Seaweed: Fact or Fancy?
by Erika Jensen
This article was first printed in the May - June 2004 issue of the Organic Broadcaster, published by the Midwest Organic and Sustainable Education Service.
If you're like
me, you look through catalogs every year at about this time, ordering
supplies. You look at things like seaweed products, humic acids,
Brix meters, and so on, and wonder how well they work. There are
a lot of ways to spend your money as a farmer, and with a limited
amount of capital, there can only be so much experimentation.
my research, seaweed products really do work, although as with any
product, you must use it correctly. In researching this article,
I was surprised at how difficult it was to get information on the
subject. The scientific studies are certainly out there, but the
only books on seaweed in agriculture are at least fifteen years
old, and not available in public libraries. The brief mentions of
kelp in gardening books like The Seed Starters Handbook aren't proof
enough to inspire one to give up several evenings in a busy schedule
to do foliar spraying. There seems to be a serious gap between the
scientific literature and the information that is disseminated to
organic farmers. Information from product vendors, although helpful,
also needs to be backed up by unbiased sources of information. Well,
here's a good start on what you need to know, available in an easy-to-digest
Although there are some inconsistencies, most of the scientific
studies I read (about a dozen, and there are lots more) find that
seaweed does actually do the things that you see advertised in catalogs.
Kelp does seem like a worthwhile purchase. The use of seaweed as
a growth stimulator is widely supported by scientific studies. There
is also some evidence to support the idea that kelp is useful in
helping plants through times of stress, including drought, disease,
and cold weather. However, very few of the scientific studies I
read involved field trials. Most were conducted in very controlled
environments in greenhouses, where the plants could be closely watched
and monitored. This doesn't come close to duplicating field conditions
on organic farms, so actual results may differ from the research
findings. Of course, kelp applications can't be a substitute for
managing soils properly-but it can provide a boost to your plants.
One body of useful information that has come out of the scientific
research is information on how kelp works. Nobody has figured out
precisely how it happens, though they have some good ideas. Kelp
has some nutrients in it-a typical analysis of kelp meal will run
about 1.5-.5-2.5, for example. It is also high in micronutrients,
such as iron, copper, zinc, molybdenum, boron, manganese, and cobalt.
But what really
does the job are kelp's plant hormones, which include auxins, gibberellins
and cytokinins. These hormones, when applied either to the soil
or as a foliar spray, cause the treated plant to undergo changes
of various kinds, including root stimulation, thicker and stronger
stems, an increase in vegetative growth, and increase in fruit or
seed yield. The hormones also seem to protect the plant in times
of stress, such as from disease and insects, cold weather and drought.
All three hormones protect against senescence (plant aging and death)
by promoting the functions and structural integrity of plant cells.
Cytokinins are primarily responsible for promoting cell divisions,
while gibberellins are involved in seed germination. Auxins are
produced in the growing tip of the plant, the apical meristem, and
control the plant's growth patterns, including cell elongation in
the stem and tips of the roots. The chemical name for naturally
occurring auxins (as from plants) is indole-3-acetic acid, or IAA.
The effects of these plant hormones are complicated and not completely
Kelp meal has
the added virtue of containing alginic acid (about 26% of dry weight),
which is useful as a soil conditioner. It improves the water holding
capacity of the soil and helps with the formation of crumb structure.
This may assist plants with drought tolerance. Part of the benefit
may be associated with the stimulation of bacteria, which produce
their own exudates, which further condition the soil.
There are many different kinds of kelp, although only a few are
available as commercial preparations. Of these, the most readily
available is Ascophyllum nodosum, also called "Norwegian Sea
Kelp"; others I've seen are California Bull Kelp and Ecklonia
maxima. As Bill Wolf of Thorvin Kelp pointed out, there are many
many different kinds of kelp. "If you walked into a restaurant,
you wouldn't ask for 'land plant salad' would you? And yet people
talk about seaweed as if it were one kind of plant." Point
well taken, Bill. As much as I have read about different kinds of
kelp, as far as I know nobody has ever compared the effects of different
kinds of seaweed. It makes sense that they would vary in effect,
at least slightly. Kelp afficinados may want to try different kinds
to see if one or another has a more beneficial effect on a particular
crop-or you might want to choose the kind of kelp that has been
noted to have an effect on a particular crop.
