides, and soybean cyst nematode. Depending on the cause, the effect on your crop yield can be negligible to potentially great. For many factors management options are available.
What to Watch For
Throughout a soybean plant’s life, except for natural maturity, its leaves can become yellow for various reasons. The most common cause is a nutrient deficiency. Other common causes are compaction, which can restrict nutrient uptake by the roots (Figure 1), soybean cyst nematode infestations, diseases, and a phenomenon called “yellow flash” caused by a high application rate of glyphosate.
Nitrogen(N) deficiency symptoms can occur during the time that rhizobial bacteria are forming the N-producing nodules and the nodules are not fully functional. Extremely dry or wet conditions can also cause N deficiencies by limiting root development, reducing access to soil nitrates, and inhibiting soil rhizobia populations. Nitrogen deficiency symptoms appear as a yellowing of the lower leaves (Figure 2, top left). A rescue N application may help preserve yield potential if timely nodulation does not occur.
Iron (Fe) deficiency or Iron Deficiency Chlorosis (IDC) occurs most often in shallow depressions or low-lying field areas where soil Fe becomes unavailable due to high soil pH from the buildup of carbonates. The nutrient is necessary for chlorophyll development, nodule formation, and metabolic processes, and is immobile in the plant. Deficiency symptoms appear as interveinal yellowing (leaves are yellow, veins are green) on the youngest, uppermost leaves between the first and third trifoliate growth stages (Figure 2, top right). The plant’s access to soil Fe may be improved with an in-furrow application at planting of an ortho-ortho-EDDHA Fe chelate product.1
Manganese (Mn) deficiency is most common on poorly drained soils, especially on clay and silt loam soils on eroded knolls where the pH is higher than the rest of the field. Manganese is immobile within plants; therefore, the symptoms, which are similar to IDC, appear more commonly on younger leaves (Figure 2, bottom left). Factors that inhibit root uptake increase the potential for Mn deficiencies. A foliar application of Mn sulfate may help improve deficiency symptoms. Future plantings may benefit from a planting time 2-by-2 banding of Mn sulfate in deficient areas of the field.
Potassium (K) deficiencies appear as yellowing along the edges of the oldest leaves because K is mobile within the plant (Figure 2, bottom right). Symptoms can spread upward to newer leaves. Under severe situations, stunting may also occur. Restricted roots or lack of K in the soil can be causes for the deficiency. During the growing season, a broadcast application of 0-0-60 with sufficient rainfall or irrigation to move the fertilizer into the soil may reduce symptoms and help preserve yield potential.2
Soybean Cyst Nematode
The common symptoms of an SCN infestation are yellowed leaves and stunted plants. The symptoms occur because the damaged roots are unable to access nutrients and have a decreased ability to tolerate heat and drought stress.
A temporary phenomenon called “Yellow Flash” can occur after a high post-emergence application rate of glyphosate under dry conditions. The symptoms occur on the uppermost, newly expanded leaves and may appear similar to Mn deficiency (Figure 3). This is more common on stressed plants and in field margins where applications may have been overlapped. The condition has not been shown to affect yield potential and leaves generally recover from the yellowing within a week after application. The symptoms may be delayed 10 to 21 days under drought conditions.
Impact on Crop
The effect that yellow leaves may have on yield potential depends on the cause, duration of the cause, and availability of an in-crop solution. Nutrient deficiencies caused by a cool environment may disappear with warmer temperatures. Foliar fertilizer applications may resolve in-crop deficiencies and reduce the potential for yield loss. However, if SCN is the problem, there is no in-crop rescue; therefore, the potential for yield loss depends greatly on the infestation level.
Management solutions to help reduce the potential for leaf yellowing include soil tests for nutrient deficiencies and SCN infestations, timely scouting to identify potential nutrient and pest problems, use of resistant SCN products, use of products with resistance or tolerance to certain diseases, such as Phytophthora root rot, foliar-applied fertility, and use of seed treatments and foliar fungicides to help deter labeled diseases.
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1Franzen, D.W. 2013. Soybean soil fertility. SF1164. North Dakota State University Extension. www.ag.ndsu.edu/.
2Mallarino, A.P. 2006. Potassium deficiency symptoms in corn and soybean: What can we do about them? Iowa State University. www.ipm.iastate.edu/.
Pedersen, P. 2007. Soybean nutrient requirements. Iowa State University Extension. http://extension.agron.iastate.edu/.
Staton, M. 2014. Identifying and correcting manga-nese 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/.
Rosenbloom, K. 2012. Possible causes of yellow soybeans. http://www.agprofessional.com/.
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