Liquid Seaweed Fertilizer – Most Popular Organic Fertilizer

Liquid Seaweed Fertilizer


Canadians can get it here

Seaweed for plants has been used by gardeners and farmers for thousands of years.

You can collect it from the beach and lay it right on your garden beds as a DIY seaweed fertilizer and mulch that quickly breaks down, supplying dozens of nutrients to the soil.

I used to do that when I lived on the west coast, but now that I’m far away, I’m using a liquid seaweed made from one of the most common seaweeds: kelp.

Actually, it doesn’t contain enough NPK to be considered a fertilizer, so it’s often referred to as a seaweed plant food. But we don’t use it for NPK, as you’ll learn below.

Here’s a video:

Kelp is an excellent soil amendment, but when we don’t have any around, a fertilizer from the kelp still brings many of the same benefits.

And a big advantage of the liquids over the solids is that we can spray them onto plant leaves to feed the arboreal food web and for direct leaf uptake.

When you do that, the plants and microbes get a nice shot of nutrition as well as the natural plant growth hormones that seaweed has become known for.

Benefits Of Liquid Seaweed Fertilizer

Big Watermelon With Liquid Kelp Fertilizer
These record-breaking watermelon were grown with my liquid seaweed fertiliser and liquid fish.

Here are some of the benefits of fertilizing with seaweed, from a paper called ‘Seaweed extract stimuli in plant science and agriculture’ (Source):

  • Improves plant vigor, root development, chlorophyll synthesis
  • Promotes earlier flowering, fruit set and uniformity of fruit
  • Retards senescence, extends product shelf life
  • Improves nutritional quality
  • Imparts stress tolerance, drought, salinity and frost
  • Alleviates disease, bacterial and fungal
  • Assists in pest control of insects, soil nematodes

Seaweed feed has been well studied and has become a mainstay of organic farming and gardening. T. L. Senn did a lot of research on using seaweed as a fertilizer back in the 60s and 70s. He published many papers and eventually wrote a great little book called ‘Seaweed and Plant Growth.’

And since then, there has been a ton of research.

In addition to the nutrients it contains, many of the benefits are attributed to the “plant growth regulators” in seaweed.

Organic farming consultants often recommend it be included in any spray application regardless of what else is being used.

Seaweed Fertilizer – Make Your Own

If you live near a beach with some seaweed on it, you can just take that and use it directly as a seaweed garden mulch, provided it’s legal in your area to remove it.

Be sure to leave most of it for the beach, though, as it has a big role to play there as food and habitat for many different species.

If you want to make a basic liquid seaweed fertilizer, pack the seaweed into an airtight container and fill it with water.

You don’t have to rinse off the salt first, although if you happen to know you already have an excess of salt, you certainly can.

Let it sit for at least a couple of weeks (or more like a couple of months in cold weather).

It smells fairly bad because it’s an anaerobic fermentation, but that’s okay. Adding a bit of Effective Microorganisms into the water can cut down on the smell.

If you let it sit long enough, most of the seaweed may decompose and dissolve, but if there’s any left, you can still throw it onto your garden as a mulch.

When you’re ready, mix it with 10 parts water and spray it directly onto your plants. It’s not nearly as concentrated as an extract or a commercially-processed liquid kelp fertiliser, but that’s okay – it still brings benefits.

Finding A Quality Kelp Fertilizer

Seaweed Fertilizer From The Ocean
Some seaweeds can grow over 2 feet per day.

There are 3 main formulations of kelp as a fertilizer:

Kelp meal. This is simply kelp that is dried and processed into a meal (not as fine as a powder – it’s closer to rolled oats) and applied to the soil.

I tend to gravitate to the efficiency of liquid fertilizers, especially when it comes to supplying trace elements, and among dry fertilizers, kelp meal is particularly expensive, but if I ever found some on sale, I’d certainly consider it.

Powdered kelp extract. This is kelp meal that undergoes further processing in order to get more nutrients out of the kelp. It’s then powdered so you can mix it with water and spray it on your plants and soil.

The application rate is much lower than kelp meal because we’re not using this to enrich the soil, but rather to feed the plants directly.

Liquid kelp fertilizer. There are different ways to make liquid kelp, but for most of them, it’s kelp extract that’s mixed with water.

If you have a small backyard garden, this is the easiest to use and it’s what I’ve always used in my small garden. It’s only now that I’m moving to a bigger property that I’m getting into powdered kelp instead because it’s more economical.

