Zinc is considered to be one of the micronutrients for plant nutrition. It is not used in the quantity to be known as a primary, or even a secondary plant nutrient, yet it is still a vital nutrient for the healthy development of a wide variety of crops.
When a deficiency of zinc appears, there are symptoms that first appear on the new growth and leaves. Interveinal chlorosis ranging from a light yellow-green striping to a whitish band shows on newer leaves, and then with severe deficiency the new leaves may be nearly white with shortening of internodes. This usually results in stunted plants. Zinc deficiencies can usually be prevented with the application of zinc fertilizers.
There are a number of industrial by-products containing zinc which are being processed and sold as zinc fertilizers, such as zinc oxide, zinc sulfate, and zinc oxysulfates (containing a mixture of both zinc oxide and zinc sulfate). However, all zinc sources are not the same in terms of quality and zinc availability. The level of water-soluble zinc in these fertilizers is mainly related to the relative contents of zinc sulfate and zinc oxide. Zinc fertilizers should be in a water soluble form to be fully effective for crops.
Drs. Westfall, Amrani, and Peterson, Colorado State University, recently conducted a test for "zinc plant availability as influenced by zinc fertilizer sources and zinc water solubility." They found that the highest dry matter production was obtained with 99.9% water-soluble zinc ZnSO4. Their regression correlation showed that the higher the water solubility of the zinc sources, the more effective the Zn fertilizer was in increasing dry matter yields.
As part of their conclusion of this study, they suggested that granular Zn fertilizers should have water-soluble zinc levels of at least 50% to be effective in supplying sufficient zinc levels for the growing crop. The total amount of zinc in a fertilizer (as is presently reported) is not enough information to determine if it will meet the zinc requirements for a crop. Farmers need to receive information about the solubility of the zinc in the granular Zn fertilizers to know if the fertilizer will be effective in preventing zinc deficiencies.
Many of the zinc granular fertilizers that are black, brown, or dark gray in color tend to be zinc oxysulfate products that may contain very low levels of water-soluble zinc. Even in some of my own experiences, I have failed to see zinc responses after applying these darker granular products. It is true, some of these products do contain more than 40% water-soluble zinc and the crops do respond. That is why it is important to inquire about the water solubility of the zinc product, before having it applied to your field.
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