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Worm Compost from Kelp

January 11, 2009

We are trying to promote worm composting in Africa, as few people here can afford commercial fertilizer, for school gardens etc. We are busy setting up a web site to discuss cheap DIY ways of composting and could use suggestions.

Has anyone ever tried worm farming with kelp or other types of sea weed?

We have a plentiful supply of kelp  literally tons thrown up along our coasts  and I once read that ancient Romans improved soil fertility by digging in sea weed. However I am worried that the sea salt in the kelp - could it be detrimental to the worms, or otherwise produce compost that is too salty for garden plants? If you know the answer, or could put us in touch with someone who might, we would be very grateful.

Cape Town

Here is a link that might be useful: http://www.working-worms.com

Comments (43)

  • s.s.tupperware

    My worms love sea weed but its freshwater, salt will need to be rinsed first...

  • african

    I don't think freshwater water weed is relevant - we are talking about Kelp which is a large sea water plant. Obviously we would hose down the kelp with fresh water toi get rid of surface salt - but does the salt permeate the body of the kelp itself and damage the worms, when eaten?

    Here is a link that might be useful: Working worms

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  • 11otis

    Hello Cape Town,

    every now and then I buy the concentrated liquid sea weed (from the sea, not fresh water) to be used as fertilizer. I have no idea how it is made but there must be solid waste left; and less salt in it if any.
    So, if you can combine these 2 projects, it's a double dipping. Good luck.


  • african

    Thanks Otis11,
    Yes that was how I got the idea in the first place. In my country you can buy a very expensive product, made from kelp, called "Kelpac" which is used in the garden in small quantities, usually as a foliar feed. Its purpose is to add a range of "trace elements" that may be lacking in the soil. This then promotes healthy plant growth and improves productivity. My thinking was exactly in line with you, to use the worms to do the work for me, but I was still a bit apprehensive of having too much raw sea salt in the kelp itself. I guess that I'll just have to get started and give it a try, unless someone already has the answer.
    Cape Town

    Here is a link that might be useful: Working worms

  • 11otis

    Cape Town:

    Go for it! You won't have to wait long to see if the worms can take the stuff (I don'tknow what to call it).
    I think it is not whether they can "eat" it or not but rather if there is any salt residue, it will irritate the worms' skin.
    Also I imagine, if there is salt residue, it will influence the time how long the microbes need to process it until it is ready for the worms.

    I guess, just plop a few worms into a bowl with that mass. Poor worms!


  • 11otis

    Cape Town,
    PS: I use the liquid sea weed to spray the foilage of my orchids and sometimes water them with it. And my other "precious" flower plants.
    If I am growing from seed, once they germinate, I spray them with the sea weed also. I have been growing lilium from seed only since last fall and they just started to germinate. So far no damping off. Knock on wood!


  • leearnold

    I would say to start a smaller vermicompost bin separate from your main bin and try the kelp. That way, if it IS something that the worms don't like, or if it kills them; you won't put your whole worm composting operation at risk.
    Alternately, you could compost the kelp in a "traditional" pile then feed the compost to the worms after it has set and been rained on a time or two. I'm not sure how much rain you get where you are.

  • african

    I have decided to proceed on a trial basis with crushed up kelp as a feed for a small worm farm, but have been advised that kelp is very high in iodine, which may not be good for plants (or humans. Any comments?
    Cape Town

    Here is a link that might be useful: Working Worms

  • pyropunk

    Howzit fellow "Souff Effricen"

    Since nobody seems to have an answer, the only thing you can do is try.
    What I would to is:
    1) Create a small setup separate from your main worm bin.
    2) Rinse and crush sea-weed before feeding.
    3) If the worms eat it, harvest the resulting VC and feed it to a variety of plants. In this way you can test which plants like the kelp VC and which ones don't.
    4) Should the worms not like the kelp, try to compost it first using a normal (or even a hot) compost setup. Proceed with steps 1) - 3)
    5) If the worms still don't like pre-composted kelp the only thing I can think of is to cook it first.
    6) If that doesn't work, ...I'm all out of ideas... :-(

    Please report back on your experiment. Maybe someone else can make use of the results.


