Micronutrient fertilizers can be either inorganic or organic materials. The inorganic materials can be further divided into water-soluble and non-soluble compounds. Organic materials are either synthetic chelate or natural organic complexes. Chelates are ring-type chemical structures formed around a polyvalent metal. Chelated micronutrients are of interest because they tend to remain soluble longer when applied to the soil, giving time for the plant to take up the desired nutrient. Chelating agents are complex chemical structures; and, usually, the initials of the compound are attached to the fertilizer material. Examples of these agents are EDTA, DTPA, EDDHA, NTA, and HEDTA. Only the more commonly used products will be discussed.
Zinc sulfate (36 percent zinc, 14 percent sulfur) is the most commonly used dry zinc material. It is a relatively water-soluble inorganic compound and is effective in the granular form. It is often applied to soil areas that are low in zinc to raise the soil zinc levels.
Zinc oxide (50-80 percent zinc) and zinc carbonate (52-56 percent zinc) are inorganic compounds and very insoluble in water. They are not effective zinc sources if granulated, but are effective when finely ground. Therefore, they can be used when fertilizer materials are manufactured. Also, they can be added to ammonium polyphosphate solutions or suspensions if equipment is available to handle these materials.
Zinc ammonium sulfate (several products available) is commonly used in fluid fertilizers. It is effective if properly applied in the soil (band application) and is less costly than chelate forms of zinc.
Zinc chelate (Zn-EDTA most common) is an effective material. The main advantage is stability and mobility in the soil. When dry fertilizers are blended and applied as a row band, inclusion of zinc as a granular zinc chelate, because of its mobility, is likely to be more effective than granular zinc sulfate. When fluid fertilizers are used, a zinc chelate does not perform much differently than inorganic sources of zinc. Zinc chelate is too expensive to use at rates needed to increase zinc levels in the soil.
Organic non-chelate zinc (several products) is often a byproduct of the wood industry. Most products are effective; and, usually, their performance is similar to zinc sulfate.
Ferrous sulfate (20 percent iron, 18.8 percent sulfur) is the most commonly used inorganic iron source. It is not effective as a soil-applied material because it quickly reverts to unavailable forms. However, ferrous sulfate can be used as a foliar spray with some success. One spray application of a 1-1.5 percent solution may correct mild iron chlorosis. Several applications one to two weeks apart may be needed for more severe chlorosis.
Fe-EDDHA chelate (6 percent iron) is the most stable iron chelate. Its main use is soil application (placed with the seed) since it is effective even in high pH soils. Product cost is the main factor limiting widespread use.
Fe-DTPA chelate (10 percent iron) is the most commonly used iron chelate. It can be soil-applied if the soil pH does not exceed 7.5. It also is used as a foliar treatment. Although effective, its cost limits use on field crops.