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Zinc fertilizer materials can differ greatly in their chemical and physical properties. They are sold as liquids, dry granules or powders. Zinc fertilizers can be classified as either organic or inorganic.
Organic zinc fertilizers consist of zinc ions combined with complex organic (i.e., carbon containing) molecules. Inorganic zinc sources do not contain these complex carbon molecules in their structure.
Dry zinc materials are sold either as granules or powders. Granules are mixed with dry bulk blends for direct soil application. Powders are usually blended with liquid or suspension fertilizers, but may be used as a coating for granular N-P-K fertilizers.
Zinc sulfate (ZnSO4) is probably the most widely used zinc fertilizer material. It is effective, economical, and easy to apply. It is frequently manufactured from spent sulfuric acid that is a byproduct of industrial galvanizing processes. The zinc content of zinc sulfate ranges from 18% to 36%. It is highly water soluble and available in both granular or powder form. It can be mixed with UAN solutions, polyphosphates, or suspensions.
Zinc oxide (ZnO) is a low-cost zinc source usually sold as a dry powder, but sometimes as a granular material. It is insoluble in water, but is relatively soluble in both polyphosphates and in fertilizer solutions containing free ammonia. Zinc oxide is often used in blended liquid suspension fertilizers, but requires careful mixing. It is not often used in dry bulk blends, because it requires good distribution and soil incorporation to be effective.
Zinc chloride (ZnCl2), zinc nitrate (Zn(NO3)2), and zinc carbonate (ZnCO3) are dry materials. They may be used with liquid fertilizers and suspensions because of their high solubility.
Zinc frits are silicate materials called "fritted glasses" that are slightly soluble in water. Zinc content varies from 10% to 30%.
Inorganic zinc materials will dissolve in liquid polyphosphate fertilizers (e.g., 10-34-0). Each inorganic zinc material has its own solubility limit in polyphosphates. The different solubilities can result in actual zinc content ranging from less than 1% to more than 2% in the final fertilizer mixture.
Inorganic zinc materials are virtually insoluble in liquid orthophosphate fertilizers (e.g., 8-24-0 or 9-18-9).
Liquid zinc fertilizers are made by dissolving an inorganic zinc material in water or other liquid. The resulting liquid solution permits easier handling when adding zinc to other liquid fertilizers.
Zinc chloride or zinc sulfate may be dissolved in water or UAN solution. Zinc content of the resulting liquids can range from about 5% to over 40%. These materials will behave exactly like dry zinc chloride or zinc sulfate.
Zinc nitrate can also be mixed with water or UAN solution. Zinc content can range from 12% to 17%.
Ammoniated zinc or zinc-ammonium complex is a liquid fertilizer made by combining zinc chloride or zinc sulfate in sequence with ammonia and water. Zinc content of the final product can range from 9% to 20% with 10% to 16% nitrogen.
Ammoniated zinc can be safely mixed with polyphosphate fertilizers at a final analysis of 0.5% to 1.1% actual zinc. The general rule-of-thumb is that 1 gallon of 10% ammoniated zinc (10% Zn) can be mixed with 7 gallons of polyphosphate fertilizer (e.g., 10-34-0).
This 1:7 ratio results in a stable mixture that will not separate under normal conditions. This ratio must be adjusted for 15% or 20% ammoniated zinc sources according to the product label to avoid settling out.
Polyphosphate molecules have the ability to “sequester” the zinc ions and keep them in solution. Ammoniated zinc will settle out in fertilizers that are 100% orthophosphate, typically clear liquid products. Problems will also occur in poor quality polyphosphates that have a high percentage of orthophosphate molecules.
Organic zinc materials can be divided into synthetic chelates and natural organic complexes. Organics can normally be mixed with orthophosphate liquids, polyphosphate fluids, nitrogen solutions, and suspensions. Some organics may require constant agitation to prevent them from settling out of the fertilizer blend.
Organics are available as dry powders with zinc content ranging from 6% to 15%. Dry powders must be mixed carefully with liquid fertilizers to prevent segregation and "balling" of the powder.
Liquid organic zinc fertilizers are often preferred over powders because of easier handling and mixing properties. Zinc content of liquid sources typically range from 5% to 10%.
Synthetic chelates are complex molecules that include EDTA, NTA, EDDHA, and HEDTA EDTA is the most common chelate used for zinc fertilizers. Zinc contents range from 6% to 13%
Synthetic chelates are considered "premium" zinc source. and are often the most expensive source per pound of actual zinc applied. The main advantage is mobility and stability across a wide range of soil pH.
Organic complexes are manufactured from naturally- occurring, organic materials that have chelating properties, but are not considered “true” chelates. These include lignosulfonates, phenols, and polyflavinoids.
These materials are typically obtained from tree bark and other byproducts of the wood industry. Sucrose-type materials, like cane sugar molasses, are used for some products (sucrates).
The final fertilizer product results from reacting an inorganic zinc source with the material. It may be marketed as either a dry or liquid source. Zinc content is variable.
Organic zinc complexes are usually less expensive than synthetic chelates. Fertilizer products may be labelled as "fully chelated", likely meaning that the complexed materials are mixed with synthetic chelates.
There are fertilizer materials that are mixtures of synthetic zinc chelates, organic zinc complexes, and inorganic zinc materials. The label will usually give specific mixing and blending instructions.