  • 11otis

    IMO, if you are going to feed the worms with the seaweed "pulp" after extracting all the good stuff for the liquid seaweed fertilizer, there will be less iodine in the pulp.

    I imagine the extracting process is similar to getting coconut milk/cream out of a coconut where water is also used to "flush" the good stuff out of the nut. There might still be a small % of everything left in it (in the pulp)even with very efficient machinery. I believe the coconut pulp is used as cattle feed.
    I am NOT a scientist, this is just my observation.

    If you don't mind, I would like to know if your experiment works, just to satisfy my curiosity.
    You can e-mail me.


  • captmatt

    I have had the same thought... Plants in general don't like salt, even plants that live in salt water. So most have mechanisms to get rid of excess salt. I also see tons of organic living material washed up on the shores of the Gulf of Mexico. I am assuming the plants have a reduced salt content than the 3% or so of sea water so I will be gathering up some of the local red sea kelp/weed and washing it down, letting it dry a bit and testing it on a few hundred worms. I'll hope to post some results in a few months.

  • african

    Thanks to all for the useful advice - but no Otis11, I think it would defeat the object of the exercise to extract the juice and then just use the remaining pulp - I want the worms to process all of the trace elements present in the kelp. I was just curious about the iodine - not really concerned. I'll take my chances.

    I've just acquired a small 3 bin set up for the kelp worm farm and will keep you posted on progress.

    Here is a link that might be useful: Working Worms

  • udargin

    I buy kelp meal to feed to my worms, it is not processed; just dried, ground, and packaged. I then rehydrate and feed it to the worms. You would also do very well if you just composted the kelp as it is an excellent organic fertiliser; or dry it in the sun and grind it (dried kelp is approx 1-0-2)
    75% water insoluable nitrogen, 25% water soluble nitrogen and Potash as well as many trace minerals. Salt levels should be fine after rinsing.
    Good Luck!

  • floridamg

    In the Aran Isles, two islands in the irish sea off the coast of Ireland, there is almost no natural soil, and the inhabitants have mixed kelp, which washes upon the beach, along with sand to make soil. I don't feed my worms kelp, because there are plenty of food scraps to feed them, but I would think that they would like it. Also put sand in the food as the worms need the grit.

  • african

    Thanks for the suggestion Mike, but grinding up dried kelp, is a tough proposition. With our hot summers, the sun dried kelp on our beaches is tougher than old leather. I'll take my chances with the fresh stuff, which is easy to mush up with a mallet.
    If the formula is 1, 0, 2 then it has no phosphate, which is bad news for me, as our soils are particularly poor in phosphates, so I'll have to add some bone meal, which is organic - I wonder if its best to feed bone meal to the worms, or just add it to the vermicasts afterwards, when I harvest the compost?.

    Here is a link that might be useful: Working Worms

  • african

    Yes, floridamg, I now see that kelp compost is actually used all over the world and is highly valued as being high in soil nutriments and trace elements.

    I have been in touch with some local people who use it in its raw form, for tree planting. They don't even compost it first or wash it to get off the surface salt. They just put a layer of soil between the tree roots and the chopped up kelp. They say the trees grew like mad.

    I'm sure that kelp vermicompost will be a winner - If the worms prosper on the kelp - there's plenty of it and its free.

    Cape Town

  • pyropunk


    Did you start your experiment yet or were you just researching so far?


  • gardenfanatic2003

    I don't know that salt from the ocean is sodium. If people are using it to amend their soil, I doubt if it's sodium, because that's detrimental to plants (and worms). What I'd be most concerned about with bedding a worm bin with seaweed would be that the seaweed will heat the bin up. I'd rinse it first and use in small quantities.

    Perhaps the best use of this wonderful free resource that you have in large quantities would be to use it as a green source for outdoor compost piles. That way you would get large quantities of compost with all the benefits of the nutrients in seaweed.


