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.

According to 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 package.

Seaweed Science
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.

How Kelp Works
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 understood.

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.

Kinds of Kelp
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.

Growth stimulation responses
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.

Vegetables
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.

Studies done 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.

Fruits
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.

Grains
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.

Other effects of kelp

Disease 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.

Shelf Life
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.

Cold Tolerance
In 1961, 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.

Farmer Testimonials:
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 evidence.

Sam Danzinger-Danzinger 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.

Greg Hetrick-Village 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!

Judy Hageman-Snug 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.

"What a 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.

How to use seaweed in production

Application rate
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.

Timing
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.

How to choose a product

Type of seaweed
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
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 to product.

Organic Status
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.

Production Methods
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.

Erika Jensen is a farmer and freelance writer living in Waupun, Wisconsin.