Except for iron and zinc, micronutrients not warranted
Plants require seven micronutrients: boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn). In Nebraska, deficiencies of iron and zinc can be common, depending on soil properties, but deficiencies of the other five micronutrients are rare.
One case each of boron deficiency has been reported on alfalfa and sugar beets, both on sandy soils. We do not know of any other confirmed micronutrient deficiencies on agronomic crops in Nebraska. With the exception of iron and zinc, the probability of profitable yield response to micronutrients is very low.
However, toxicity associated with misuse of micronutrients is a risk and has been confirmed in Nebraska.
With relatively high crop prices this year, there has been increased interest in products, including micronutrient fertilizers, that are marketed to promote crop growth. Some dealers have even gone to the extreme of mixing such products with commonly used fertilizers, often without informing producers before the invoice is sent.
Do high yield crops need exceptionally high soil availability of micronutrients? More yield means more nutrient removal. However, high yield crops have the potential to be highly efficient in nutrient uptake and utilization. High yield crops are freed from much stress, such as soil water deficits, that low yield crops encounter. High yield crops typically have healthy and extensive root systems capable of efficient nutrient uptake and healthy plants capable of efficiently converting nutrients and carbohydrates to yield. This high efficiency has been repeatedly shown for high yield corn, especially for nitrogen, but also for phosphorus and potassium. There is no evidence to the contrary for micronutrients.
Zinc deficiency is most likely to occur on low organic matter, calcareous soils, but can occur on other soils. Corn is relatively sensitive to zinc deficiency, and the zinc soil test is a reliable indicator of the probability of profitable response to zinc application. When selecting between inorganic (e.g., zinc sulfate) and chelated (Zn EDTA) zinc products, consider the rates and relative costs. Inorganic zinc (not the product) is applied at two to four times the rate of chelated zinc. The chelated form should be considered for deficiency correction only. If soil test zinc is above 0.8 ppm and application is to build or maintain a high level of availability, use an inorganic form.
Iron deficiency is most common on calcareous soils. Soybean, dry bean, and sorghum are especially sensitive. Planting tolerant crops and varieties is important to management of iron deficiency. Seed dressing with iron EDDHA at 0.2 lb/ac iron has been as effective as applying 50 lb/ac iron as iron sulfate. Use this as a guideline in comparing costs.
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