Updated July 4, 2018:
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Executive Summary: Iron fertilizer evaluation and improvement
R. Jay Goos, Professor, Department of Soil Science, NDSU
Many iron fertilizers do not work in our soils, as they are quickly converted to insoluble iron oxide (rust), and are thus ineffective at alleviating iron deficiency chlorosis (IDC) in soybeans. However, a handful of chelates, FeEDDHA, FeEDDHSA, and FeHBED can "hang on" to the iron tightly enough to keep it soluble. However these products are difficult to manfacture, and commercial sources are a mixture of isomers and condensates....which means that their quality varies from product to product. With so many iron products for sale now, the first objective of this study was to evaluate eleven iron fertilizers for sale in the region with a greenhouse study. Soybeans were grown for two 4-week "crops," with a control, and these 11 iron fertilizers applied at a rate of 1 milligram of Fe per pot.
One product, "Marathon + Greenboost" was not effective. The label said that it contained 9% Fe, and our analysis yielded less than 0.5% Fe. All other materials were highly effective in alleviating chlorosis for the first 4-week crop. All of the materials, except for Marathon + Greenboost, produced plants without chlorosis (chlorophyll readings greater than 30 are dark green). However, it is when a second 4-week crop is grown, that we can observe the "staying power" of the fertilizers. Products like Iron Up, Soygreen, Soygreen Liquid, and Ferrale Evo gave the plants with the most chlorophyll for the second crop. The chelate FeEDDHSA has some handling advantages over FeEDDHA (dissolves more easily), but it was estimated that at least 20-25% more iron would be needed to equal the iron uptake given by a high quality FeEDDHA product.
One problem with products like FeEDDHA is that they are mobile in the soil, and can be leached away from seeds or seedlings with rainfall. The second objective of this study was to evaluate ways of slowing this leaching. We are evaluating the effects of granulation and also additives to FeEDDHA solutions to slow movement. Space does not allow the presentation of any data, but so far the most promising material is a polymer additive to FeEDDHA solutions that has a gelling action and seems to slow the movement of iron.
This work is important to North Dakota farmers because IDC is a widespread and destructive disorder on poorly-drained soils. After variety selection, use of an effective iron fertilizer is an important control measure.