2021 is the first year I’m selling kelp extract in addition to the liquid kelp and I’ll be curious to see which one people choose.

Wherever you get your seaweed fertilizer and whichever form you choose, good seaweed products are processed with NO heat and NO high pressure, so as to keep as many of the beneficial components intact. 

The other thing to think about is whether the manufacturer is sustainably harvesting the seaweed, because overharvesting is an issue in some areas. 

That can be more difficult to figure out, but seaweed harvesting is coming under more scrutiny, and the good harvesters are starting to get recognized.

I always contact the manufacturer to ask what they’re doing to make sure their process is sustainable. If they have a good answer instead of brushing me off, that’s a good start. Then I go to the internet to see if anyone else has any more info on the company.

The powdered kelp fertilizer I’m just starting to use is from Thorvin, harvested in Iceland. Yes, that’s a bit far from North America, but they’re doing an excellent job of sustainably producing the kelp using geothermal energy.

The liquid seaweed fertilizer I use is from Neptune’s Harvest, also sustainably harvested in the cold waters on the coast of New England, far away from major urban centers.

How To Use Seaweed Fertilizer


For the kelp extract, I use 1 teaspoon per 1000 square feet. This can be done every 2 weeks for the whole growing season.

For me, that works out to about 90ml per 1000 square feet per year.

The dilution rate is 1:3000 parts water, which is 4 gallons for 1 teaspoon.

To get that huge dilution in my hose-end sprayer, I dissolve the kelp in 25 times as much water (1/2 cup of water per teaspoon of kelp) and then set the sprayer to spray 2 Tbsp per gallon of water.

Here are their suggested application rates:

Mix 1⁄4 teaspoon with one gallon of water, which covers 250-300 square feet. Spray plants or water in every two weeks, or at least three times during the growing season. For best results, spray as a fine mist covering all foliage, including the underside of leaves in the early morning or evening.

  • Vegetables, fruits, root crops, trees, shrubs, flowers. Mist all leaf surfaces every 2 weeks. 
  • Lawns. 1/4 tsp/gallon of water for 250-300 sq. ft.; 10 oz/gal for 1 acre
  • Transplanting & rooting solution. 1/3 tsp/gal – Place cuttings in solution until roots develop, then plant. Dip transplants in a solution and plant. Water with solution.
  • Seed treatment. 1/3 tsp/gal – Lightly mist seeds before planting or soak the seeds, or bulbs, for 5-10 seconds.


For the liquid seaweed fertilizer, in my garden, per 1000 square feet, I use 1/2 cup, 8 times per year. 

That works out to nearly 1 quart per 1000 square feet per year (my guess is that’s about 110ml of kelp extract).

If you want to spray it weekly, use 1/8 cup (2 Tbsp) per 1000 square feet instead.

The dilution rate can be between 1:50 and 1:100. I go with 1:50, which is a gallon of water for every 5 Tbsp of seaweed. I just use a hose-end sprayer set to spray 5 Tbsp per gallon.

Here are their suggested application rates:

  • House Plants: Use 1 tablespoon per gallon of water every 1-2 weeks.
  • Outdoors: Use 2 Tbsp per gallon of water every 1-2 weeks.
  • Lawns: Use 1 gallon for 8000 sq ft every month.
  • Seeds: Use 1 teaspoon per cup of water for soaking seeds.

They suggest applying until the soil is saturated or as a foliar feed in the morning or evening until the leaves are wet.

They recommend 3 gallons of product per acre, 4 times per year. That works out to about 1 quart per 1000 square feet per year.

Seaweed is great mixed with other fertilizers and microbial inoculants.

You Can Get It Here


In summary, this seaweed fertilizer:

  • Provides many benefits, but is especially known for improving plant health and helping plants deal with heat, cold, wind, drought and disease.
  • Is manufactured sustainably, without heat and pressure.
  • Is organic (OMRI-Listed) and undoubtedly one of the most popular organic fertilizers available.

Both the liquid and the powder are excellent. I tend to recommend the liquid for smaller gardens and for indoors, just because it’s so easy, and the powder for bigger gardens, just because there is the extra step of dissolving the powder, but the money savings are worth it for bigger gardens.

As a free bonus when you order today, I’ll also enroll you in my online Biostimulants course.


