When soybeans turn yellow at an early stage of growth, there are several possible explanations.
Nitrogen (N) deficiency. In fields that have been extremely wet or extremely dry, or under severe early heat stress, rhizobial nodule development can be delayed, resulting in N deficiency. As soil moisture levels return to more normal conditions (if a short-term stress), 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 N fixation goes to the new growth first.
Soybeans double cropped after wheat can be N deficient for a short period of time shortly after emergence 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 small amount of N (no more than 30 lbs. acre) 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 temporarily inactive. 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 where soybeans are planted into new acres with no history of soybean production. In recent years, there have been reports of inoculated soybeans planted on “virgin” fields that have failed to produce nodules, resulting in N deficiency. An examination of the root systems showed very few or no nodules. Previous studies show that a rescue application of 90 to 120 pounds of N per acre gave good returns in these situations. A rescue application should be considered only if N deficiency symptoms are confirmed, and applications should be made as soon as possible to increase N uptake.
Iron (Fe) chlorosis. Soils that are too wet can also induce temporary symptoms of Fe chlorosis. With Fe 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 Fe chlorosis so certain varieties may show more of the symptom than others.
Excess nitrate in the soil can exacerbate problems of Fe chlorosis in fields with high soil pH and prone to causing Fe chlorosis problems. This can be particularly noticeable during early soybean growth.
An interesting phenomenon that occasionally has been observed is that 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 from Fe chlorosis than the rest of the field. Recent studies have shown that soil nitrate concentrations in these wheel tracks are typically lower, so Fe chlorosis symptoms are alleviated compared to the rest of the field. 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 on Fe chlorosis in the volume of soil
Potassium (K) deficiency. Another cause of yellowing could be K deficiency. Contrary to Fe deficiency, K deficiency is typically more common later in the season. 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 N, you can see symptoms both in fields that are too wet or too dry. Most of the time, the symptoms will fade with improved soil conditions that allow good root growth, unless the field is truly deficient in K. 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, root insects and disease etc.) can lead to reduced growth and potential leaf yellowing. With a restricted root system, the growing plant can’t access the nutrients 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.