Seaweed Fertilizer – Does it Harm the Environment?

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Robert Pavlis

Seaweed fertilizer and seaweed extracts are becoming more popular but there are concerns that harvesting and manufacturing it harms the environment. In this post I will look at how seaweed makes it to market and determine if it is an eco-friendly process.

Seaweed Fertilizer - Does it Harm the Environment?
Seaweed Fertilizer – Does it Harm the Environment?, photo by Scripps Institute

What is Seaweed?

There are some 10,000 species of seaweed but only a few species are harvested for food, cosmetics, fertilizer and carrageenan products (ex agar). It is not a true plant, being more closely related to diatoms, algae and downy mildew, all belonging to the kingdom Chromista.

Kelp, one of the more popular harvested seaweeds, is photosynthetic and has a biochemistry that is very similar to land plants. It has a root-like structure called a holdfast that allows it to attach to things like the sea bed or man-made rope. Stem (stipe) and leaf-like structures (blades) grow off the roots. These blades are very plant-like and grow as much as 2 feet a day in ideal conditions. Kelp reproduces by microscopic spores not unlike those found on ferns. It grows naturally in large beds called forests which are located not far from the shoreline. Strong winds and waves knock off pieces which then float to shore.

Other seaweeds are commonly referred to as rockweed. They are similar to kelp but tend to grow more slowly.

Different types of commonly harvested seaweed, photo by Scottish Government
Different types of commonly harvested seaweed, photo by Scottish Government

Why is Seaweed Used as a Fertilizer?

Chemically, kelp is very similar to plants and is therefore a good source of organic matter for the garden. As it decomposes, it releases nutrients which plants can use. It is an attractive fertilizer for two reasons; it’s organic and it may contain biostimulants which are special substances that can enhance plant growth.

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I’ll review the efficacy of seaweed as a biostimulant in another post.

Seaweed extracts have an NPK of about 0-0-1 . Products with higher values are mixtures of other fertilizers and seaweed. This is quite a dilute fertilizer that sells for a relatively high price. It makes no sense buying the product for the nutrients. In its liquid form it doesn’t even add bulk organic matter to the soil.

If this product is worth buying it is for the biostimulants, not its nutrient content.

Harvesting Seaweed

Seaweed is obtained from three sources; beach collection, wild collection and farming.

Beach Collection

Beach collection can be profitable in areas with large kelp forests. Large amounts of material get washed on shore where it is easily collected. This is not a significant commercial source of seaweed but it can be a good way for gardeners to get cheap organic matter and collecting it has limited effect on the environment.

Wild Collection

Rockweeds are generally collected along shorelines and although wild collection only accounts for about 5% of the global seaweed production, there is now concern about over collecting in some areas. When collection is done in a sustainable way it will not harm native populations.

California is a good example of sustainable harvesting. They have tight regulations that harvesters must follow when harvesting their giant kelp beds.

Farmed Seaweed

The majority of harvested seaweed now comes from sustainable farming. This is done in one of two ways.

Natural kelp forests can be harvested so that they are not harmed. Boats are used to cut off the top portion of the kelp, leaving the lower section to continue growing. Because of its rapid growth this is a viable way to harvest. Kelp farmers are also managing and expanding the forests, reversing some of the damage that has been done by society.

Harvesting seaweed in Asia, photo by Jean-Marie Hullot
Harvesting seaweed in Asia, photo by Jean-Marie Hullot

A much more common way to farm seaweed is to grow it artificially. Young plants are attached to ropes and set in shallow water. Periodically, the farmer pulls up the ropes and cuts off the extra growth leaving the holdfasts attached to the rope, where they continue to grow. This form of farming is very popular in Asia where much of the production is used for food, but it is now being done globally, including North America. It is expected to become a vey large industry.

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How is Seaweed Fertilizer Made?

Harvested seaweed is normally washed and then either dried or treated wet. A variety of treatments are used including enzyme extraction, ultrasound, and various chemical extractions including water, acid or alkaline solutions. This is then filtered to remove particulate matter including a lot of the solid organic matter.

There are many processes for doing this and most are proprietary. This, along with the fact that the original raw material varies greatly, means that each product is different. Except for NPK values it is hard to compare one brand to another.