  • I ship in the U.S. only. I ship 7 days a week.
  • In the continental U.S., shipping is $15.
  • All of my products have a 1 year 100% money-back guarantee.
  • If you have a question about a product, leave it in the comment section below I'll try to respond within a few hours.
  • Dry fertilizers and compost tea brewers ship separately so they will arrive on their own maybe a day or 2 apart from my other products.
  • I send a percentage of every order to Thrive For Good and other similar organizations. They're working mostly in Africa to help communities grow organic, medicinal food for themselves, and then use the surplus food to generate income for themselves as well as feeding the orphans in their communities.

Free $25 Bonus When You Buy Today

When you buy this liquid seaweed fertilizer, you get enrolled in my online Biostimulants course.

The course includes 10 videos totaling about 75 minutes where I chat about seaweed, fish, sea minerals, molasses, rock dust, and how to use them all.


  1. Early this morning, I sprayed my zucchini with this liquid seaweed, sprayer set at 5T per gallon. Now the leaves look damaged. The edges look spotted and slightly wilted. They looked great before. Is this common? Did I miss a directions?

    1. Hmmm, I’ve seen some damage with the fish before – never with the seaweed. On the bottle they say to mix it 1:128, which is more like 2Tbsp per gallon of water, but on their website when I look at the application procedure in more detail, they often say 1:10, which is like 25Tbsp per gallon of water. I’ve always done 5Tbsp with no problems, although it never hurts to use more water. I’m very sorry for your troubles Elizabeth. Zucchini leaves are pretty tough – I bet they’ll recover soon.

      1. Thanks, Phil. I am optimistic. I’ll let you know what develops. 🙂

        1. Well what happened, Elizabeth?

          1. The leaves developed symetrical markings which persisted, but the plants looked wonderful in posture, seemed really happy. Things went fine from there. Wish the season weren’t ending soon.

        2. Hey I’m doing hydroponics what do you recommend the serving for 6 gallons of water?

          1. The label doesn’t say. I’ve read 1 Tbsp per gallon of water, but I can’t say for certain.

  2. aamir raza says:

    is it possible that after applying neptune 2-3-2 or 2-1-3 i do not remember that for the first two or three days the plants do not look good the flowers that were there seem dried up,the leaves also not appear too strong but a couple of days later things start looking up and eventually more than they were before it us happening now rose mums hawthorne jasmine basil are looking weak started application two or three days ago could you help thanks

    1. It could be if you didn’t dilute it enough. Dilute at least 50 times (at least 1 gallon of water for 5 Tbsp of neptune’s harvest.

  3. James McCarthy says:

    What should I use for a soil that has virtually no Nitrogen and very little Phosphorous?

  4. I live in Wisconsin so I can’t harvest fresh seaweed. What is the best Kelp/Seaweed you can purchase online? Thanks for any info. I know Neptune is ok but are there better options?

    1. I’d say the best overall value is Neptune’s Harvest, which is why I’m selling it on this page. They’re using Thorvin seaweed, which is one of the best in the world.

  5. Olwyn Rutter says:

    Hello there,
    This is hilarious. I have just moved from Victoria, BC and now live on Crete Greece. Trying desperately to find a source of orgnic liquid seaweed fertilizer. Don’t suppose you ship to Greece do you? If so, what would the cost be? The product I am looking for is to be used on organic hydroponic vegetables. I wish I had left all my clothes out of my cases and just brought gardening supplies instead!
    Look forward to hearing from you.

    1. Sorry Olwyn, I can’t ship overseas. There must be someone in Europe who can help.

  6. Curious how often you spray your plants in particular vegetables.

    1. I do everything monthly, which is great, although I would do weekly in very small amounts if I had the time.

      1. Jerry Foster says:

        Is there a high temp when it’s not good to spray the sea weed,I live in Florida.

        1. I’ve never come across anything about this, other than that it’s best to apply in the morning when it’s not quite as hot. Plants still need nutrients when it’s hot, though. Sorry I can’t give you more specific info on temps.

  7. Md Moniruzzaman says:

    I use organic wow soluble kelp powder. Is it different than liquid seaweed? Which is more beneficial liquid seaweed or powder seaweed?

    1. It’s different but they’re both good. I use liquid because it’s ready to use and also because it’s made with fresh kelp instead of powdered kelp. But the powdered is still beneficial. I’ve never seen any research comparing the two.

      1. Md Moniruzzaman says:

        Thank you for the prompt reply. I’m interested to use both and check myself which makes difference.

