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There are several metallic elements that are considered essential nutrients. They are referred to as "micronutrients" because they are required by plants in very small amounts when compared to major nutrients, like nitrogen or phosphorus. The primary micronutrients include zinc, iron, manganese, copper, and boron.
Micronutrients typically have a "critical" soil test concentration, below which there is a significant impact on yield. There is an"optimum" concentration range that is greater than the critical concentration. No yield impact is expected with soil test levels in the optimum range or greater. Excess levels are potentially toxic and can affect plant growth or yield.
The general approach toward a soil test level that is below the optimum concentration is to recommend a one-time micronutrient application to "build" the soil to the optimum concentration or slightly above. This is expected to keep the soil test between the critical and the optimum concentrations for about three to four years as the crop harvest removes the available micronutrient fractions. The soil test may then be at or below the critical level, requiring another fertilizer application to "rebuild" the soil test.
It is important to note that plant availability for certain micronutrients may not be directly correlated to the soil test value. Other factors not directly related to the soil test may have a significant effect on plant availability for certain micronutrients. This can include soil pH, organic matter, soil moisture, parent material, or other factors. It may prudent to follow up with plant tissue analysis to confirm deficiency or sufficiency of a given micronutrient.
Note - iron: Soil test results are NOT used to make iron fertilizer recommendations as other with other micronutrients. The iron soil test alone is not a reliable indicator of crop response. The occurance of "iron deficiency chlorosis (IDC)" is related to other factors like crop species sensitivity, soil pH, presence of reactive carbonates (i.e., "excess lime"), and soil moisture status. Variety selection is probably the most useful IDC mangement strategy. There are specific products for soil application at planting that are useful. Foliar applications of certain products may also be useful.
Also see the following Crop Files: