Update:
The project has been completed. Its final product is a regional extension-type publication that is in the final formatting stage before posting it to a web site and hardcopies are printed. We expect it to be available and free for downloading at the Iowa State University Extension Publications web site by the end of May, 2017. It also will be available at the Soybean Research & Information Initiative. We reviewed peer-reviewed published information about soybean response to micronutrients in the North Central region. We also reviewed difficult to find articles in conference proceedings, research bulletins, and project reports as well as unpublished results of recent projects in the states represented by the co-investigators. We looked for information such as method of micronutrient fertilizer application, major soil properties or series, use of soil and/or plant-tissue testing, number of trials and years, and occurrence of a yield response. We shared to producers and crop consultants the results of recent and most relevant research conducted in the region. During the last six months, we defined the main concepts, content, and format of the publication.

The publication reflects what research has found for decades in the North Central region. The likelihood of soybean yield response to micronutrients is small across the entire region. Yield increases from fertilization with some micronutrients are likely, however, in specific soils or conditions that are infrequent in several states. Deficiencies of several micronutrients are commonly observed in coarse textured, very low organic matter, highly organic, or high-pH (calcareous) soils. Soybean iron deficiency chlorosis (IDC) is frequently observed in calcareous soils present mainly in the western areas of the region. Research on fertilization with Fe during many years has shown little success at increasing soybean yield to levels similar to those in unaffected soils. However, recent or ongoing research mainly in Kansas and Minnesota with newer products (o-o-EDDHA chelated Fe fertilizer) is showing good potential to alleviate the impact of Fe deficiencies. Use of tolerant varieties is the most certain way for minimizing the incidence of IDC and increasing soybean yield in these soils. Manganese deficiency in soybean is observed mainly in coarse textured soils or certain soils, typically with high pH and organic matter, in Indiana, Michigan, and Ohio. On the other hand, application of micronutrients where the soil supply is adequate may cause toxicity and yield reduction. The most common example is B, and both toxicity and yield reduction have been documented in Minnesota.

There have been difficulties at developing interpretations for soil or tissues tests that could guide fertilization. This is because of a lack of widespread deficiencies essential for test calibration research and because the plant-availability of some micronutrients, mainly B, Fe, and Mn, is greatly affected by short-term changes in environmental conditions that make testing unreliable. These include mainly soil moisture and oxidation-reduction conditions controlled by soil aeration. For these reasons, sufficiency levels of soil or plant-tissue tests for soybean for the North Central region are scarce and the existing ranges are very wide. Recent field calibration research for B, Cu, Mn, and Zn in Iowa, Kansas, and Minnesota including more than 100 trials showed that the upper ends of most existing sufficiency ranges are well into the adequate range and, therefore, their use reduce the probability of a deficiency but often encourage unneeded fertilization. For example, the existing soil-test interpretation ranges in the region would encourage B and Zn fertilization of many fields in which a yield increase is unlikely, but soil-test interpretations for Cu and Mn seem adequate. For tissue tests, however, use of existing sufficiency ranges for B, Cu, Mn, and Zn suggested fertilization of many fields where a yield increase was not observed. Another important conclusion of the project was that high-yielding soybean removes high amounts of micronutrients with harvested grain, but the yield level or the yield potential alone is not a good indicator for the need of additional fertilization with micronutrients.

Therefore, major concepts included in the publication are the inadequacy of published sufficiency levels for some micronutrients for soil or tissue tests and an urgent need for field calibration research in areas or soil types where deficiencies are more likely to develop reliable interpretations for soil and tissues tests. Decisions about micronutrient fertilization for soybean should consider soil and tissue test results, along with the soil or environmental conditions that traditionally have been identified with a higher likelihood of deficiencies and economic yield responses to fertilization. These include mainly coarse textured, calcareous, organic, or severely eroded soils.

The final product of this project is a North Central region extension-type publication that is in the final formatting stage before being posted to a web site and hardcopies are printed. We expect it to be available and free for downloading at the Iowa State University Extension Publications web site by the end of May, 2017.

The publication reflects what research has found for decades in the North Central region. The likelihood of soybean yield response to micronutrients is small across the entire region. Yield increases from fertilization with some micronutrients are likely, however, in specific soils or conditions that are infrequent in several states but more frequent in a few others. Although high-yielding soybean can remove high amounts of micronutrients, the yield level and yield potential are not a good indicators of the need for fertilization with micronutrients. Soybean iron deficiency is commonly observed in calcareous soils present mainly in western areas of the region. The use of tolerant varieties and recently developed fertilizers are good options to alleviate the impact of Fe deficiencies, although typically the yield level achieved is less than in field areas that are not calcareous. Manganese deficiency in soybean is observed mainly in coarse textured soils or in certain soils, typically with high pH and organic matter, in Indiana, Michigan, and Ohio. In these soils and conditions manganese fertilization is a recommended practice. Deficiencies of other micronutrients are much less common. On the other hand, application of some micronutrients to soils with already adequate levels may cause toxicity and yield reduction. A typical example is boron, for which there is a narrow range between deficiency and toxicity.

There have been difficulties at developing interpretations for soil or tissues tests that could guide fertilization mainly because of a lack of widespread deficiencies essential for test calibrations. Also, the plant-availability of some micronutrients is affected by short-term changes in environmental conditions. Good examples of these conditions include soil moisture and oxidation-reduction conditions controlled mainly by soil aeration. Therefore, research-based existing interpretations of soil and tissue tests for micronutrients in the North Central region are scarce. Moreover, use of published interpretations in the region or from other regions reduce the probability of a deficiency but often encourages unneeded fertilization. Therefore, there is an urgent need for field calibration research in areas of the North Central region where deficiencies are likely in order to develop more reliable interpretations.

Decisions about micronutrient fertilization for soybean should consider soil or tissue test results, but also should consider the soils and environmental conditions that traditionally have been identified with a higher likelihood of deficiencies and economic yield response to fertilization. These include mainly coarse textured, calcareous, organic, or severely eroded soils.