Seaweed has been shown to increase yield for such vegetable crops
as potatoes, tomatoes, peppers and fruits like grapes, and grains
such as wheat and barley.
In a study done with potatoes in 1976, potato yields increased about
13% for one variety, King Edward, when the potatoes were sprayed
at the rate of 1 gallon per acre. In a study done on chard with
Ecklonia maxima, total plant yield (roots and leaves) increased
by more than 111% over the control. Interestingly, the leaves from
the sprayed plants had an increase in chlorophyll over the control.
on tomatoes, peppers and cucumbers with Ecklonia maxima were performed
by applying the seaweed in two ways: as a foliar spray, and as a
root drench prior to transplanting. In all the studies, the plants
with the highest yield were the ones that received both treatments.
For example, when tomato plants received a .4% seaweed spray and
a root drench, they showed a 28% increase in total weight of fruit
harvested; tomatoes also came in slightly earlier and about 10%
larger. In another study done on tomato roots, roots treated with
seaweed were more than twice as extensive as the control. In studies
done in Australia, (most likely with Ecklonia maxima) seaweed applied
as a foliar spray or in the soil increased root growth in wheat,
sunflowers, beans, corn, peas and grasses.
In a study done by Acadian Sea Plants, Limited (a product vendor)
with the species Ascophyllum nodosum two varieties of grapes were
treated with a foliar spray at critical times such as pre-bud break,
pre-bloom, post-bloom and sizing stages. At the most optimal concentration,
there was a 25% yield increase over the control, and also an increase
in size and firmness. However, Brix levels tended to be slightly
lower in treated grapes.
A trial in 1973
in the United Kingdom examined the effect of seaweed sprays on plum
trees. The extract significantly increased fruit set. How significantly
depended on variety-from 40% to 86% increase. Similar trials with
similar results were performed on cherries, apricots and peaches.
In a study done in 1986 with Ecklonia maxima, wheat was sprayed
three times (during the early stages of plant growth) and seeds
were soaked in a seaweed and water solution before planting. Treated
plants were more robust and had darker green leaves than the control,
and senesced more slowly. Perhaps because of this, kernel yield
increased significantly, accompanied by an increase in vegetative
matter. In a similar study done with barley, a 1:500 dilution (the
optimum concentration) produced a 60% increase over the control.
In another study done on wheat, the stem diameter more than doubled
over the control. This can mean less lodging and more straw for
small grains farmers.
effects of kelp
and Pest Resistance
Kelp has also been found to increase pest and disease resistance,
perhaps due to the changes that it effects on the plants. In a study
done in 1961, TL Senn found that seaweed sprays will reduce the
severity of powdery mildew on cantaloupe. He also mentions in his
book Seaweed and Plant Growth, that studies done with red spider
mites showed a 40% reduction in insect population. This may be due
to the chelated metals that seaweed contains, or because the gibberellins
in seaweed suppress red spider mite reproduction. W.A. Stephenson,
in his book Seaweed in Agriculture and Horticulture, reports that
seaweed reduces the incidence of botrytis in strawberries and damping-off
on lettuce seedlings.
Again because of plant hormones, seaweed has been shown to have
effects on the shelf life of fruits. In a study done in North Carolina
on peaches using A. nodosum, trees were given 2lbs of kelp meal
per tree applied in the soil around the tree, and sprayed with a
foliar spray every 4 weeks until harvest. The treated peaches showed
a 50% reduction in the number of fruit that rotted over a 21-day
period. Although I wasn't able to find information on other fruits
or vegetables, there might be similar results for other crops.
