A number of soybean fields are showing a slight yellowing of leaves, and some growers are concerned that soybean might need more fertilization. Yellowing of leaves at this time is likely the result of stressful growth conditions in the last few weeks, when plants were not growing very fast. Many fields are now at V3-V4 and are starting a phase of rapid shoot growth. As the shoot is rapidly growing, the root system, which in some instances has not developed very well, cannot yet meet the increasing nutrient demands, causing a--likely temporary--deficiency. Sometimes rapid growth can cause a temporary dilution of nutrients in the plant, resulting in leaves showing a slight deficiency.
In some fields the pale-green to yellow leaves are the result of very wet soil conditions. Soybeans don't thrive very well in wet soils and are more sensitive to this condition than corn. This problem shows up in low areas of the field and on poorly drained soils where water tends to sit for prolonged periods after rain.
Soil water content is critical not only to supply the water needs of the crop but also to dissolve nutrients and make them available to plants. Temporary nutrient deficiencies can be observed when excess water in the soil depletes oxygen and builds up carbon dioxide. While oxygen is needed by roots to grow and take up nutrients, high levels of carbon dioxide are toxic and limit root growth and activity.
Low light intensity during cloudy days reduces photosynthetic rates and nutrient uptake by the crop. Since it sometimes occurs at the same time that soils are waterlogged or temperatures are cool, cloud cover can exacerbate the limiting of the crop's capacity to take up nutrients. While the opposite condition--dry weather--is not so much of a problem for soybeans this year, it can have similar results. When the surface layer of the soil becomes too dry and the crop's root system is small and shallow, root activity and nutrient movement to the root may be limited.
Another likely explanation for the slight yellowing seen in some fields is a reduction in nodulation and/or the number of active nitrogen-fixing nodules as a result of stressful growing conditions. Since in most cropping systems soybeans depend on this symbiotic relationship to supply much of the needed nitrogen, it is likely that available nitrogen is somewhat reduced. As growth conditions continue to be favorable, the root system will catch up, and the deficiencies should disappear.
It is common to think that all crop problems are to be solved through management practices--that if we have done something, the crop has to respond to it. While this may sometimes be true, environmental forces can have an important impact on nutrient availability and the capacity of the crop to grow, even when you have done all you can to provide adequate soil fertility and good conditions for growth. When environmental conditions are challenging, there is typically little that can be done--in an economically feasible way--to solve the problem. The good news is that adverse environmental conditions are usually temporary, and when they occur early in the growing season, they don't typically translate into yield loss for soybeans.
Remember that plants obtain most of their nutrients and water from the soil through the root system. Any factor that restricts root growth and activity can restrict nutrient availability--the nutrients are plant-available in the soil, but the crop's ability to take them up is restricted. If you are seeing slight yellowing in your soybean fields but you followed an adequate fertilization plan in preparing for the current crop, I predict that the yellowing is temporary and suggest that you need not be overly concerned.