By Jonathan Kleinjan, South Dakota State University Extension
Many soybean fields in eastern South Dakota are showing symptoms of Iron Deficiency Chlorosis (IDC) again in 2018. As soybean plants begin to develop true leaves, interveinal yellowing can be observed (Figure 1).
Figure 1. A soybean plant affected by IDC. Note the yellowing between the veins in the leaves.
This yellowing, or chlorosis, is caused by a shortage of iron (Fe) in the plant, resulting in reduced chlorophyl production. The first trifoliate leaves will remain green because there is enough Fe stored in the plant to support early leaf and plant growth. Iron is immobile in the plant. Therefore, symptoms are exhibited in the newest plant tissue.
If Fe deficiency persists, leaf tissue and even the plant growing point may die. In addition to being important for chlorophyll production, Fe is important for energy transfer within the plant cells, enzyme production, and root nodule formation.
Soybeans can recover from IDC if 1) it is not too severe and 2) environmental conditions improve, allowing root systems to uptake more Fe. Unfortunately, even in mild cases of IDC, any yellowing will most likely cause yield losses.
Most soils have a contain an abundant supply of Fe. The problem is, most of this Fe is in a form that plants cannot use. Plant available Fe is reduced by: high soil pH, high soil carbonate concentrations (CaCO3 or MgCO3), poor soil aeration (waterlogged conditions), and high concentrations of soil nitrate (NO3-).
Any of these factors alone or in combination with one another can interact to decrease soil available Fe. Soybean roots release inorganic acids and enzymes to increase Fe availability in the soil. IDC is generally not a problem in soils with pH values less than 7.
So, what can producers do to prevent and/or manage for IDC?
Plant varieties with a good IDC score. No varieties are completely tolerant but there are large differences. Some varieties are simply better at excreting root acids to increase soil available Fe. This is the most important management consideration when dealing with IDC.
Increase plant populations in problem areas. Some research has shown that increasing planting population can reduce the severity of IDC simply by increasing the amount of soybean plant roots producing exudates.
For example, if a producer normally seeds at 140,000 seeds/acre, they may want to increase to 200,000 seeds per acre in problem areas.
Rotate away from soybeans for a time. Some producers have mentioned that 4-5 years of continuous corn production will greatly reduce their IDC problem when they shift back to soybean production.
The reasoning is not entirely understood but it could have something to do with the acidity of the nitrogen fertilizers used in corn production reducing soil pH over time.
Improve soil drainage to reduce anaerobic conditions and help move soil carbonates and salts deeper into the soil profile.
Plant a companion/cover crop. Research has shown that planting oats at 1.5 bu/acre with soybeans and killing it before it reaches 12” in height can increase soybean yields in IDC affected areas of the field.
Reduce stress to plant as much as possible. ‘Burner’ herbicides can especially harm the crop when it is suffering from IDC.
Apply 1-3 lb/acre of iron chelate (preferably ortho-ortho chelate) in-furrow at planting. If possible, apply only to the areas likely to be affected by IDC.
Unfortunately, not much can be done once IDC shows up in a field. Foliar applications of iron products such as FeEDDHA can be applied at 1-2 lbs/acre with a spray focused on the soybean rows, but results may be inconsistent and fail to provide an economic return. The best thing to do is plan to avoid problems in the future by applying a combination of the management practices mentioned above.