DIY Seaweed Fertilizer

The internet seems to have turned us into a society of hacks. It offers the secrets of how to do anything – including DIY seaweed fertilizer. Not living anywhere near the ocean, it never occurred to me that this was possible. Like most gardeners, mine comes out of a jar, instantly melting in water. But the other day, someone asked us about adding seaweed to the DIY fish fertilizer recipe on a post from last month. Guess he was so excited about the fertilizer project that he overlooked the ingredients not including any seaweed. But after pointing that out I got to wondering… so how do you make your own seaweed fertilizer?

To be water soluble, it would have to be an extract, otherwise you’d have seaweed meal. The top result on Google search for how to make seaweed extract leads you to believe that you can make it from any kind of seaweed. And you can do it in less than half an hour!

Not seaweed fertilizer; it's extra weak seaweed tea!

Just soak whatever kind you find offered in the produce section of your grocery store in hot water for 20 minutes. You’ll know it’s done when the water colors. Dilute it 1:5 in water, and you’re good to go. Well, they’re great at SEO, but not making fertilizer. There’s a lot more to coaxing the good stuff out of those leaves and stems than steeping it in hot water for such a brief time. A weak tea at best, and probably lacking as the food thickener the author implies it offers. This is like applying the liquid left from steaming or boiling green vegetables to your plants. Green bean water is a viable free fertilizer, but cutting it by 5 parts makes no sense. It’s already diluted and won’t burn plants… as long as you let it cool down.

If this is all it takes to create an excellent plant nutrient from seaweed, then why do you suppose the companies that do the extraction spend millions of dollars to build a processing plant? Even compost or worm tea needs to brew for at least 24 hours to be a garden benefit.

Thinking that the 60 trace elements, numerous natural growth hormones, multiple vitamins, and disease control properties of large seaweed (aka kelp) can be transferred to heated tap water in 20 minutes flat without any other agent involved is uninformed at best. More beneficial than plain water, sure, but by how much? And the initial heat can harm any microbes left on that seaweed sanitized for human consumption. Kelp does contain every nutrient that your land plants would ever need, but it’s going to take more than this to release it from the biomass for instant access by your crop. Seaweeds are some of the most powerful plants on this planet, but this isn’t the way to capture their awesome benefits.

A lot of people make fermented seaweed tea. They call it seaweed extraction, but not one of them is pressing the good stuff out of the collected aquatic vegetation. To pull the oils and juices out of leaves, nuts, and what have you – requires chemistry or cold pressing, which is squeezing it out under heavy pressure. From what I gather, there are only 4 facilities equipped to do this worldwide, and the cost to build one is incredible. Here’s one guy’s attempt using commonly available machinery:

So, instead of actually extracting the wonderful essences of seaweed, the crafty do it yourself processes the whole thing. This method is to store harvested and rinsed seaweed into a tote tub or garbage can. Cover it with water, put the lid on loosely, and let it steep for 2-3 weeks. It’s done when the chunks melt.

If all you want is some free natural fertilizer, I suppose this will give you a source of plant nutrition that has extra properties like micronutrients, auxins (growth hormones), disease resistance, and some microbial activity. But technically, this popular stewed seaweed practice gives you something more like a botanical tea compost. But it’s anaerobic, left to sit for periods of time that are cause for worry over toxic microbes populating. The longest an aerobic compost tea should brew is 48 hours because of the dangers involved with the lack of oxygen. Knowing that, I would be pretty hesitant to apply this stuff to my veggies – even if I did live in the perfect isolated spot to gather pristine dead seaweed on my stretch of waterfront.

There are also some that lactose ferment either fresh seaweed or kelp meal. However, according to organic guru Lumperdawgz of marijuana grower forum fame:

“Taking kelp meal (or fresh kelp which is a really bad idea, BTW) and adding that to water with lactobacillus (like EM-1 or Carandang’s BIM method) you’ll end up with fermented kelp which is not seaweed/kelp extract by any definition. Not even close as far as it relates to polysaccharides, secondary metabolites, enzymes, et al.”

