5 Best Seaweed Fertilizer Liquid Or Powder | How And When To Use It

From ancient times, humans have used seaweed as a food source, medicine, and a means to enrich crops. Seaweed, despite the name, isn’t a weed or even a plant. It’s a very large type of algae that grows in abundance in the Earth’s oceans. With over 10,000 different species of green, brown, and red seaweed in existence, it’s no wonder that it has become a gardener’s best friend. 

Before the introduction of synthetic fertilizers, seaweed was gathered and spread around the base of various crops. Breaking down over time and releasing a broad spectrum of micronutrients that are vital to plant growth and vitality. To save you time and expense, I’ve personally put a wide variety of seaweed fertilizers to the test, to determine which is the best seaweed fertilizer liquid or powder.

What Is Seaweed Fertilizer? 

Seaweed is a completely natural material that has become a familiar staple in organic gardening. Highly prized as a plant fertilizer, it has the ability to improve soil structure and moisture retention. Providing a nutrient-rich environment for plants and essential soil microorganisms, alike.

A 2020 study, published in the International Journal of Environmental Science and Technology (IJEST), focused on why seaweed makes such a potent and effective fertilizer. Unlike other organic or synthetic options, seaweed, in particular, was revealed to contain natural growth hormones that stimulate lush, vegetative growth on ornamental plants. Crops benefit from the same in addition to a higher yield.

Whether used as a tea or powder application, seaweed fertilizer is versatile enough to be used directly on plants or mixed into compost for an added boost of magnesium, potassium, zinc, iron, and most importantly, nitrogen. 

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How is Seaweed-based Plant Food Made 

With proper licensing, seaweed is gathered from ocean shores in bulk, dried, then allowed to ferment and break down into a slimy liquid. Once the material is broken down, it can then be made into a “tea” or allowed to dry and then pulverized into a powder. Which is often easier to store and can easily be reconstituted with water. 

Seaweed Fertilizer Benefits For Plants 

Seaweed offers a broad spectrum of benefits for plants in a completely organic way, especially edibles. This marine kelp is packed with valuable trace minerals like iron, manganese, zinc, and copper, which are then transferred to fruits and vegetables through plant absorption. 

Applying seaweed tea directly to the soil or misting foliage imparts high levels of carbohydrates that are essential for healthy, strong plant growth. Stronger plants are more resistant to environmental stresses, pests, and diseases. As the need to adopt more sustainable gardening practices increases, finding the best seaweed fertilizer for your garden will set you on a course for success. 

Which Is Better Seaweed Or Fish Fertilizer? 

When considering an organic source of nutrients, some question the difference between seaweed and fish emulsion. Both are rich in what plants need but what makes the difference in your soil? Seaweed contains adequate levels of nitrogen but they’re not particularly high. Making it perfect to add to compost (which is already rich in nitrogen).

If you don’t use compost and you’re looking for higher levels of nitrogen, then a seaweed and fish emulsion combo might be an effective option. Fish fertilizer is essentially ground-up fish parts. So, it’s naturally going to be higher in nitrogen. 

Liquid Seaweed Fertilizer N-P-K Ratio 

In the products I’ve tested, it’s clear that the NPK ratio is lower than in more conventional feeds. However, I never regard this as being an issue. I use a lot of compost in my veggie beds, so the extra boost of nutrients works beautifully and without the fear of burning my plants with too much nitrogen.

Now, in areas where you wouldn’t typically add compost (like your lawn), these would benefit from an organic feed combination that offers higher levels of nitrogen, potassium, and phosphorus. 

The NPK in seaweed, alone, is measured at around 1-0-4. One part nitrogen to four parts potassium. Even though phosphorus is plentiful in the earth’s oceans, seaweed doesn’t absorb it. However, fortified seaweed powder that does contain added phosphorus is available.  

Seaweed Fertilizer Powder 

Seaweed powder is produced by allowing fresh seaweed to completely dry out. Then, ground down to a fine powder, with no loss of nutrients or effectiveness. This dry form allows for the addition of phosphorus, whereas liquid forms do not. 

Powdered seaweed can easily be worked into the soil for a slow-release effect or mixed with water and applied for quicker intake. 