  8. Could this be used on house plants? If so, what would be the proportions to dilute? Thank you – Adrienne

    1. Absolutely. For houseplants, use 1 tablespoon per gallon of water every 1-2 weeks.

  9. Can you use this to spray leaves that will be harvested, I.e., lettuce p, spinach, etc. does it change the flavor?

    1. The dilution is such that I’ve never noticed a thing, and I tend to spray up until close to harvest time.

  10. Kelp does have hormones that make it grow 2 to 3 feet a day. Transferring hormones from one plant to another plant, Does it really Work?
    Maybe its not the hormone in kelp that is making your plants grow bigger? Maybe it is the vitamins and nutrients that are making plants grow bigger? I am doing an experiment on several different plants tomato, cucumber, melons, herbs and few flowers. Half plants with vitamins and half plants without vitamins, with all same soil, inoculated with mycorrhizae before placed in pots/planters. Have been feeding my plants multivitamins every time i water them (1 multivitamin per gallon of water totally dissolved). The plants that i been feeding vitamins are double the size than ones i have not fed vitamins. If there is any deficiency in my plants i’ll feed them very little fertilizer just make sure plants are as healthy as they can be for my experiment. Kelp is loaded with vitamins/nutrients. Before i become a true believer that the hormones is making plants grow bigger i would have see the results with just the hormone itself without vitamins/nutrients kelp produces.

    1. Thanks for sharing, Mark. I think you’re right that the nutrients are part of the picture. But in many of the great books on using seaweed as a fertilizer (which you can find online from Acres U.S.A.), they really stress the hormones, not that the hormones gets “transferred” to the plants – just that they influence plant growth, which we know for sure. Perhaps it’s hormones and nutrients. Sounds like a great experiment you’re doing!

  11. 3 Qs re: fertilizing to get an Overview of how to best view & do fertilizing when using the Natural Big 4 inputs you advise. Kelp — Sea Minerals — Fish Fertilizer – Compost Tea/BioAg.

    1. Do they provide everything your garden will need with respect to fertilizing (once the soil structure & minerals have been balanced)? Is there any reason to “additionally” give your plants an “all around organic based fertilizer” such as retail OMRI organic products that are labeled as General Garden Fertilizers, that have things like alfalfa, feather, fish bone? In otherwords, is it possible to ONLY use the “Big 4” and have an excellent garden without the added expense and work of additional products? (I realize these Big 4 are “food” for the microbes and not the plant directly).

    2. Do these 4 products actually correct an imbalanced “soil” so if you are not sure of your soil balance — over time using these Big 4 will bring it naturally into balance?

    3. I find it very useful to not just “follow” directions but to understand the “WHY”. So with that knowledge it allows me to apply wisely with intent and be able to explore when things go haywire. You have done wonderful work in this blog (& lessons) providing so much of that. If you have the time, would really like to know for each of these 4: Kelp, Sea Minerals, Fish Fertilizer, Compost Tea/BioAg – what is it that they mainly contribute and importantly can they be over applied – i.e. the benefits & their general parameters? For example, I’ve heard Fish Fertilizer is primarily for Nitrogen for green color & growth of foilage and there is no concern of it being over applied. It would help with the confusion to have a “brief” coherent overview to better understand the “parts” when gardening with natural products to use them properly and bring them all together successfully.

    Thank you.

    1. 1. The big 4 will get you part of the way there, but not all the way. But I wouldn’t add an “all around organic based fertilizer”. If I were wanting to go further, I would soil test and then add the specific nutrients in which I was deficient. That said, there are other useful products out there. For example, if you soil test with Crop Services International, they’ll usually recommend an additional 1 or 2 microbial inoculants that they have and an additional 1 or 2 micronutrient fertilizers. There are thousands of products out there so there are bound to be other useful ones.

      2. No, they won’t correct soil imbalances much at all. They just make sure your microbes and plants have a little of everything, which is helpful, but the soil imbalances will still be a limiting factor.

      3. On the right side of this page (or the bottom if you’re on mobile), you can read a little explanation I wrote titled “Sea Minerals Vs Seaweed Fertilizer Vs Fish Fertilizer” where I do my best to share the differences. In the end, those 3 are quite similar, just all with slightly different make-ups, and I imagine my explanation probably isn’t enough for you. There are books from Acres U.S.A., especially ‘Fertility From The Ocean Deep’ and ‘Food Power From The Sea’ that may give you some more info. I expect they all could be overapplied but I’ve never tried 🙂 And if you go to the compost tea page, you can read (on the right side) the box titled “Compost Tea Vs SCD/EM Vs Mycorrhizal Fungi” where I make a similar comparison. You can overapply BioAg but not compost tea, although I don’t really have a good explanation as to why.

