DEAR NEW FARM:
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?
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.
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.
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.
us with comments, suggestions and questions.