  • tclynx

    Kelp meal is very useful. Especially when harvested from relatively pollution free waters. They use kelp meal as a feed additive for livestock. They make sea weed extracts for fertilizing and spraying on plants. Many people simply collect it and use it directly in the garden without even washing it first. All depends on how salt sensitive the particular plants are, do remember that almost all commercial fertilizers are salts of some form or other. Sodium Chloride is what most of us think of as table salt and is the primary salt in sea salt though not the only element of course.

    How much the iodine would affect things is all related to dosage. Iodine might affect the bacteria needed to make a worm bin work well but if the bin is already healthy and active, it might not take so long for the bacteria to manage to start breaking down the kelp.

    You don't have to wait for the kelp to be turned into worm castings to get some benefit from it. It would probably make a good mulch in the gardens as well as something good to add to compost bins and a good soil amendment for the gardens.

    Good Luck and eager to hear how your worms like the kelp!

  • african

    Hi pyropunk,

    Hoe gaan dit? I have just started putting kelp into a small bin that was previously being fed on general waste. A bit early to make anything of it, as there is still fair bit of old food mixed with the bedding. I'll let you know

    Cape Town

    Here is a link that might be useful: working worms

  • african

    I have had the experiment with kelp going for just about a month. Sad to report, things don't look too good. The worms are not prospering - I would judge that there are only half as many as I started with and the survivors seem sluggish and half dead. No babies.

    With worms, it is not always easy to be absolutely sure of the true cause , when you get a loss of population - but I would say that the kelp is the prime suspect at this stage.

    In the interests of science, I'm going to carry on for a while longer.

    Here is a link that might be useful: Working Worms

  • 11otis

    I hope the readers won't mind if I post my opinion/comment to Cape Town's posting. I won't be able to back it up with quotes or links from anywhere. It will be just my personal opinion.

    From my own experience and reading posts from other people, it appears that worms experience some shock when they are moved to a different environment. Some times the new home has a similar environment like the one they came from, then they would adjust fine. If not, many of the adults will die off and hopefully leaving cocoons before they do so. The babies that hatched in the new home will adept to the environment since they wouldn't know any other. So Cape Town, don't give up on them.

    Would it help if you "pre"-compost your kelp before you add it to your bin? That way you know for sure the kelp has the microbes. And once the bin is in full swing you can eliminate that step. Does the kelp decompose at all?
    BTW, how big/small (you did mention "small")is this bin?

    Good luck.

  • cathd66

    I'm no expert (although Irish, so maybe kelp as a fertiliser is in my genes!) but my instinct is that kelp should probably be pre-composted before feeding to worms. All seaweeds are typically high in protein- (nitrogen) and would therefore benefit from hot composting to make maximum use of available nutrients. I know I wouldn't feed raw grass clippings to my worms for fear of protein overload. In a small system you'd also get overheating- though you might get away with it in a large windrow for example.
    The link I've given below is one of the best references I've ever found on the subject of large scale vermicomposting, and one of its systems might be adaptable to your kelp.
    I was thinking particularly that if you made a pile with worms and more conventional foodstuffs, then extended it into a 'windrow' (read the manual!) by adding kelp to one side every few days, the kelp would compost and the worms would move into it at their own rate. (you might need to mix it with some kind of 'brown' material like cardboard/ paper)

    Here is a link that might be useful: vermiculture manual

  • pyropunk


    that is a good document you found! Almost as good as WemG ;-)


  • organicpepper_grower

    I have a huge pile of sea weed, it has been spread out all winter, I put it all in one big pile and now its time to compost it but I was think the same thing to feed it to my worms. Where it has been spread all winter would there be much salt content left?

  • kansastropic

    I'd say never feed kelp even if it was rinsed because it absorbs salt. I know it because I was going to feed it to my worms and they avoided it. It left me curious so I tasted it. I didn't get sick but it was mildly salty. Just remember this a worm's body is as tender as the inside of your mouth. They can eat some plants that are poisonous to humans such as amaryllis leaves and the red berried elderberries. (I've tasted a small quantity of them too.)

  • organicpepper_grower

    I'll just compost it then, i dont want to kill my worms

  • african

    Thanks for the advice Otis11 and cathd66. I have come round to believing that pre-composting is probably a very sensible route to follow and I'm going to set up a stack.

