Transition time for soybeans

Jeff Burbrink writes about soybeans.
Posted on June 26, 2013 at 1:00 a.m. | Updated on June 26, 2013 at 5:24 p.m.

Jeff Burbrink

Around Area Farms

Did you notice some of the soybeans in the county turned yellow last week? There may have been several reasons for that color change.

Many of the beans were in the vegetative second or third stage last week. That is a transition time for beans. Up until that time, the bean plant has depended upon the cotyledons, or seed leaves, for nutrients, particularly nitrogen. As the plant matures, the cotyledons dry up and fall off the plant. If soils are cool, the nitrogen fixing nodules on the root may not be actively working yet, and the bean plant’s upper leaves can yellow due to a short-term deficiency of nitrogen.

It is fairly easy to see if this is why beans are yellowing. Simply dig a plant from the soil and examine the inside of a nodule. Nodules that are white on the inside are not fixing nitrogen yet, while those that are red inside are fixing N for the plant. Remember to dig the plant from the soil, don’t pull it out. Pulling the plant may cause the nodules to fall off the roots.

Yellowing due to the slow development of nodules is temporary. But yellowing can occur due to several other reasons. Soybean cyst nematodes can yellow a plant by injuring the roots. This yellowing will get more severe over the course of a summer. Manganese and iron deficiency can also cause yellowing, and are sometimes related to the pH of the soil. When pH gets too high on mineral soils, these nutrients can be unavailable to the plant even if present in adequate amounts in the soil. A tissue test can help to confirm this symptom, since many of these problems are very similar to each other in appearance.

Jeff Burbrink is an Extension educator in agriculture and natural resources. Write to him at 17746 E. C.R. 34, Goshen, IN 46528; call 533-0554; or fax 533-0254.


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