The element iron (Fe) is required to form chlorophyll, the green pigment in plants.  When iron uptake from the soil is limiting to plants, plants become iron-deficient.  The most common symptom is interveinal chlorosis in newly developed leaves, where the leaf tissue turns yellow while the veins remain green.  This deficiency, termed iron deficiency chlorosis or IDC, can cause moderate to severe yield reductions in soybeans (PHI, 2009; Pioneer, 2009).

IDC occurs to some extent in soybeans that are grown on the high-pH soils in the Black Belt region of east Mississippi (MSU Info. Sheet 873, 2011).

IDC is not caused by iron deficiency in the soil, but rather by the plant’s inability to extract it from the soil.  Soybean plants obtain iron from the soil by releasing acids that solubilize the iron in soil to a form that is readily taken up by the roots.  In high pH soils with high levels of bicarbonates and soluble salts, this process can be limited by the chemical reactions between these materials and the iron (PHI, 2009).

Soil pH is not a good indicator of where IDC will occur and does not correlate well with IDC.  However, there is a direct correlation between IDC and high concentrations of calcium carbonate and soluble salts in soil.  Thus it is important to measure the levels of these materials in soil on sites planned for soybean production (Monsanto, 2010; Pioneer, 2009), and take remedial action (see below) if those levels suggest the potential occurrence of IDC.

The best strategy for managing IDC is to select a soybean variety with tolerance (Helm et al., Agronomy Journal, Vol. 102, 2010;Monsanto, 2010; NDSU, 2011; PHI, 2011; Pioneer, 2009; UMN, 2011).   The Louisiana State University 2012 Soybean Variety Trial publication gives IDC ratings for MG IV and MG V varieties that are grown in the Midsouth.  Ratings of private varieties against IDC made by the originating company (e.g. Pioneer, 2009; Pioneer, PMN, 2012; Asgrow-Dekalb, 2012) are likely the best source for selecting varieties with IDC tolerance.

Iron chelate fertilizer placed close to the seed at planting can be effective for getting iron into the plant, but its cost (>$14/acre) should be considered.  Using the proper type of iron chelate to ensure that iron is available to the plant for a long enough period of time to prevent IDC is important (KSU, 2011; PHI, 2011; UMN, 2011).  Results from research or recommendations for applying iron chelate to the seed are mixed ( Liesch et al., "Agronomy Journal", Vol. 103, 2011; PHI, 2011).  Applying iron as a foliar fertilizer is unpredictable in its effect or will not correct the problem ( Liesch et al., "Agronomy Journal", Vol. 103, 2011; NDSU, 2012; PHI, 2011).

Wiersma, in an article published in "Crop Science" (Vol. 52, 2012), presents evidence that iron-efficient and iron-inefficient soybean varieties have seed iron contents that are distinctly different from each other, and the maximum iron contents in seed of each of the variety classes are seldom exceeded.  Thus, soybean plants tend to maintain iron in the seed within genetically controlled limits.  Furthermore, he concludes that:

• Seed iron content is useful for identifying soybean genotypes that have resistance to iron deficiency;


• Using iron content of soybean seed is equivalent or superior to using visual chlorosis score as an indicator of resistance to iron deficiency;


• Conventional plant breeding can be used to increase seed iron content in order to improve resistance to iron deficiency;


• Iron content of soybean seed that are to be planted can be used to successfully predict IDC; and


• It should be possible to measure iron content in seed from a chlorosis nursery and relate this trait to genotypic resistance to iron deficiency.  Soybean breeders should explore this methodology to ascertain its usefulness as a selection criterion for developing varieties with resistance to IDC.

An excellent source of issues related to and remedies for soybean production on sites that have known IDC-inducing conditions can be viewed on the PMN webcast presented by Dr. Daniel Kaiser in 2011 (available with free USB subscription).

Composed by Larry G. Heatherly, Nov. 2012, larryheatherly@bellsouth.net