  12. Hi Phil,

    I started using the liquid seaweed fertilizer a few weeks ago on my houseplants , and now I am noticing some small mushrooms growing in their soil, and some fungus gnats! Could these be related? I haven’t changed the frequency of the watering.

    Do you have any tips on this? Thanks.

    1. Hmm, that’s surprising. I suppose it could have something to do with the seaweed but I’m not sure what. Mushrooms fruit when they’re ready to. I’d be surprised if the seaweed would stimulate fruiting but perhaps. Mushrooms aren’t generally bad, though, so probably no worries. The fungus gnats are bad, though, and again, I’d be surprised if the seaweed encouraged them, whereas I wouldn’t be surprised if seaweed actually discouraged them. I recommend you stop the seaweed for a month to see what happens, and then start again to see what happens. Personally, I’m using it twice a week on my indoor lettuces right now and they’re loving it 🙂

      1. Thanks, Phil. I’m going to follow your advice and pause for a bit on the seaweed

  13. Hi Phil,
    Is the blackstrap molasses you sell, organic?

    1. It used to be, but no, not the one I carry right now. It’s non-gmo and unsulfured but not organic.

  14. mary huggins says:

    love the product. we’ve been using it regularly since we got it. not much happened at first and then we had a rain and the flower are abundant. one question i have is the when we bought the plant i thought the flowers would be purple. they are not. they are white. is there something we can do to make them purple? i know we add a product for hydrangeas to be blue…so just wondering. thanks

    1. I’m not sure which type of plant you’re referring to but almost all flower colors can’t be changed.

  15. I am trying the Bio Ag and Liquid Seaweed this year along with molasses. I truly appreciate how detailed you are about the application rates but I am still a bit confused. I bought the hose end sprayer you sell. I want to spray a mixture of Bio Ag, Seaweed and Molasses once a week on about 350 sq foot garden. What would I add to the sprayer and what would I set it on?

    1. Yes, there’s still room for improvement with my application rate recommendations. My calculator can help, but there’s room for improvement with it, too. If you’re spraying weekly, I would spray a scant 1 Tbsp of each. I would first dissolve 1 Tbsp of molasses in 2 Tbsp of warm/hot water. Then, I’d add 1 Tbsp of liquid seaweed and 1 Tbsp of Bio Ag. It’s not much product because you’re spraying so often and the area is so small, so it can be hard to get the last 1-2 Tbsp out of the sprayer, so you may want to add a couple of additional Tbsp of water to increase the total volume of liquid in the container. I would start by setting the sprayer to spray 1 Tbsp per gallon of water and see how that works. If there’s not enough water pressure to suck up the products, I’d increase that setting as high as 5 in order to get the product out.

  16. — what a great site — Phil, Thanks for a sharing all your experience. Just some info …When using seaweed extracts, it is very important to follow the dosage and application directions carefully. Seaweed extracts can be applied as a foliar spray or as an additive to the nutrient formula, but foliar application seems to be the most efficient. Just remember not to use it as a foliar spray on flowers. Also, be careful not to use too much! Hormones are very powerful, even in small amounts, and using too much will have the opposite effect on the plants. Too much auxin restricts root growth, too much gibberellin reduces germination rates, and too much cytokinin can burn leaves ( auxin, gibberellin, cytokinin are the hormones in Seaweed extract).
    Seaweed extracts are particularly rich in cytokinins. Cytokinins stimulate rapid cell division and the production of new cell walls, so cytokinins are particularly important for new growth, cytokinins, result
    in the production of greater root mass.
    In my opinion moderation is the key.

    1. Excellent comment, thanks for sharing, Naveed.

  17. How does a variety of seaweeds in a liquid fertilizer compare to just kelp? I’ve found a product (in the UK) that’s sustainably harvested and cold-pressed, but that contains a ‘special blend of kelp and wrack species, Laminaria hyperborea, Laminaria digitata, Fucus vesiculosus and Fucus serratus’.

    1. I’m not sure how much of a difference it would make, but I don’t see any downside.

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