Horticulturist Joseph Albano (right) measures chlorophyll levels in leaves of marigolds treated with the chelating agent EDDS, and technician Chris Lasser measures iron levels in iron chelate samples. Photo by Stephen Ausmus.
require certain elements for normal growth, including small amounts of
micronutrients like iron, manganese, copper, and zinc. Fertilizers that
provide these micronutrients often include certain synthetically
produced organic compounds known as “chelating agents,” raising concerns
about whether their runoff into waterways increases levels of heavy
Agricultural Research Service scientist with the U.S. Horticultural
Research Laboratory (USHRL), in Fort Pierce, Florida, is testing a
relatively new, natural, and “greener” chelating agent as an alternative
to the synthetic ones.
make sure plants take up sufficient quantities of the micronutrients
they need, growers use fertilizers formulated with chelating agents.
These compounds bind with the micronutrients so they are available in
the root zone. “Chelating agents also allow growers to maintain
concentrated fertilizer stock solutions that contain soluble
micronutrients. Growers can dilute these solutions and inject nutrients
into their irrigation water,” says horticulturalist Joseph Albano.
effectiveness of a chelating agent to supply micronutrients to the
plant is a balancing act that depends on soil or potting mix pH level.
If the pH is too high, the metals become insoluble and are unavailable
to the plant, even with the use of chelating agents, and this leads to
nutrient-deficiency disorders. If pH is too low, metals become soluble,
and in combination with chelating agents, they may become toxic.
Marigolds, vinca, impatiens, and geraniums are particularly susceptible
to nutrient disorders caused by iron and manganese toxicity, referred to
as “bronze speckle” or “micronutrient toxicity syndrome.” Symptoms of
the syndrome can vary, but leaves of affected plants often develop
chlorotic (yellowing) and necrotic (dead tissue) speckling that can
progress to total leaf necrosis. The development of this disorder
greatly decreases the value of ornamental plants.
and DTPA are the most common chelating agents used in formulating
soluble fertilizers for floral and nursery crop production because they
can “deliver” micronutrients within the recommended pH range for growing
most of these crops. But they pose problems. When complexed with iron
and exposed to sunlight, they degrade quickly, so growers need to store
chelate-containing fertilizers in opaque containers. A more pressing
concern, though, is that they are not readily biodegradable and persist
in the environment for some time. They can also extract metals from
sediments, and their use is believed to add to the amounts of iron and
other heavy metals in waterways. EDTA is a particularly popular
chelating agent and is used in many industrial processes and municipal
products in addition to horticultural fertilizers. But concerns about
its use in Europe have prompted calls there for the use of alternatives
thinks he has found a “green” alternative for U.S. growers. Known as
“EDDS,” the chelating agent is just as effective as EDTA and DTPA at
delivering micronutrients to plant roots. But EDDS is biodegradable,
meaning that it will not persist in the environment and thus is less
likely to take up and transport heavy metals in soils and waterways.
EDDS is “gaining favor in Europe because it degrades in weeks as opposed
to months,” Albano says.
a series of greenhouse studies, Albano grew marigolds for 47 days in
containers filled with standard soil-less potting media. He used
fertilizers formulated with EDDS, EDTA, or DTPA. Each of the three
treatments was chelated with iron so that Albano could assess how much
iron the marigolds were taking up, their overall health and growth
patterns, and how quickly the iron-chelates degraded when exposed to
results showed that iron-EDDS degraded more quickly when exposed to
light, but that there were no significant differences in plant health,
growth patterns, or in the micronutrient levels found in plant tissue.
The report, published in HortScience,
was the first peer-reviewed study to evaluate EDDS as a chelating agent
in fertilizers used on a horticultural crop, Albano says. He has also
coauthored a chapter on chelating agents in a book on best management
practices for container-grown horticultural crops. The work is expected
to encourage use of EDDS as an environmentally friendly chelating agent
in formulating fertilizers used in the production of floral and nursery
Source: U.S. Dept. of Agriculture