I am a volunteer worker in Senegal, West Africa. The soil that we are raising millet and peanuts on is lacking in zinc. Since I want to respect appropriate technology, I am looking for an organic source of zinc to put on a hectare of land. Is there any?

James Hanes



We asked Dave Wilson, research agronomist for The Rodale Institute, for his help on this one. Some of Dave’s answer considers the fact that many of the farmers reading his response will also have to consider NOP standards. Here’s what Dave had to say:

Four different types of compounds are used as zinc fertilizers and these compounds vary considerably in their zinc content, their price and their effectiveness for crops. The four different sources are: inorganic salt compounds, synthetic chelates, organic complexes, and inorganic complexes.

Inorganic salt compounds include zinc oxide, zinc carbonate, zinc sulfate, zinc nitrate and zinc chloride. Of these, zinc sulfate is the most commonly used source and is available in both a crystalline form and a granular form, but the granular form has a low solubility and is not so effective immediately after application as the crystalline form. The fertilizer must be thoroughly incorporated so the plant roots will come in contact with the zinc.

Synthetic chelates like Zinc-EDTA are regarded as being the most effective sources of plant micronutrients. Zinc-EDTA is considered to be 2 to 5 times more effective than zinc sulfate but is also about 5 to 10 times more expensive. Disodium Zinc EDTA is the most commonly used chelated source of zinc and is more stable than Calcium EDTA. Other chelates such as zinc citrate may be cheaper than Zinc-EDTA but also less stable. These chelates can be applied to soils usually in a band or used as a foliar spray. They are mobile in soils and will move with the soil water to the plant roots.

Organic complexes are usually less expensive than the synthetic chelates, such as Zinc-EDTA, but they are also less effective. These sources need to be placed in the root zone to assure root-zinc contact.

The Inorganic Complex used most often as a source of zinc is ammoniated zinc sulfate solution (also a source of nitrogen and sulfur). This is often combined with ammonium polyphosphate as a starter fertilizer. Ammoniated zinc chloride solution is another inorganic complex fertilizer. These sources need to be placed in the root zone to assure root-zinc contact.

Livestock manures and composts should not be overlooked as zinc sources if these resources are available. Application of good quantities of organic manures (farmyard manure, compost, green manure and green-leaf manure) usually supplies the required micronutrients to crops. Compost teas made from these manures and compost may be tested for Zinc concentration and possibly could be a source of zinc used as a foliar spray.

According to the OMRI Listing for USDA Certified Organic, there are 18 acceptable zinc sources in organic production (see www.omri.org/omri_datatable.htm). Depending on the speed of your computer, it may take several seconds for the database table to appear. At the top of this table, click on “crop products” in the dropdown window marked “All items”. Type in “zinc” in the window to the right of the dropdown window, hit the flashlight icon to the right of that window, and the database will come up with all the zinc products that are listed.

All of these products listed on the OMRI list have an "R" for "Restricted” status. Typically if you can show to your certifier, with documentation—such as soil-test analysis results or plant-tissue analysis results—that you have a "zinc" deficiency in your crops because of low levels in your soil, then the certifier can authorize the use of one of these products. But the emphasis is on "certifier approval" and documentation.

When a soil test indicates the need for Zinc, typically small amounts are needed for optimum yield. Plant analysis, however, should be used in combination with soil testing before arriving at firm recommendations as tissue analysis is an effective evaluation of a plant's zinc nutritional status. Zinc deficiencies are usually associated with concentrations of less than 20 ppm of zinc in the dry matter.



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