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.
* 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.
Excess nitrate in the soil can exacerbate problems of iron chlorosis in fields with high soil pH and prone to causing iron chlorosis problems. This can be particularly noticeable during early soybean growth.
An interesting phenomenon that occasionally has been observed is that the soybean plants in slightly more compacted soil (for example in the wheel tracks associated with the last tillage pass) will be greener and display less yellowing than the rest of the field. Recent studies have shown that soil nitrate concentrations in these wheel tracks are typically lower. The areas of compacted soil have less oxygen, likely resulting in more denitrification. Areas of higher soybean population in the field can also show greener conditions. Higher plant populations and greater root density can reduce the negative effect of higher soil nitrate concentrations in the volume of soil.
Why do higher levels of soil N tend to exacerbate problems with iron chlorosis? The reason for this is the subject of debate among plant physiologists, and the answer isn’t yet clear. But the effect seems to be real.
* Potassium deficiency. Another cause of yellowing that is being seen in some fields is potassium deficiency. At this time of year, deficiency symptoms include an irregular yellow mottling around leaflet margins. The yellow areas coalesce to form a more or less continuous, irregular yellow border. Again, as with nitrogen, you can see symptoms in both too wet and too dry fields. Most of the time, the symptoms will fade with improved soil moisture conditions, unless the field is truly deficient in potassium. Potassium deficiency can also be caused by soil compaction, which limits root growth and development.
* Rooting restrictions. Anything that restricts expansion of the root system (e.g. extremely wet or dry soil, compaction layers, sidewall compaction, etc.) can lead to reduced growth and potential leaf yellowing. With a restricted root system, the growing plant can’t access the nutrients (iron, potassium, nitrogen before nodulation) it needs to make more leaves. As a result, many of the nutrient deficiencies described above can show up in fields where you might not expect them based on a typical soil test.