No. 15 Article 5/July 6, 2007

Yellow Soybean Leaves

A number of soybean fields have been showing a slight yellowing of leaves, creating concern that soybean might be in need of additional fertilization. The yellowing of leaves at this time is likely the result of stressful growth conditions preceding the last few weeks in which most soils were fairly dry and plants were not growing very fast. After the rain and warm conditions over the last 2 weeks, soybean plants are now in a rapid-shoot-growth phase. As the shoot is rapidly growing, the developing root system is not yet able to meet the increasing nutrient demands, causing what is likely a temporary nutrient deficiency or, due the to rapid growth, a dilution of nutrients in the plant. 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. Because in most cropping systems soybean depends 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. As growth conditions continue to be favorable, the root system will "catch up," and the deficiencies should disappear. If an adequate fertilization plan was followed in preparation for the current soybean crop, I predict that the slight yellowing we see in some of our soybean fields is temporary in nature, and it is not necessary to become overly concerned at this point.

Finally, in some areas where rain was more abundant and the crop was not stressed by lack of water, some people have expressed concern about the possibility for root rot to have occurred. Although this is possible, I would expect the visible symptoms to be much worst than a slight yellowing of the canopy. In cases where root rot is suspected, I would recommend checking the root system of the crop to determine whether or not there are rotted roots. But again, from a fertilization standpoint, application of fertilizers is not going to correct the problem if indeed there is root rot damage.--Fabián Fernández

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