A number of soybean fields are exhibiting a slight yellowing of leaves, and some growers are concerned soybeans might be in need of additional fertilization, says Fabian Fernandez, University of Illinois Extension specialist in soil fertility and plant nutrition.
"The yellowing of leaves at this time is likely the result of stressful growth conditions in the last few weeks in which the plants were not growing very fast," Fernandez notes. "Many soybean fields are starting a rapid shoot growth phase. Meanwhile, the root system, which in some instances has not developed very well, is not yet able to meet the increasing nutrient demands, causing what is likely a temporary nutrient deficiency. This rapid growth can cause a temporary dilution of nutrients in the plant, resulting in leaves showing slight nutrient deficiency."
In some fields, the pale-green-to-yellow leaves are the result of very wet soil conditions. Soybeans don't thrive well in wet soils and are more sensitive to this condition than corn. This problem is most commonly seen in low areas of the field or on poorly drained soils where water tends to stay for a prolonged period of time 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 the plant," Fernandez adds. "Temporary nutrient deficiencies can be observed when excess water in the soil depletes oxygen and builds up carbon dioxide levels. Although oxygen is needed by roots to grow and take up nutrients, high carbon dioxide levels are toxic and limit root growth and activity."
Low light intensity during cloudy days reduces photosynthetic rates and nutrient uptake by the crop. Because low light intensity sometimes occurs at the same time soils are waterlogged or temperatures are cool, cloud cover can exacerbate the capacity of the crop to take nutrients.
"While this is not much of a problem for soybeans this year, the opposite condition (dry weather) can have similar results," he says. "When the surface layer of the soil becomes too dry and the root system of the crop is small and shallow, dry conditions can limit root activity and nutrient movement to the root."
Another likely explanation for the slight yellowing seen in some fields is the fact that under stressful growing conditions, a reduction of nodulation and/or number of active nitrogen-fixing nodules could have occurred.
"Since in most cropping systems soybean depend on this symbiotic relationship to supply much of the needed nitrogen, it is likely that soybean plants are experiencing some reduction in the amount of available nitrogen," he adds. "As growth conditions continue to be favorable, the root system will 'catch up' and the deficiencies should disappear."
When environmental conditions are challenging, Fernandez says there is normally little that can be done to economically solve the problem. The good news is that those adverse environmental conditions are often temporary and unlikely to result in yield loss for soybeans.
"Most people think that if there is a problem, it has to be solved through management practices," he notes. "While this may be true in some situations, there are still environmental forces that can have an important impact on nutrient availability and the capacity of the crop to grow."
If an adequate fertilization plan was followed in preparation for the current soybean crop, he predicts that the slight yellowing seen in some soybean fields is temporary and not something to become overly concerned about at this point.
For more information, check out the June 30 edition of The Bulletin, an online publication written by U of I Extension specialists in crop science, at http://ipm.illinois.edu/bulletin.