One of the things we may observe in soybeans in June is yellowing, generally not field wide like nitrogen-deficient corn but more spotty and localized on some regions of the field landscape. Usually the crop grows out of it as the weather warms, soils dry and roots expand into a larger soil volume. However, there are a number of possible causes for yellowing, including:
- Failure to form nodules on roots
- Nitrogen immobilization
- Iron chlorosis
- Glyphosate flash
- Soybean cyst nematode
- Nutrient deficiency
Nitrogen shortages: After a cool, wet spring into June your soybeans may yellow. I refer to this yellowing as a malaise that impacts soybean plants because they aren’t fixing nitrogen. In fields that have been extremely wet or extremely dry, or under early stress, rhizobia nodule development is delayed, resulting in N deficiency. Soybeans planted into high-residue scenarios using no-till after corn or wheat may show similar temporary symptoms, since N is immobilized as organic matter and begins to decompose. That is why a starter (2 x 2) weed and feed or top dress application can help prevent this problem—use 20 to 30 lbs. per acre.
Root restriction: Compaction layers can restrict root growth—reducing aeration, water and nutrient supply—resulting in yellowing.
Chlorosis: Iron chlorosis is a yellowing of plant leaves caused by iron deficiency (the veins remain green). Iron deficiency may also occur in high pH, alkaline and calcareous soils or soils with high nitrate levels. The iron essentially gets tied up in a form in plants that makes it unavailable. There are several starter fertilizers that have chelated iron that can overcome this iron deficiency. Planting covers in the fall or spring that take up the free nitrate can remedy the situation. Iron chlorosis is usually short-lived in most situations. It is rarely a risk in Illinois.
Glyphosate flash. Many of the early Roundup Ready® varieties exhibited a yellow flash after application as glyphosate stressed the metabolic engine. Today you rarely see this since most varieties now carry the second generation trait. But once in a while we still can see these symptoms. The plants will grow out of it just like after a PPO application.
SCN: Soybean cyst nematodes infest roots in June. As they feed this causes stress on the plant—resulting in yellowing. Symptoms are usually hot spots associated with high egg counts, similar in appearance to lack of nitrogen. As with iron chlorosis or N deficiency, the plant may grow out of it as roots expand, depending on the severity of the infestation.
Nutrient deficiency: Two nutrient deficiencies can cause yellowing—potassium and sulfur. Potassium deficiency appears as yellowing around the leaf edges, which usually occurs later in the season. Sulfur deficiency can occur any time in the season and appears on young leaves (while N deficiency often appears on older leaves).
Kansas State University agronomists Dorivar Ruiz Diaz and Ignacio Ciampitti reviewed the causes of yellow soybeans here.
Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at firstname.lastname@example.org or ring him at 402-649-5919.