    Update: The worms seem livelier on their kelp diet and there are a number of babies making their appearance now, so I am more optimistic.

  • snoggerboy

    african - I am getting some dried Kelp shipped to Johannesburg and will try some in my worms. If you want to break down a bit fast then try to freeze and thaw it a few times. It'll be soft quickly.

  • wormy_acres

    'african': Any update on how it has gone the past 3 months since your last post?

    I'm visiting relatives in California right now, and saw huge amounts of kelp washed up on the beach yesterday. I tasted a bit (just curious), and it didn't seem excessively salty. Most plants and animals maintain a similar salt balance whether they're from saltwater or freshwater environments -- fish from the ocean don't taste saltier than those from freshwater. The advantage of saltwater is that nearly all trace minerals are readily available. Should be some great worm castings if it works out.

  • newbie101lt1

    i added some kelp to my Nite crawlers bin today. These guys are in dirt for your info and inside. I will let you know how things go with the kelp and them. Only have about 50 of them

  • snoggerboy

    Been soaking Kelp, keep it with some compost to age then feed to worms together then use it in water or in the bottom of planting holes.

  • alabamanicole

    This is an old post, but a fantastic topic.

    Kelp is high in sodium as well as iodine. Sodium is not bad for humans or plants -- in fact it's necessary -- but an excess of sodium is. I would have some concern about using a LOT of kelp in soils, but only because it might damage the plants and require additional irrigation so that plants can maintain their proper osmotic state. I would not be concerned about excess sodium in the fruit. The uptake of sodium into the fruit is pretty low. That might be an issue for a person with hypertension, but the key factor in excess dietary sodium is large quantities of sodium in processed food. In short, it's a rich western disease. People who can't afford fertilizer are not living off frozen dinners and potato chips.

    Iodine, on the other hand, is much harder to get in excess and a deficiency of it can be very serious. That's one of the reasons why it is added to table salt in the United States: folks far from the ocean weren't getting enough iodine. It sounds like the OP's region doesn't have this problem, but historically goiters (the most obvious symptom of iodine deficiency) were also a widespread problem in non-coastal Africa.

    I would not be concerned about adding iodine to the soil if it were in the form of raw kelp. If the soil is overworked, it is likely to be depleted of iodine anyway.

    That said, my gut instinct would be to do as the Romans, Irish and other cultures did and incorporate kelp directly into the soil. These cultures were well aware of composting, and chose not to do it for some reason. My guess would be that there may be some additional benefit to the slow breakdown of kelp. Or it could simply be economies of space and labor and not a fertility increase.

    If the kelp needs to travel inland, then it may be more efficient to compost it near the coast and then transport the finished compost. It might be a good business model for an entrepreneur with almost no start-up costs involved. Given how ridiculously renewable kelp is, this compost would likely be much cheaper than imported petroleum-based fertilizers.

    I love my worms, but I don't see any obvious benefit to feeding them the kelp. Let the natural soil microbes break down the kelp, and let the worms re-process the fruiting plants grown in kelp.

    It would be a good subject for a trial -- raw incorporation versus kelp compost versus kelp vermicompost. I would include soil testing yearly to monitor the levels of sodium in the soil to be sure none of the methods contribute to long-term reduction in soil fertility.

  • antoniab

    My brother-in-law gardens in Alaska, on an island in Prince William Sound. I asked him about this. He uses seaweed that he gathers after a rain from the beach directly on the garden as a mulch, though he will often gather much more than he can use and pile it to decompose and use later to add to the soil, since it is too expensive to ship in soil, and there is almost none on the island itself. The kids on the island bring him worms they find or have left over from fishing and he trades them worms for strawberries in the summer, so his greenhouse has lots of worm activity, though no actual worm bed. He says he does not see that the worms are harmed at all by the fresh seaweed, but rather sees them crawling into it straight away after he dumps bait worms into it, rather than into the soil. They seem to prefer the seaweed.
    So. this is second-hand, and not at all scientific.

  • african

    To follow an experiment of using kelp for worm composting, check out the blog below.

