Possible causes of yellow soybeans
There are always some fields of soybeans about this time of year that are turning yellow. There are several possible explanations.
* Nitrogen deficiency. In fields that have been extremely dry (or extremely wet, although that’s not a problem this year), rhizobial nodule development can be delayed resulting in nitrogen deficiency. As the soils receive rain (or dry out in wet years), the nodule forming bacteria will go to work and the deficiency symptoms will quickly disappear. With N deficiency, it is usually the lower leaves that are chlorotic or pale green. Within the plant, any available N from the soil or from nitrogen fixation within nodules on the roots goes to the new growth first.
Soybeans doublecropped after wheat can be N deficient for a short period of time until the beans become well nodulated. As the wheat straw decomposes, some of the soil available N will be immobilized, making it unavailable to the young soybean plants. Applying a little N at planting time to soybeans planted into wheat residue is the best way to avoid early-season N deficiency.
Hail damage can also cause N deficiency in soybeans at times. If the foliage is damaged enough so that the plant can’t provide enough food for the rhizobia on the roots, the rhizobia will slough off the roots or become inactive for a while. If this happens, the plants may temporarily become N deficient. Plants normally recover from this as regrowth progresses and photosynthates are translocated to the nodules.
Nitrogen deficiency due to a failure of soybeans to nodulate properly has also been a problem at times as soybeans expand into new acres with no history of soybean production. Over the past three years “virgin” fields which had been inoculated before or at planting have failed to produce nodules, resulting in nitrogen deficiency. A quick examination of the roots system showed very few or no nodules. A rescue application of 90 to 120 pounds of N per acre gave good returns in these situations.
click image to zoomDoug Jardine, K-State Research and ExtensionIron chlorosis. The upper leaves become chlorotic. * Iron chlorosis. Soils that are too wet can also induce temporary symptoms of iron chlorosis. With iron chlorosis, the top most leaves will turn yellow, but the veins remain green. This problem is usually more serious in soils with highly alkaline pH. Additionally, soybean varieties have varying tolerance to iron chlorosis so certain varieties may show more of the symptom than others.