By Dave Franzen
NDSU Extension Soil Specialist
Growers and crop consultants in the Valley are well acquainted with iron deficiency chlorosis (IDC) through years of experience. Not everyone in the Valley (few, really) follow the best advice in dealing with it, but many growers have made great strides in the past 20 years in being able to live with it. A fully explained narrative of the causes of IDC in soybean is available in the soybean fertility circular available at https://www.ndsu.edu/fileadmin/soils/pdfs/sf1164.pdf
In brief form the following is what is happening to fields of soybeans:
For IDC to be a hazard, the soil must be calcareous. This means that soil pH will be greater than 7, and if a calcium carbonate equivalent test is performed, free carbonates are greater than zero; sometimes greater than 20% by weight in just the surface six inches of soil. Carbonates themselves in a dry to a little moist soil will not result in serious IDC. However, if the soil is wet enough to dissolve some of the carbonate and form the anion bicarbonate, IDC will appear. Soybean roots exude acids, which acidify their root system. There are probably many reasons for them to do this, and most plants do this for many reasons. A particularly important reason for soybean roots to do this is that the iron (Fe) exists in soil as the oxidized ion Fe+3, which is extremely insoluble. Reduced Fe (Fe+2) is extremely soluble, as evidenced by the iron removers in many rural homes. Most people also understand that without an iron removal system, sinks, tubs, shirts, nearly everything ground water touches turns orange. This is because the reduced iron (you can find as much reduced iron in water as a 2 penny carpenter nail per quart) immediately becomes one trillion times less soluble when exposed to oxygen and falls out as Fe+3 oxide.
The soybean root also exudes an iron reducing substance, which takes F+3 and transforms it to F+2, making it a trillion times more soluble. This happens unless there is something in the soil to interfere with the reducing substance, which requires an acid pH environment to work. Bicarbonate neutralizes acidity around the soybean root, making it much, much harder for the soybean root to take up iron.
So the rainfall in the center and western part of the state this year has resulted in bicarbonate in the soil solution and caused IDC in regions previously unaffected.
The following are steps that growers should implement to reduce IDC in the future-
- The most important step is to choose an IDC tolerant variety-not Iowa tolerance, but North Dakota tolerance.
- Secondly- Choose fields carefully. Avoid fields with wetness issues, high carbonate levels and high salts (EC greater than 1.5 mmohs/cm). Also avoid fields with high residual nitrate.
- Third- Grow the soybean in wide rows- 15 inch to 30 inch rows. For some reason, when soybean plants are closer to each other IDC is greatly reduced.
- Fourth- Consider a companion crop of a bushel of oats or barley (barley is better on salty soils) to reduce soil moisture and residual nitrate. If conditions are dry, the companion crop can be killed out early; if a wetter season, wait until the 5th leaf stage to kill out.
- Fifth- If a tolerant variety is used, application of an ortho-ortho Iron EDDHA fertilizer with water in furrow at planting will usually reduce IDC.