As soybeans begin to mature, areas to target for IDC management reveal themselves

Iron deficiency chlorosis

Iron deficiency chlorosis (IDC) is a technical phrase for plants that have yellow leaf tissue between veins because they have not been able to take up enough iron (Figure 1). Iron is needed for plants to make chlorophyll - the green pigment that plants use to capture the sun's light energy. Through photosynthesis plants use this energy to combine carbon dioxide from the atmosphere and water from the soil to make carbohydrates. IDC is favored by saturated soils with high calcium carbonate and soluble salt (and sometimes nitrate) contents.  

Figure 1. Iron deficiency chlorosis (IDC) symptoms on soybean (photo: Angie Peltier)

Each spring, after that period of poor growth and development caused by IDC, soybean plants in northwest Minnesota typically outgrow all but the most severe IDC symptoms. Provided that there aren't too many factors other than IDC contributing to lower yield, a careful review after harvest of yield maps may help one to pick out those areas of the field that earlier suffered from IDC.

Targeting inputs

Inputs need to at least pay for themselves. For example, when soybeans can be sold for $15 per bushel and an input costs $30 per acre, the yield of the portions of the field treated with that input must produce at least 2 bushels more per acre when compared to the untreated portions of the field.  Generally speaking, one improves the probability of an input paying for itself when applying that input strategically. While one might not plan to plant an IDC tolerant soybean variety only on those field areas on which soybean crops are most likely to suffer from IDC symptoms, applying in-furrow the maximum label rate of chelated iron (a form of iron that soybeans can more easily take up) on only those field areas prone to IDC improves the chances of that input paying for itself. 

Perhaps some are thinking to themselves, 'what if I don't have a yield monitor?' or 'what if I don't want to wait until harvest to figure out my chelated iron needs for the next time I grow soybeans in that field?'  Another great way to get a reminder of where in a field soybeans had previously had the most severe IDC symptoms is to look at the crop as it begins to mature. Ironically, these areas of the field will remain green as their maturity as been delayed, while the rest of the crop begins to turn yellow and drop leaves (Figure 2). 

Figure 2. Soybean field in which the plants in the background have begun to yellow with maturity and the green plants in the foreground have delayed maturity due to early-season IDC symptoms (photo: Angie Peltier).