Possible causes of yellow soybeans
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.
click image to zoomDorivar Ruiz Diaz, K-State Research and ExtensionField of soybeans with iron chlorosis, showing greener areas in the wheel tracks. 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.
Dorivar Ruiz Diaz, K-State Research and ExtensionYellowing around leaflet margins from potassium deficiency. * 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.
click image to zoomDave Mengel, K-State Research and ExtensionChlorosis of the lower leaves from potassium deficiency shows up first on lower leaves. * 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.