Explaining the causes

• The soil contains plenty of iron (Fe) to satisfy soybean needs. But the Fe is not in the available form for plant uptake because high calcium carbonate in the topsoil keeps the soil pH at high levels (around 8). This favors Fe to remain in the insoluble and unavailable form.

• Extended periods of wet soils (from high water table, snowmelt, heavy rains) create conditions for less oxygen and more carbon dioxide (CO₂) to accumulate in soil. CO₂ is a precursor for bicarbonate production, which suppresses Fe solubility. Wet soils can also lead to excessive manganese uptake by crops.  There is NO need to apply manganese fertilizer to fields in North Dakota in general, but especially not on soils giving IDC.

• Accumulation of excess soil nitrates on less productive, IDC susceptible spots. Excess nitrates intensify IDC.  Use of cover crops, or planting a companion crop like oats or barley in spring has been suggested, to reduce soil nitrate levels and surface wetness.

• IDC is more severe in saline soils.

• Sometimes the seed you buy isn't as resistant as the seed company says.  A simple suggestion is to compare the 5 to 10 weakest or most IDC-susceptible varieties in NDSU trials to the ratings by the respective seed companies and draw your conclusions.

The Teachers asked: “why was 2016 a “bad” chlorosis year?”

No single answer can explain the prevalence of IDC on a field. Some or all of the factors listed above were the primary causes of IDC in some of the fields in North Dakota. Any long period of drought may have caused higher loss of water from soil through plant uptake and loss from the leaves (evapotranspiration) than the soil received water from rainfall.  This situation favors upward movement of soil water carrying salts with it.

Control measures

• Avoid fields that are susceptible to IDC. An attractive market price of $10/bu of soybeans that produced low yields on a susceptible field can be much less profitable than a 70 bu/ac barley crop that sells at $3/bu.

• Plant a resistant variety. Don’t rely solely on the seed company’s IDC ratings; Consider the NDSU IDC ratings as well, and talk to your neighbors too.

• Row spacing. There are indications that when dealing with IDC problems, it is more favorable to use 22 or 30” row spacing instead of 7”.

• Try barley cover crop at soybean planting: Consider broadcasting barley at about 1bu/ac, and lightly work it in; or seed with a two-compartment seeder unit. Kill the barley at 3-leaf stage when the soil is dry, or at 5-leaf when soil is wet.

• Use iron chelate (FeEDDHA). Apply 2 to 3 lb/ac of a proven product of FeEDDHA in-furrow at planting of a resistant soybean variety.

Read more on this article by Dr. D Franzen, and Dr. R Jay Goos in the Prairie Grains 2016 publication, issue 150.

Jasper Teboh, Ph. D.
Research Soil Scientist