    Here is a link that might be useful: Worming With Kelp

  • Joyousfree

    Thanks for posting that link! It's great to see what you are doing. I hope you continue to keep us posted.

  • african

    After several months with this experiment, the only honest conclusion is that Kelp and Worms do not go well together in the composting process.

    See the working worms website blog diary for the details of this experiment.

    The final observation was -

    "I now believe that raw kelp is not suited to be combined with worm farming. However, for those living near the coast, kelp will always remain a very useful, free and easily collected resource, that is of great nutritional value for your garden plants and is moreover an important provider of trace elements for promoting rapid healthy plant growth. Just keep the composting processes separate and then you can always combine the mature kelp and worm products later, when ready for planting or mulching".

    Here is a link that might be useful: Worming with Kelp

  • pskvorc

    First; excellent thread, and second; THANKS for following up here!

    Based on what I continue to sift from 'between the lines', I am coming to the conclusion that there are basically three Eisenia fetida "foods" - horse manure, "melon", and "mold". Regardless of what one starts with OTHER THAN horse manure and melons, "it" has to be broken down either through fermentation or "molding", for it to be available to worms for food. THEREFORE, is it reasonable to "pre-mold" kelp to "prepare" it for use as worm food?

    I offer this question in the form of suggestion only because I appreciate your desire to use a readily available natural resource. Of course I understand that "pre-molding" the kelp adds a level of complexity to the over-all system you are trying to develop, but a readily available, inexpensive source of "food" for the critter you want to culture is a BIG deal. What happens to kelp when it is put in a SEALED, PLASTIC container - bag or "bin"? Does is get "moldy"? If so, were it I, I would try some "moldy" kelp.


  • mendopete

    Several year back I tried feeding kelp to my worms. I was given a pick-up truck load of bull kelp after a storm. I rinsed and chopped about 25lbs for the worms, the rest went directly into the garden.
    I tried pocket feeding my bin, but the worms stayed away. It eventually "melted" away, with no worms below...
    The remainder of the chopped kelp went into a plastic bucket which was sealed. 4 months later I opened it to add some to my tea. It was the most horrid smelling nasty mess I have ever created. I could not add it to my bin and dumped it far from the house. The worms may have liked it if given the chance, but I did not want to smell THAT smell ever again.

    Good luck and happy wormin'


  • PRO

    Oh my. I agree with everything in the pskvorc post.

    "First; excellent thread, and second; THANKS for following up here!" Yes.

    My first thought was to rinse the seaweed. But mendopete did rinse and it was a no go.

    In a sealed container what it sounds like you are talking about is bkashi composting. This uses a starter.

  • Chad Currin

    Everything about vermicomposting is about finding balance. Too much kelp would be too much green material or "green manure" not only would the salt be part of the issue but lacking the other elements it takes for a happy, balanced worm bin. Needs of each worm species are different too, but likely you can use some kelp but you would also need lots of things like dried leaves, shredded paper and plenty of air exchange. Green plants produce both heat and moisture as they break down... this could cause a gas buildup or overly wet conditions. Alternately, make the vermicompost and then use the kelp to provide trace minerals and microbe food for a nice brewed vermicompost tea fertilizer.

  • Tony Serhal

    send me some dried kelp please for it is the best thing for your soil? little salt will not harm the worms. sea salt is not going to be a big issue and you can wash your kelp if you want . best of luck

  • Sean (Zone 9a, The Netherlands)

    Well these topics never die! :) I have a stacked vermicomposter and yesterday, I just chopped up some seaweed that I had collected and rinsed. We’ll see how they get on but I’m optimistic: back in the UK, I’d used seaweed before in the garden and the worms went crazy for it! People at the allotment there told me that laying down seaweed means that if you disturb the soil underneath, you’ll see it teaming with worms and I proved it to be true. In the worm tower, I also have a lot of brown material (cardboard and coconut coir) to balance it out. My worms used to die regularly and I then discovered that the cause was overheating- I had no idea how hot it gets in there, particularly as I only now have a terrace and the sun and internal heat were causing a massacre. :( Since then, I’ve learned!

    I’ll add my own update in a few weeks.

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