Organic Seaweed Fertilizer 

Organic gardening has been successfully practiced for thousands of years and a strong return to it appears to be on the horizon. Seaweed is a completely natural option that is easily combined with other earthly elements to create a custom nutrient combination to fit the needs of every garden. 

Best Seaweed Fertilizer Reviews 

There are a lot of options out there to choose from, so how do you decide? To save you the time and expense of testing different ones yourself, allow me to present my top five picks for best seaweed fertilizer.

Out of the many that I compared, these ranked the highest based on ease of use, price point, and performance. I’ve included options for both liquid seaweed fertilizers as well as powders and provided benefits and drawbacks to make decision-making easier for you.  

1. Grow More Seaweed Extract

Grow More Seaweed Extract

As opposed to simply fermented kelp, Grow More offers a seaweed extract fertilizer. The benefits of this lie in all the multifaceted components of seaweed being extracted and fortified with essential minerals that help plants thrive.

In the areas where I tested this, the added minerals really enhanced the structure of my soil. While at the same time supporting vigorous root formation and increased resistance to heat waves and frost damage. And the 1-Gallon jug means more applications for less money. 

Check out GrowMore Seaweed Extract latest price here.

How To Use: Apply the recommended dosage every 2-3 weeks with a hand sprayer, starting at the first signs of growth in spring. 

Customer Reviews: Consumers have used this on everything from orchids to large fruit trees and are in love with the results. And with such a small amount needed per feeding, it’s fantastic value for money. Making this my best buy and earning an impressive 4.7 / 5 stars.


  • Versatile enough for use on houseplants, vegetables, trees, shrubs and lawn
  • Great value for money with just 1 capful per use for most applications


  • Not recommended for use on small seedlings

2. Simple Grow Solutions Seaweed Fertilizer with Humic Acid 

Simple Grow Solutions Seaweed Fertilizer with Humic Acid

This option demonstrates that both powdered and liquid seaweed feeds can be fortified with additional nutrients. This water-soluble concentrate combines all the benefits of seaweed with humic acid and organic carbon. Increasing yields and color vibrancy in veg beds, flower borders, and lawns. 

As a fast-acting liquid, I saw immediate results in foliage vibrancy and an abundance of blooms. On my lawn, a deeper green color, over time. A word of caution, though. If there are weeds in your lawn, this will also feed those and could result in these weeds even flowering and spreading. I would recommend de-weeding, waiting 30 days, and then applying this.  

How To Use: Water veg and flower areas thoroughly to ensure smooth absorption. Then follow label instructions for space size and different plant requirements. Lawns should be watered for 20 minutes within 24 hours of application. See the latest prices for Simple Grow Solutions Seaweed Fertilizer here.

Customer Reviews: When nothing else seemed to work, consumers across various gardening forums are sharing their amazement at how fast their lawns, trees, shrubs, and flowers burst into life, after applying this product. 4.6 / 5 stars.


  • Evident results in just 24 hours
  • Effective on lawns, trees & shrubs, flowers and vegetables


  • Formula may also stimulate weed growth

3. Down to Earth Organic Kelp Meal Fertilizer Mix 1-0.1-2 

Down to Earth Organic Kelp Meal Fertilizer

Down to Earth is a reliable brand that can’t be beaten when it comes to organic gardening practices. This fertilizer is a great one to have on your potting shed shelf and can be used for a variety of plants in and around your garden including veggies, herbs, flower beds, as well as perennials.

This one is going to support the development of your plants during every step of their growing cycle from transplanting your seedlings, to healthy root and foliage development, and protection from the elements.

This finely milled seaweed is rich in nitrogen and potassium and it gave my squash and pepper plants an immense boost in productivity. What you also get with the Down to Earth brand is peace of mind that you’re not going to burn your veggie plants or add anything that would offset the delicate nutritional and microbial balance in your soil.  

Check Down To Earth Kelp Fertilizer’s latest prices.

How To Use: For new seedlings, mix ½ tsp into the soil prior to planting, then water well. For established plants: sprinkle 1-2 teaspoons around the base of each plant per month. 

Customer Reviews: Home gardeners are so pleased with how well this certified organic product performs on all manner of fruits and veggies. The highest scoring is geared toward fruit and citrus tree fertilizing, especially. 4.6 / 5 stars.