TL Senn did experiments at Clemson on frost tolerance with tomato
plants. Untreated and treated tomato plants were exposed to temperatures
of 29 degrees. The control plants were all killed and the treated
plants all survived. Again, it seems likely that plant growth regulators
have something to do with the effect, and possibly also the micronutrients
provided by seaweed.
If you're like me, the testimonial of another organic farmer can
make all the difference in the world in deciding whether or not
to use a product. In my quest for information, I talked to several
organic farmers who had interesting stories to tell about kelp.
For the most part, these farmer testimonials back up the scientific
Farm, Durand, Wisconsin
A dairy producer who markets his organic milk through the Organic
Choice LLC, Sam has had excellent results spraying seaweed extract
on his hay fields. The product he uses is the species A. nodosum,
purchased through Acadian Sea Plants Limited. Sam has an old co-op
truck sprayer, which he uses to spray his hay at the rate of 20
gallons to the acre. He sprays in the evening, from 7-10 pm, and
he can cover about 75 acres in a night (he has about 250 acres total
of hay ground). He sprays twice, once when the hay is about 6-8"
tall and again one week later. This usually occurs at the end of
May, or about one week before cutting. "I've had excellent
results with my hay," reports Sam, noting a 4-6" difference
in the height of the hay. Although he hasn't done formal trials,
he notices the difference in the headland areas where the sprayer
misses the crops.
Another benefit he's observed is that the sprayed crops seem to
have much less leafhopper damage. The kelp spray also seems to bring
the sugars up in the hay, and it seems quite palatable to his cattle.
Edge Farms, Nelson, Wisconsin
Greg is a dairy farmer who loves seaweed so much that he sprays
everything he grows, including corn, winter wheat, succotash (a
mix of oats, barley and wheat), and hay. He uses a seaweed extract
powder, purchased through Acadian Sea Kelp. Although the price is
right, the powder is difficult to mix up, and becomes slimy. He
mixes about 15 pounds of dry powder into his 500-gallon tank. Unlike
some other growers, his goal is to spray both the soil and the plant,
with the goal of using the spray as a soil amendment as well as
a foliar feed. Along with the seaweed, he adds organic sugar, calcium
hydroxide and sometimes boron, as needed.
Greg has been using seaweed for the past two years and is pleased
with the results. "It seems to give the plants a good boost.
They look better when I use the kelp. It also seems to keep the
weeds in check, particularly velvetleaf." He points to the
trace minerals as the main source of benefits to the soil microbiology
and plants. He went on to mention that 2002 was his best corn year
ever; his yield averaged about 196 bushels/acre. Although 2003 was
a dry year, he still did surprisingly well, around 100 bushels/acre.
The forages that he grows seem to be "extra high in energy"
and he seems to get more milk from the cows. Can seaweed really
help with weed control? Although it's possible, it's not an effect
that has been noted by other growers, and I don't know of any scientific
studies that would support the information. If you try it on your
own farm, experiment on a small plot first!
Haven Farm Belleville, Wisconsin.
Judy Hageman uses a kelp spray on her hoophouse tomatoes and flowers.
Previously, she used a combination of fish and kelp, but it attracted
raccoons and Judy found that "they will just destroy anything
to try to get at the fish smell." She uses 1 tablespoon per
gallon for watering seedlings, and a more concentrated solution
of about 1 tablespoon per quart (plus some compost tea) at transplanting
time. Afterwards, the tomato plants are foliar fed with the more
diluted solution (1 T per gallon). Ideally, Judy would like to spray
about once per week, but sometimes because of time constraints she
sprays every two weeks. The flowers get the same treatment as the
tomatoes, except that they do not receive the foliar feeding. She
uses seaweed extract purchased through Jung's for about $25 per
gallon. Since it is locally available and she doesn't have to pay
shipping, that's still a bargain.
difference it makes," says Judy. Plants are stronger, greener,
and less susceptible to disease and aphid damage. "Since we
have been using seaweed we haven't had any problems with white fly,
either." Last year they ran out of seaweed, and so didn't fertilize
all of their tomato plants, in effect doing an informal trial. "The
seedlings that didn't get the seaweed didn't transplant well, and
their color was not as green. Even though we sprayed later with
seaweed, the plants never caught up with the other ones." Results
are also good with the flowers. Judy has been raising very healthy
flower transplants for many years. Although she has never used floral
preservative, her flowers last an exceptionally long time.
