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
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
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.
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
According to the OMRI Listing for USDA Certified Organic,
there are 18 acceptable zinc sources in organic production
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.