Micronutrients are “small but mighty” elements required by plants to complete their life cycles. Plants take up less than one pound of micronutrients per acre, with only a few ounces required for optimal crop production. While micronutrients are needed for optimal plant growth, they may not need to be applied.
Essential micronutrients include boron, copper, iron, manganese, and zinc. For some plants, molybdenum, chloride, or nickel may be important. Elements like cobalt sodium, silicon, and selenium are also considered beneficial, as they may not impact plants directly but can affect processes in the soil that have an impact on plant growth and development.
Since plant requirements for micronutrients are small, most soils supply adequate amounts. Many micronutrients are metals, and their availability can be impacted by soil pH. Solubility and availability of some micronutrients decreases as pH increases. However, high pH soil does not necessarily mean that micronutrients need to be applied.
Crops vary in their need for certain micronutrients. Knowing which crops are sensitive to a specific micronutrient deficiency is key. The two micronutrients which are most likely to be deficient in Minnesota are iron and zinc.
Iron is most commonly deficient in soybean grown on soils high in lime. The deficiency of iron in soybean is commonly called iron deficiency chlorosis, and, interestingly, is not related to a deficiency of iron in the soil. Instead, it is a result of soybean’s poor ability to extract iron from the soil. Iron deficiency chlorosis can be corrected if specific chelated sources of iron are applied. However, the primary method to deal with iron deficiency chlorosis has been selecting a soybean variety which better tolerates high lime conditions.
Zinc deficiency is more likely to occur in corn and dry edible beans. Soil testing for zinc is the best option to determine where a response is likely. A response to zinc is possible when the DTPA zinc soil test is at 0.75 parts per million, but is more likely at 0.5 ppm or less. Broadcast application of dry zinc fertilizers can be expensive, so accurately assessing where a response will occur is the best course of action.
Broadcasting zinc can benefit crops and increase the zinc soil test, benefiting future crops. A common practice in Minnesota is to include a chelated zinc source when starter fertilizer is applied with the planter. While banding zinc is thought to be more efficient, the use of chelated zinc across all corn acres may not make economic sense.
For other micronutrients, there has been limited evidence of copper responses in small grains grown on organic soils in Minnesota. Corn, soybean, and sugarbeet response to boron has been researched but has not been shown to increase yield. Manganese has not been shown to increase corn or soybean yield even though glyphosate-tolerate soybean varieties have been shown to go through temporary deficiency of metals like manganese.
When thinking about micronutrients, remember that while “small but mighty” for plants, they are not always needed for crops. Ensuring that a micronutrient is deficient is crucial to increase both yield and profitability.
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Support for this project was provided in part by the Agricultural Fertilizer Research & Education Council (AFREC).
This article was first published in July 2019.
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