Is Harvesting Seaweed Eco-friendly?

Some sources have claimed that seaweed fertilizer should not be used because it is harming our natural seaweed populations. Although a small amount of fertilizer is produced from irresponsible harvesting, the majority is made from farmed sources where harvesting is eco-friendly.

What are the other impacts on the environment?

Seaweed is grown in salt water, requires no fresh water for growth, and requires no fertilizer input. In fact, it has been shown that farming seaweed actually reduces fertilizer runoff from land based farming, reducing nitrate and phosphate pollution in oceans. Growing seaweed allows us to collect these pollutants and return them to soil where they can grow crops. “Seaweed farms also create safe, healthy nursery grounds for young fish and crustaceans that improve wild population levels. ”

Since seaweed grows so fast, it also pulls a lot of CO2 out of the air/water and some feel it can have a significant effect on global warming. It may be much more effective than growing trees on land.

Processing seaweed does have an environmental impact. It needs to be washed to remove excess salt and this requires fresh water. A variety of chemicals are used to decompose the seaweed and release the nutrients. The most commonly produced product is a liquid that contains very low levels of nutrients. What this means is that we are shipping water around the country, and that is not eco-friendly. It is available as a powdered meal, which is much better for the environment.

In summary, the harvesting of the seaweed is eco-friendly but the production and shipping of the liquid form is not. However, the claim that using seaweed fertilizer is harming natural seaweed populations in unfounded in all but a few cases.

Should You Use Seaweed Fertilizer?

As a fertilizer I see absolutely no reason for using it. It contains almost no nutrients, it harms the environment by shipping water around the country and it’s the most expensive fertilizer you can buy. It’s even more costly than fish fertilizer.

Is it worth buying for the biostimulants? I’ll address that in the next post.

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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!

14 thoughts on “Seaweed Fertilizer – Does it Harm the Environment?”

  1. Actual seaweed has to have some kind of nutrients – I see plenty of plant material growing from the rotting stuff in the beaches around where I live. I bring home tubs full of the stuff that washes up onto the beach for my compost after storms. Wouldn’t it have as much nitrogen as other green materials?

    • This post is not about seaweed – it is about products sold as seaweed fertilizer. The NPK values I have given to you come right from the bottles of the products.

  2. Great article! Farming i should think wouldnt be seen as something not harmful as the qrea used for farming makes that part lacking the natural plants and life. Like a plantation of trees for lumber isn’t a forest and if before the plantation it was a firest it will impact the environment the flora and fauna.

    Many do like the idea of using seaweed for their plants. In the baltic seaweed contains lots of cadmium so not only have they low level of npk so you need lots to make an impact you will fast get above the recommended Cd levels. But in blogs and social media it’s often recommended as pure gold. The government don’t recommend to use it and has tested the effects in soil but who cares. In the olden days people did use “släke” seaweed that has been gathered on the shores so it must be great!

    I hope you will have a look on bat guano in the future and it’s ecological effect in comparison to what you gain from it

  3. “It contains almost no nutrients …” Is it not possible that seaweed and seaweed fertilizers are good sources of micronutrients? It seems to me that the adage “everything is everywhere” is more likely to be true in the ocean, where there is extensive mixing over large areas, than in the soil.

    • Micronutrients are usually very low compared to the macronutrients. If there are no macronutrients, there would be even less micronutrients.

      A source of micronutrients is not something promoted by manufacturers.

          • “Microscopic amounts are of little use to plants.” I’m not sure what microscopic means in this context, but surely it is in the nature of micronutrients that they are needed in very small quantities. Isn’t that what what micro means?

            In any event, the quotation and link in my previous comment was not intended to make a claim about any particular product or micronutrient; rather, it was in response to your statement that “micronutrients is not something promoted by manufacturers.” Apparently it is in at least one case.

            I’ll leave you in peace now. Thanks for your patience.

        • That may be – and it might not be.

          Lets say brown seaweed has more boron. How much boron is in commercial seaweed extract? Show me a product that lists it as an ingredient.

          Assuming such a product exists, now show me a field study (not a lab study) that provides evidence the higher boron from seaweed is of value.


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