Seaweed Fertilizer Benefits – Are They Real?

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Robert Pavlis

The use of seaweed fertilizer is becoming more popular, but does it really work?

It is a very dilute, expensive fertilizer so it does provide plants with some nutrients, but the claims for it go well beyond that. Seaweed also contains a number of other substances called biostimulants which are claimed to help plants grow, ward off disease and produce higher crop yields. The biostimulants in seaweed fertilizer might be more valuable than the NPK nutrients.

Given the high cost of seaweed extract, it’s important to know if these biostimulants really work.

Seaweed Fertilizer Benefits - Are They Real?
Seaweed Fertilizer Benefits – Are They Real?

What are Seaweed Extracts?

I have described the process of harvesting and processing seaweed in another post called, Seaweed Fertilizer – Does it Harm the Environment?

There are a couple of key points that have to be understood for the current discussion.

  1. There are many kinds of seaweed and any discussion about biostimulants needs to take into account the specific species being used. Not only that, but a given species can have quite different properties depending on where it is harvested.
  2. There are numerous processes used to convert seaweed into extracts and most are proprietary. Without knowing the process it is difficult to know which biostimulants are present in the final product. Manufacturers lead you to believe that their product contains them all.

What is a Biostimulant?

The European Biostimulants Industry Council describes them as “Substances and/or microorganisms whose function when applied to plants or the rhizosphere is to stimulate natural processes to benefit nutrient uptake, nutrient use efficiency, tolerance to abiotic stress, and/or crop quality, independently of its nutrient content.”

For the purposes of this post they are chemicals, excluding basic nutrients, that make plants grow better.

Plants naturally make a wide range of substances that help it grow. It follows that we should be able to process a plant, extract these biostimulants, and apply them to other plants in order to make them grow better. Seaweed is not really a plant, but it has a very similar biochemistry to plants and therefore it is reasonable to assume that extracts from it will act like biostimulants and make other plants grow better.

Growing Great Tomaotes, by Robert Pavlis

Applying Biostimulants to Plants

There are two general ways to apply biostimulants to plants; foliar spray and soil drench. Each application technique can have different effects on plants. Substances applied to leaves may not actually enter the leaves or they may enter the leaves in concentrations that are too small to be effective. Biostimulants applied to soil, have to make it to the rhizosphere without being degraded by microbes, and then they have to be able to enter the roots and be transported to other parts of the plant.

Claims by Commercial Products?

Commercial products and some garden writers make all kinds of claims and I could easily list 50 benefits including “slug repellency”. Many of these claims are made using flawed logic that goes something like this.

Plants make biostimulants that help them grow.

Therefore, applying extracted seaweed biostimulants must also make plants grow.

There is no doubt that biostimulants exist and that they help the producing plant grow better. What is much less clear is the effect these have once extracted and applied to another plant.

The commercial products I looked at rarely provide any proof that their product works, or if they do, they refer to some studies about basic biochemistry within a plant and use the above flawed logic. None provided evidence that applying seaweed extract will improve plant growth.

Since the general public is still not very aware of biostimulants, many of the products on the market are sold as a fertilizer, not a biostimulant. Quite a few products are made from other types of fertilizer with a pinch of seaweed added, mostly for marketing purposes. Who can resist a product containing organic seaweed?

What Does the Science Say?

A lot of research has been done on seaweed extracts. Much of it is lab based which is a good way to understand biochemical processes but not a good way to confirm the benefit of real world applications. Other work compares the application of seaweed extracts to the application of water as a control. Clearly seaweed does contain nutrients, so a comparison with water only proves that fertilizer enhances plant growth. However, even if we exclude these types of research there is still significant evidence that biostimulants affect plant growth.

Soil Science for Gardeners book by Robert Pavlis

A field experiment applied two types of kelp to beans and found a 24% increase in harvest. An extract of phytohormones (cytokinin, auxin and gibberellin) also increased yield, but not as much as the seaweed extract.

A foliar application of seaweed extract known to have cytokinin activity produced a significant increase in the yield of potatoes.

Seaweed extracts suppressed clubroot in broccoli (Plasmodiophora brassicae) grown in Australia. It was also found to be effective against white blister disease on broccoli leaves, and a fungal pathogen (Sclerotinia minor) in a variety of crops.

A 2009 review of the subject concluded that, “numerous studies have revealed a wide range of beneficial effects of seaweed extract applications on plants, such as early seed germination and establishment, improved crop performance and yield, elevated resistance to biotic and abiotic stress, and enhanced postharvest shelf-life of perishable products. However, the mechanism(s) of actions of seaweed extract-elicited physiological responses are  largely unknown.’

A more recent review (2019), that specifically looked at rockweed (Ascophyllum nodosum), reached similar conclusions. “The existing evidence for A. nodosum extracts as biostimulants in agriculture is promising.” They also point out that we still have many questions that need answers before seaweed extracts can be productively used in agriculture.

  • What is the optimal rate of application?
  • When should it be applied, and at what intervals?
  • How are results affected by the type of crop and climate?
  • Which extraction methods are best and are there better ways to extract the biostimulants?

There is a significant body of evidence to support the use of seaweed extract, but to date the actual mode of action is largely unknown, although we are getting better insights.

Heavy Metal Concerns with Seaweed

All plants absorb heavy metals from the soil and that is why it’s important for gardeners to limit the amount of heavy metals they add to the garden. These are far more toxic than most synthetic chemicals, in part because they don’t decompose.

