Richard Taylor, Extension Agronomy Specialist; rtaylor@udel.edu
Looking at the double-crop soybeans in some areas of Sussex County, I noticed that there are pockets of yellowed beans in many fields, especially in those areas that have received many of the recent rainfall events. I thought it might be worthwhile to review the difference between manganese (Mn) and nitrogen (N) deficiency.
The N deficiency is caused by saturated soil conditions that cause the soybeans to slough off their nodules leading to N deficiency. We often see Mn deficiency in small patches in a field, usually where lime applications have overlapped or where the soil is lighter than the general field so that even a correct lime application rate causes high soil pH — inducing Mn deficiency in the area.
Wet spots or drowned-out areas in a soybean field often are also small patches in the field which can lead to confusion as to whether it’s N or Mn deficiency you see. The difference is in the type and location of symptoms. Manganese deficiency shows up first on the newest leaves and is characterized by what is called interveinal chlorosis where the veins remain green but the tissue between the veins turns a very pale green or, more likely, yellow or white. If the deficiency starts early in the growth cycle of the plant and remains untreated, much of the plant will show the symptoms. If the plant grows for a long period of time before a foliar Mn application occurs, the lower affected leaves might not receive enough of the foliar spray to green back up, although the newer upper leaves will turn green again.
Plants impacted by wet soil conditions causing the Bradyrhizobia nodules to fall off the root system will show classic N deficiency symptoms in which the lower leaves will turn yellow and the yellowing will progress up the plant if N is not supplied. Since anaerobic soil conditions, especially during hot summer conditions, can cause the roots to die, the progression up the plant can be very rapid. Also, since low wet spots in fields are often small, irregular areas, the yellowing symptom can easily be mistaken for spotty Mn deficiency. Although close inspection of the affected areas allows you to distinguish between the two conditions, you can easily mistake one problem for the other if driving by in a vehicle or if viewing the areas from a long distance away.
So before you waste your fertilizer dollars, it’s worth the time to walk through your fields and check out those yellow areas to see if they are caused by Mn deficiency or are showing N deficiency due to localized saturated soil conditions.