High Rates of Fe-EDDHA and Seed Iron Concentration Suggest Partial Solutions to Iron Deficiency in Soybean

Received for publication December 16, 2004. Plants require a continuous supply of iron (Fe) to maintain proper growth. Low rates of Fe chelates applied to reduce Fe deficiency in soybean [Glycine max (L.) Merr.] probably do not satisfy this requirement. Our objective was to evaluate the effectiveness of high rates of Fe-EDDHA in reducing Fe deficiency when applied to susceptible and resistant cultivars grown on soils where soybean historically has exhibited mild to severe Fe deficiency. Four cultivars (two resistant, two susceptible) and six rates of Fe-EDDHA (0, 2.24, 4.48, 6.72, 8.96, and 11.20 kg ha^–1) were evaluated at one location in 2002 and two locations in 2003. Severity of Fe deficiency varied markedly across environment and cultivars. Unifoliolate relative chlorophyll concentrations indicated that Fe deficiency can occur early in plant development and that planting seed Fe concentration (seed [Fe]) may be insufficient for early growth. Responses to higher rates of Fe-EDDHA were environment and cultivar specific and occurred over an extended period, manifest at maturity. At lower rates ((less than)6.72 kg Fe-EDDHA ha^–1), resistant cultivars exceeded susceptible cultivars in plant height, seed number, and grain yield, whereas at higher rates, susceptible cultivars often had values similar to resistant cultivars. Both resistant and susceptible cultivars exhibited linear responses to increasing rates when grown in harsh or intermediate environments, suggesting that even at very high rates of Fe-EDDHA, Fe deficiency limited plant growth and grain yield. Seed [Fe] changed very little in response to added Fe. Plotting relative grain yield versus seed [Fe] for each environment illustrated the narrow range of seed [Fe] associated with wide ranges in relative yield and the large difference between resistant and susceptible cultivars regardless of relative yield.