Leaf Yellowing in Soybeans

Yellowing of leaf tissue can occur in soybean plants. Nitrogen deficiency, manganese deficiency, potassium deficiency, and soybean cyst nematodes have been shown to have a role in turning soybean leaves yellow.

Nitrogen (N) Deficiency

Soybean plants can go through a period when their leaves are a light-green color before the nodules supply adequate N and a dark-green color returns. If proper nodulation, sufficient nutrients, and moisture are present, soybean can recover from this yellow phase at a fairly rapid pace as the season progresses (Figure 1).

Nitrogen deficiency can be identified as a yellowing or chlorosis of the lower leaves in the canopy as N is remobilized to the new growth. The two sources of N available to the plant are the soil and N fixation. The first choice for a N source is the soil. This process requires less energy compared with N fixation. The N taken from the soil can account for up to 50% of the total N needed for growth.¹

Manganese (Mn) Deficiency

Symptoms of Mn deficiency are identified by interveinal chlorosis (yellowing) on the newest trifoliates (Figure 2). Manganese is immobile in the plant, so symptoms will generally appear on the younger leaves, although older trifoliates often show symptoms as well. Soil pH affects the availability of Mn; as soil pH increases, less Mn is available to the plant. Manganese deficiency is not generally found in soils with a pH below 6.2.² The deficiency is most common on poorly-drained soils, especially clay and silt loam soils on eroded knolls where the pH is higher than the rest of the field. Roots must reach Mn to absorb it. Wet soils, extremely dry soils, cool weather, soil compaction, root diseases, N deficiency, and herbicide damage can limit root growth causing a Mn deficiency.

Potassium (K) Deficiency

Potassium deficiency symptoms develop because plants cannot extract adequate K from the soil. Potassium deficiency appears as yellowing of the leaf margins on older leaves that usually begins at the leaf tip and extends down the margins toward the leaf base.³ With severe deficiency the leaf edges may become brown (the tissue dies) and affected plants will appear stunted, although the newest leaves may be normal. The most common reason for severe deficiency is soil-test K is lower than optimum for vegetative growth. Depending on the severity of deficiency, yield potential may be reduced.³

Soybean Cyst Nematodes (SCN)

Serious infestations of SCN cause soybean leaves to become yellow in somewhat circular patches of the infested areas. Plants will often be stunted in addition to the yellow leaf appearance. Symptoms may look similar to nutrient deficiencies, compaction, or drought damage. In severe cases drought-stressed plants may actually die.

Yellow Flash From Herbicide Application

Yellow flash can sometimes occur with an application of a high rate of glyphosate such as where spray overlap occurs or where the spray boom is being charged. Symptoms of yellow flash are more likely to occur when rapid uptake, translocation, and metabolism of glyphosate occur during warm, humid weather.⁴

A few days after glyphosate is applied, all leaves on the soybean plant may remain green except the newest leaves — at the top — that were less than 0.5 inch long at the time of spraying. These leaves continue to grow and expand, but chlorophyll production is reduced, leaving a yellow color (carotenoid pigments become visible in the absence of chlorophyll) that can last up to a week. In the case of yellow flash, glyphosate does not cause green leaves to turn yellow; rather, temporary yellowing is likely due to the degradation of glyphosate to a compound known as AMPA.⁴ AMPA was found to be related to reduced chlorophyll levels in soybean.⁴

Field areas with micronutrient deficiencies (iron and Mn) may be under additional stress and have more pronounced yellow flash symptoms. Yellow flash usually appears one to two weeks after application and is gone by three weeks after application.⁴ There has been no evidence for yield loss from yellow flash.

For additional agronomic information, please contact your DEKALB® Brands Seed Representative.