to use seaweed in production
Application rate varies considerably depending on which species
of seaweed you are working with, what you are spraying and which
brand of product you purchase. Just like with various kinds of fertilizers,
there is an optimum application rate. Either too much or too little
can cause problems. Too little, and you won't see a response to
the product; too much can induce a toxic effect. The manufacturer's
information will perhaps be your best guide, but I would encourage
farmers to do a little experimenting on their own. If your product
manufacturer recommends the same rate for all vegetables, this may
not be enough information about the optimum rate. Also, different
varieties may respond differently to seaweed applications. At the
very least, keep your eyes open and notice what is going on-better
yet, do an informal trial and keep notes. If you don't have time
for it, delegate the project to an intern or employee.
If you are choosing
to apply kelp to the soil, the most cost-effective method will be
in-row application. Dry kelp fertilizers can be applied at planting
time either by mixing it with seed (as in a grain drill) or by running
the fertilizer through the insecticide box on your planter.
As with other kinds of foliar feeding, time of day makes a difference.
During the evening and early morning, stomata in the plant's leaves
are open, and the plant is more receptive to foliar feeds. If you
are not able, for whatever reason, to set aside some time in the
early morning or evening for spraying, foliar feeding is probably
not for you.
Timing the spray
to particular events in the plant's growth cycle is crucial. Many
sources I read recommended spraying at times of high plant stress
or plant need. These times include germination, transplanting, pre-bud
formation, bud formation, and immediately before frost. When I talked
to the folks at Peaceful Valley Farm Supply, they recommended spraying
every ten days to two weeks. However, this may be too time-consuming
for many farmers. In some trials, different varieties sprayed on
the same day yielded different results. This may be because the
varieties had different maturity dates, and so the plants were at
different stages of growth when they were sprayed. If you have three
or four different crops on your farm, your life remains fairly simple-but
if, for example, you grow several different kinds of peppers in
the same bed, you may need to spray them all at different times
to get the absolute best results. This may be simply impractical
for many farmers.
to choose a product
Of the seaweeds available, A. nodosum is by far the most readily
available. You will have more choices between products, production
methods, and it will be easier to find an organic product if you
use A. nodosum. However, so much interesting research has been done
on E. maxima that I admit I'm tempted to try it.
Cost varies significantly depending on what product you are purchasing.
Depending on your scale of production, your other inputs and gross
per acre, this may or may not be the primary factor in choosing
a manufacturer. For larger acreage, it makes sense to look for seaweed
sold in larger amounts, because the manufacturer will give you a
discount for purchasing in bulk. Application rate is a major factor
in analyzing the cost, and this rate varies considerably from product
Not all seaweed products are organic. Apparently the organic status
depends on non-active ingredients and production and processing
methods. Organic producers will need to check with their certifying
agencies to make sure the products they use are acceptable.
There isn't enough information out there about how production methods
affect the quality of the product. As far as I can tell, no comparative
studies have been done on production or processing methods. The
manufacturers of some products, such as Kelpak, use a cell-burst
process that does not use any heat and supposedly retains more of
the plant hormones. Although it makes sense that the cell-burst
process would be better, this is not supported by any scientific
data that I've seen.
Is seaweed worth it? I think so, and I plan to use it in my own
garden for the first time this year. I have tried fish with seaweed
products before, but not pure seaweed extract. Although I probably
won't be able to do a scientific study, I plan to do an informal
trial with a control and keep track of the results at least mentally,
if not on paper. If there are other farmers out there who are interested
in seaweed, I'd encourage you to try it and send your results to
the Organic Broadcaster so that we can learn from each other's experiences.
We need more information about how seaweed works on organic farms,
so that we can better help each other.
is a farmer and freelance writer living in Waupun, Wisconsin.
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