Fertilizers
Micronutrient Fertilizers
The micronutrients are removed from soil by crops in relatively small amounts. Therefore, deficiency of micronutrients iron, manganese, zinc, copper, boron, and molybdenum is not as common as those of nitrogen, phosphorus, and potassium. Sources of micronutrients are classified as inorganic or synthetic chelates. Micronutrient sources vary considerably in physical state, chemical reactivity, cost, and availability to plants. Soil application is the most common method of micronutrient application for crops. NPK fertilizers are commonly used as carriers of micronutrients.
Chelates
Some micronutrients are “chelated” in which a metal nutrient ion is encircled by the larger organic molecule to neutralize it electrically in order to increase its availability. Chelated fertilizer stability is desired because it means the chelated micronutrient will remain in a bioavailable form for a much longer time period, thus increasing micronutrient use efficiency in crop production. Chelates of zinc, manganese, iron, and copper have become standard products for foliar application in agriculture. Chelating agents for various micronutrient metals include EDTA, DTPA, HEEDTA, and EDDHA. EDTA is the most common synthetic chelating agent and is used for both soil and foliar applied nutrients.
Micronutrient versus Macronutrient Fertilizers
Micronutrient fertilizers differ from managing macronutrients, such as nitrogen. First, the solubility and plant availability of micronutrients is affected by media pH to a much greater extent than is macronutrient solubility. Second, the difference between acceptable concentrations of micronutrients and concentrations that are either too low (deficiencies) or too high (toxicities) is small compared to a broader range for macronutrients.
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