  • Long-term, effective nutrition for vegetables and fruit, including citrus.
  • Certified organic for natural gardening


  • Slightly higher price point compared to other similar fertilizers

4. Neptune’s Harvest Organic Hydrolized Fish & Seaweed Fertilizer 

Neptune's Harvest Organic Hydrolized Fish & Seaweed Fertilizer

This water-soluble option is perfect for plants that require a little more nitrogen. The balance of fish emulsion and seaweed offered my nitrogen-loving citrus trees a boost of natural nutrients while also improving soil that I’ve found difficult to amend by hand.

Heavy-feeding vegetables and those that need a high nitrogen intake such as tomatoes, peppers, squash, cucumber, and sweet corn can produce a higher yield with regular feeding throughout the growing cycle as this fertilizer has been shown to build up natural sugars in crops. Click here for Neptune’s Harvest Organic Fish & Seaweed Fertilizer

How To Use: For vegetables, flowers, trees, and shrubs, combine ⅛ cup of fertilizer per 4 liters (1 gallon) of water. Then, apply the entire amount to the target area.

Customer Reviews: Gardeners who made the switch to purely organic gardening are raving about the similar results they’ve seen with this product when compared to synthetic options. 4.7 stars. 


  • Effective combination of fish and seaweed for abundant blooms and a higher yield
  • Performs well on lawns and with seed germination


  • Storing diluted fertilizer is not recommended

5. Sea Magic Dry Soluble Seaweed Extract 

Sea Magic Dry Soluble Seaweed Extract 

Sea Magic is an appropriate name for what I’ve found to be the easiest to store and prepare for application. This can be purchased in the middle of winter and stored in a cool dry place until you’re ready to use it. Then, simply mix into 4 liters of water and there’s your liquid concentrate for the season!

For organic gardeners looking for pure seaweed, this is for you. No additives or processing agents. Just humble seaweed in a powdered, water-soluble cake. Ready to provide everything from houseplants to berries with all they need to thrive.  

How To Use: Dissolve dried seaweed in water and apply to plants every 2-3 weeks. 

Customer Reviews: Rose experts can’t believe how many blooms they got when using this product and vegetable growers seemed stunned by their yield. Happily granting Sea Magic 4.6 / 5 stars. Check out prices for Sea Magic Soluble Seaweed Extract here on Amazon.com


  • Can be used alone or in combination with an all-purpose fertilizer
  • Makes upto 50 gallons of nutritious feed for just one price


  • Fertilizer cake may need to be cut into proportioned sizes for equal feeding

Which Plants Like Seaweed Fertilizer 

Most plants love seaweed in one form or another. Houseplants thrive and grow strong. Flowering plants produce more blooms, fruit is sweeter and less prone to rot and disease and vegetable yields are higher. Even high-acidity plants benefit as the best seaweed fertilizer liquid or powder will work to increase acidity and add iron to the soil. 

Even non-flowering trees, shrubs, and lawns love seaweed. Just be careful, weeds love it too. If using this on your lawn, you may want to apply a weed killer a few weeks prior to application.

How Do You Fertilize With Seaweed? 

Depending on which one of these you find the best seaweed fertilizer for you, it may be as simple as diluting a powder or cake in water and watering as usual. Or, you can sprinkle it directly on the soil or spray it across your plants for effective foliar absorption. Some may only need to be applied every so often. While others may require more frequent applications.  

Verdict: Best Seaweed Fertilizer 

It appears the verdict is in! You can’t go wrong with one of nature’s most reliable organic fertilizers and applying it couldn’t be easier. Whether in liquid forms like Grow More’s Seaweed Extract that can be used as a foliar spray and will encourage root development and lush foliage. Or Simple Grow Solutions Super Seaweed with added Humic acid that will encourage your plants to absorb more of those micronutrients found in soil.

Alternatively, choose Kelp Meal, the easy-to-store granules from the all-natural brand Down to Earth. A great organic choice for all-around gardening needs from herbs to veggies, flower beds to perennials and can simply be diluted in water.

The use of seaweed as a fertilizer may have been an ancient resource but it can still hold its own and will serve you well in modern-day gardening. I trust that with your newfound knowledge, you’ll put it to good use as part of your natural and organic gardening experience.