Are those missing things important to what you’re trying to grow? That’s up to you to discover and decide. The thing is, not everything you can make at home will give you the same benefits as a product commercially produced from the same initial ingredient. And when it comes to your plants, particularly those that provide you with food, sometimes it’s better to pay a few dollars to get safe, reliable results. Dry seaweed extract isn’t as expensive as you might think. The scary sounding price tag of about $25 for 8 ounces of soluble powder makes over 140 gallons. That’s like 17 cents per gallon. How is that not cheap? And it comes in an easily stashed compact container. The product also has a really long shelf life. But that backyard stew in a tub will go bad in about 3 months, and how will you all those gallons of extra liquid?

Commercial seaweed extraction is done through a variety of methods, and what you’re after as a fertilizer, trace elements, or growth regulator in the finished product dictates the process used. In some cases, the actual species of seaweed is important too. Agricultural or horticultural seaweed products are created by high pressure water extraction, solvent extraction, microwave-assisted extraction, and supercritical CO2 extraction.

Lower priced soluble seaweed fertilizer is processed with hydrolysis using potassium hydroxide or sodium hydroxide solvents, and sometimes with the addition of heat. These are dark brown to black powders and liquids, while the pricier clear version is cold-pressed. So, why pay more? The heat and hydrolysis cause some loss of beneficial things in the seaweed that remain when cold-pressed. Meanwhile kelp meal, though slow to break down, is likely missing nothing and has added benefits.

Another thing to consider is that different types of seaweed have different crop benefits. If you visit the FAO link at the bottom of the page, you’ll find some good info on that. Then you’ll begin to see why those who manufacture horticultural seaweed products bother to name what kind of seaweed the stuff in the jar or bag came from. It matters. It gives you different benefits, and can indicate whether it’s an all purpose nutrient, or best used for growing leaves, or flowers and fruits.

And finally, just collecting brown seaweed when it washes onto shore isn’t always the wisest thing to do. This isn’t 1939. Everything that runs off the land winds up in the ocean, especially if the collecting spot is near a tributary outlet. In that case, your seaweed has been taking in everything that ran off farmland and cities from watershed start to final dumping point… in the ocean. Probably not the kind of thing you want to incorporate into your soil, or spray on food crops. If you’re going to harvest beached seaweed for garden uses, you need to travel to an uninhabited area that’s a long ways from a river flowing into ocean waters.

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  • Marc de Ruijter says:

    Hi Tammy,
    Your article was a bit of an eye-opener to me as far as the seaweed tea is concerned. Ofcourse not all nutriënts are in the liquid after a few hours or days!

    However, i disagree strongly with your answer to Ctace, above. First, most experts i have read do not see salinity as a problem at all, and if you are concerned about salinity: just rinse it off.
    But more importantly, if you just add the seaweed to your garden as a mulch: after a year or so it will be composted! And this has the added benefit of a slow-release fertilizer which is exactly what you want and you can never overdose on that.

    May the force grow your plants.

  • maggie says:

    What time frame are you able to harvest seaweed?

    is biofouling a problem?

  • Xena says:

    I’ve got a friend who makes a seaweed/kelp tea at home in a bucket with an air stone and used it in his vegetable garden as well as on his cannabis plants and Judging from what I see of his plants I’m a believer that his method is working well enough for me to try it.

  • Joshua Harrison says:

    Seems like the author is paid to write articles to promote petrochemical fertilizers instead of organic diy gardening. Definitely not buying what shes trying to sell.

    • Tammy Clayton says:

      Thanks for stopping by, Josh. Perhaps you should read the article again tho – because it’s not promoting any petrochemical fertilizers at all. It’s about seaweed as a fertilizer and why you need to a) collect it from safe sources, and b) do more than steep it in water for a few minutes to a few days to harness the good stuff inside the plant.

      Personally, if I lived where access to kelp is easy, I’d collect it and dry it. Then pound it into powder and mix it into the soil so all it’s lovely ingredients were available to my plants. Adding it to the compost pile would also take place.

  • Ctace says:

    What about putting seaweed in the ground and soil on top and plant the vegetables or flowers?

    • Tammy Clayton says:

      It seems to me that this approach might raise your soil salinity – which is not a good thing for many garden plants. Plus it’s not composted.


Tammy Clayton

Contributing Writer at Garden Culture Magazine

Tammy has been immersed in the world of plants and growing since her first job as an assistant weeder at the tender age of 8. Heavily influenced by a former life as a landscape designer and nursery owner, she swears good looking plants follow her home.