Crops grown in compost made from seaweed collected on beaches, compared to soil, did not show increased levels of lead, copper or mercury, but did show elevated levels of cadmium which were above EC limits. Keep in mind that growing in compost would increase heavy metal levels well above that received by an occasional foliar feed.

Washington State has strict policies about testing fertilizer (including biostimulants) for heavy metals and reports that after 22 years, “with rare exceptions, these products pass standards by a wide margin.”

Commercial seaweed fertilizers contain between 10 and 50 mg/Kg. “”With the application rates suggested by the manufacturers, the application of these fertilizers is 2 orders of magnitude lower than the maximum permissible sewage sludge load for arsenic.”

Any plant based material you add to the garden contains heavy metals, including compost, manure, and wood chips, as do synthetic fertilizers. Seaweed does not seem to be a cause for concern, unless you are collecting large amounts of it.

What Does All This Mean for the Gardener?

If you are trying to fertilize plants, don’t use seaweed products. They are expensive and provide very low levels of nutrients. If your soil needs nutrients buy fertilizer.

Seaweed extracts are made in a sustainable way, in most cases, although shipping the liquid form all over the country is not. If you buy it, at least buy a solid form.

Does it work as a Biostimulant? There is a lot of evidence that “it” does, but as gardeners, we are left with a significant problem. Given the many sources of seaweed and the variety of manufacturing processes, we have no idea which product actually works. Just because it is labeled “seaweed” does not mean the product does anything in the garden. For all you know the important biostimulant your plant needs has been eliminated during manufacturing or the product used the wrong seaweed.

Seaweed is not one product – it’s many different kinds of products and so far manufacturers tell us nothing about their product. And even if they did, the science has not progressed far enough to help us pick the right product.

There are many claims for seaweed; increased growth, reduced effects from environmental stress, reduced pests and diseases and even faster seed germination. However, none of these benefits can be tied to a particular product.

What about testing seaweed extracts on ornamental plants? There is almost none, and remember that with ornamental plants, better growth is not always a good thing. I don’t want my evergreens to grow faster!

Does it improve soil? Some studies do show an improvement in soil properties, but almost any organic matter will do the same thing, and at a much reduced price. At the end of the day, soil improvement depends on the quantity of added organic matter and seaweed products add very little. I would not use them to improve soil.

The only place that might warrant the use of seaweed extract is the vegetable garden. But given all of the unknowns and no way to properly select a good product, I’d pass, until we have better science.

If you do want to give it a try, at least take the time to run controls.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

13 thoughts on “Seaweed Fertilizer Benefits – Are They Real?”

  1. Hello, I use the organic material I have in my surroundings, I live near the sea so it makes sense to put a small amount of sea weed into the compost pile and water plants occasionally with sea water, however I wouldn’t if I didn’t live by the sea.

  2. I had a veggie garden for years. Every year was a learning experience. I started using seaweed fertilizer and each year my garden improved. Never did I have transplant shock using seaweed. Plants were healthier and fruit was plenty! I AM a true believer in seaweed fertilizer.

    • I consider myself an organic gardener, or a better term would be “environment gardener”.

      I don’t follow stupid rules just because an organization says we should. But I do as little harm to the environment as possible.

  3. I am a farmer in the Willamette Valley in Oregon. Conventional farmers who grow green beans for processing use kelp extract applied with the seed at planting time and a kelp based cytokinin product just before flowering to boost blossom set and yields. These cheap farmers would not spend the cash on it if it didn’t work. Here is a link to a kelp extract manufacturer’s research on fruits, veggies and forage/grain. I use both the kelp extract and the cytokinin on many veggie crops and they work well. They are not fertilizer. Happy reading

    • The link goes to a company selling these products, and on that site I see NO scientific studies to support anything.

      Marketing works because people believe it – use of a product is no evidence the product works.

  4. Here in the UK it is mostly marketed as a source of micronutrients and trace elements (Ca, Mg, Fe, Mn, Zn, Cu, B) which are often lacking in other fertilizers. But these would be present in most soils, maybe with the exception of Ca and Mg in acidic sandy soils.
    Many people give it to (pot grown) tomatoes, some say it makes them sweeter or that it helps prevent blossom end rot (although tomato fertilizers usually have Ca and Mg in them).
    I like to use it for roses, to be sure they have everything they need to fight diseases.
    I also give it to permanently potted plants occasionally, to supplement nutrients that can be lacking in their regular fertlizer. Hard to say if they need it – they are in a loam-based compost, so not a completely soil-less medium.

  5. In Australia Seaweed is widely recommended on gardening TV shows as a plant tonic & they state it is not a fertiliser.

    Claims are made its good for:
    Improved cell wall growth.
    Prior to transplanting to get over shock
    After transplanting to get over shock
    Newly sown seeds
    Planting out cuttings
    Planting out seedling

    Whether any of these claims are baseless I don’t have a clue

    Myself I put a cap full together with a cap full of liquid fertiliser in a 9 litre watering can for my outdoor root bound pot plants

  6. I’ve had minor success with Maxicrop seaweed powder. It produces quite a bit of liquid, and since I only have a small garden, one container lasts for years.
    I’ve done some side by side comparisons over the years, giving seaweed to some plants but not others, and the yields on my San Marzano tomatoes are larger, the fruit slightly bigger, with seaweed.
    So is that the growth stimulants, or just the extra potassium? Hard to say.
    But I’m sticking with it.

  7. I have never found seaweed to improve the health and growth of my plants. I have found that diluted urine works well, and it’s free.

    • Paul, I am great proponent of urine in the garden & in compost as per my comments on other topics. Its such a waste to flush urine down the lavatory!


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