ISSUE 7   June 15, 2006

SOYBEAN IRON DEFICIENCY CHLOROSIS

It didnít take long for iron deficiency chlorosis (IDC) to appear in soybean fields in the eastern North Dakota once the first trifoliate leaf emerged. True chlorosis appears as the figure shows- yellowing between the veins, with the veins remaining green most of the time, unless the conditions are extremely severe. General yellowing is also possible when soils are saturated, or nodulation is poor. General yellowing is not caused by iron deficiency.

The main cause of IDC is presence of carbonates in the soil. One of the solubility products of carbonates, bicarbonate, neutralizes the acidity around the soybean roots that are important for the activity of an iron reducing substance secreted from the roots. The iron reducing substance transforms native iron with an electron valence of III (FeIII, ferric iron) to iron with an electron valence of II (FeII, ferrous iron). This is important for the soybean, because FeII is a trillion times more soluble and available to the plant than FeIII.

Anything that interferes with iron uptake, or the ability of the soybean to overcome the hardship caused by bicarbonate interference with iron transformation will increase the incidence, severity and longevity of IDC.

There are a number of factors that do not cause IDC by themselves, but contribute to greater severity and longevity when they are present along with carbonates. These factors include high soluble salts, cool weather, wet soils, herbicide stress, high soil nitrates, and poor variety choice.

Application of iron fertilizer products has resulted in inconsistent results in the past. It may be possible to green up plants with an iron application, but the end result will probably be disappointing. Add to that the small-scale nature of the IDC areas throughout the field, and the possible benefits deteriorate quickly.

It is unusual for an entire field from fence-row to fence-row to be IDC. If this occurs, it is either because soil conditions, particularly soluble salts, are generally high (above 1 mmoh/cm) and/or the variety is not adapted to conditions.

When IDC is observed, it is important to take note so that appropriate varieties are used the next time the field is in soybeans. If it is possible to take an aerial photo of the field, in years to come when it is possible to variably seed a field based on whether areas are susceptible to IDC or not, the photo will help to define zones that would benefit from a more tolerant variety. Have the photo taken from at least 5,000 feet in an angle pointing straight down to the field (nadir). Satellite imagery might be helpful if the resolution is 10-m or less.

Dr. Dave Franzen
NDSU Extension Soil Specialist
david.franzen@ndsu.edu


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