This time of year, soybeans may begin showing signs of chlorosis or other leaf discoloration in all or parts of the field. There may be many causes of discoloration. Nutrient deficiencies are one possibility.
The following is a brief description of the symptoms of some of the most common nutrient deficiencies in soybeans.
Nutrient deficiency symptoms
Nitrogen. Lower leaves are chlorotic or pale green. Within the plant, any available nitrogen (N) from the soil or from nitrogen fixation within nodules on the roots goes to the new growth first. Soybeans prefer to take up N from the soil solution as much as possible, since this requires less energy than the nitrogen fixation process. Both sources of N are important for soybeans since they are a big user of N.
Iron. Iron chlorosis, occurs in calcareous soils with high soil pH. The classic symptom is chlorosis (yellowing) between the veins of young leaves. Iron is not mobile within the plant. A side effect of iron deficiency can be N deficiency, since iron is necessary for nodule formation and function. If iron is deficient, N fixation rates may be reduced. Iron deficiency occurs on calcareous soils because at high levels of calcium, iron molecules become tightly bound to the soil particle and unavailable for plant uptake. In addition to high pH, plant stress can favor the development of iron chlorosis, and therefore the severity can vary significantly from year to year in the same field.
Figure 1A. Iron chlorosis in soybeans. The upper leaves become chlorotic. Photo by Dorivar Ruiz Diaz, K-State Research and Extension.
Figure 1B. Closeup of iron chlorosis in soybeans. Photo by Dorivar Ruiz Diaz, K-State Research and Extension.
Magnesium. Lower leaves will be pale green, with yellow mottling between the veins. At later stages, leaves may appear to be speckled bronze. This deficiency may occur on very sandy soils.
Manganese. Stunted plants with interveinal chlorosis. Can be a problem in soils with high pH (>7), or on soils that are sandy or with a high organic matter content. Manganese activates enzymes which are important in photosynthesis, as well as nitrogen metabolism and synthesis. Symptoms are hard to distinguish from iron chlorosis.
Figure 2. Manganese deficiency symptoms are similar to symptoms of iron chlorosis in soybeans. Photo by Dave Mengel, K-State Research and Extension.
Molybdenum. Plants turn a light green color due to lack of nitrogen fixation. This deficiency is not common, but can occur on acidic, highly weathered soils.
Figure 3. Molybdenum deficiency in soybeans. Symptoms are similar to nitrogen deficiency. Photo by Dave Mengel, K-State Research and Extension.
Phosphorus. Phosphorus deficiency may cause stunted growth, dark green coloration of the leaves, necrotic spots on the leaves, a purple color to the leaves, and leaf cupping. These symptoms occur first on older leaves. Phosphorus deficiency can also delay blooming and maturity. This deficiency may be noticeable when soils are cool and wet, due to decrease in phosphorus uptake.
Potassium. Soybean typically requires large amounts of potassium. Like phosphorus deficiency, potassium deficiency occurs first on older leaves. Symptoms are chlorosis at the leaf margins and between the veins. In severe cases, all but the very youngest leaves may show symptoms.
Figure 4. Potassium deficiency: chlorosis of the lower leaves. Photo by Dave Mengel, K-State Research and Extension.
Sulfur. Stunted plants, pale green color, similar to nitrogen deficiency except chlorosis may be more apparent on upper leaves. Plant-available sulfur is released from organic matter. Deficiency is most likely during cool wet conditions or on sandy soils with low organic matter content.
Mobile Nutrients: These nutrients can be transfer from older tissues to youngest tissues within the plant. Symptoms are noticeable first on lower, oldest leaves.
Immobile Nutrients: These nutrients are not easily transfer within the plant. Therefore, symptoms occur first on upper, youngest leaves.
Possible causes of nutrient deficiencies:
For more information, see K-State Research and Extension publication MF-3028, Diagnosing Nutrient Deficiencies in the Field at: http://www.ksre.ksu.edu/bookstore/pubs/MF3028.pdf
Dorivar Ruiz Diaz, Nutrient Management Specialist