Kelp vs Seaweed- What’s The Difference?

Last Updated on August 28, 2022 by Sam

What is the difference between kelp vs seaweed? And why are they used as fertilizer? Isn’t kelp a seaweed? What form should I pick (seaweed powder or liquid extract)? I ask myself all those questions when I look at the different organic and/or natural fertilizers available in the gardening market. 

I’ll try to answer all those questions in the next lines in the hopes you may choose better about kelp and seaweed with information on your side. 

Seaweed vs Kelp- Is There Any Difference?

Seaweed is a word used to name many species of aquatic plants, specifically oceanic ones (hence the sea- prefix). Seaweeds can grow like underwater forests, kelp beds (a type of brown seaweed) cover up to 25% of the world’s coastlines. Seaweeds create ecosystems that can host thousands of other sea creatures such as plankton, seals, fish, jellyfish, and more. Phytoplankton is also considered a seaweed because of it’s immense growth potential. 

Seaweed vs Kelp- Is There Any Difference

Seaweed is a term that includes many types of algae, including Kelp. Seaweeds grow in many marine environments (oceans, rivers, or lakes), but especially Kelp grows in shallow and cool saltwater along rocky coastlines.

Some of these seaweeds are even called sea-vegetables because they’re edible to humans. For example, nori algae (used by the Japanese in sushi and other traditional dishes), dulse, kombu, and wakame. Some seaweeds are medicinal, used mostly to treat skin problems. For more info on seaweeds and kelp, click here to open the National Ocean Service.



Include many types of algae of red, brown, and green.Brown type of seaweed
Grow in many marine environmentsOnly grows in saltwater, along rocky coastlines in depts of 6 to 90 ft. deep
Many different sizes, some of them are microscopicLarge size
Used as a liquid fertilizer from natural seaweed sourcesUsed as a natural fertilizer and source of trace minerals and usually mixed with fish meal
 Can be found in powder or liquid kelp

GS Plant Foods Organic Liquid Kelp Fertilizer

Kelp as a Garden Fertilizer

But how does kelp or seaweed help my garden soil and veggies? In the beaches, these plants prevent sand erosion and maintain the ecosystem healthy by feeding animals, as well as providing shelter. 

In the garden, kelp and seaweeds turned out to be a great fertilizer for any kind of plant/soil nutrition deficiencies. Seaweeds are not particularly high in nitrogen or phosphorous, but they contain up to 60 different minerals which are micronutrients (trace elements) for plant health promotion. It also has pest deterring properties. Pets don’t like the smell of it, which results in keeping them away from the garden. 

Seaweeds, including kelp, certainly amend nutrient-depleted soils and improves structure, water retention rates and ultimately end up decomposing, and in the process, they add organic matter to your garden beds, pots, and composts or other fermented fertilizers.

Learn more How To Use Kelp Meal Fertilizer

Neptune’s Harvest Organic Hydrolized Fish and Seaweed Fertilizer

Foraging Seaweed for Fertilizer

If you forage kelp or other seaweeds for fertilizers, follow these tips for better results.

  • Collect only from the coastlines below the high tide mark, in small quantities, only what you need.
  • Know that there’s no need to wash the salt away, it contains minerals and nutrients.
  • Cut the plants by hand or grass scissors, sometimes they hide small stones, sand, or even shells.
  • Use fresh or dry, both ways are equally nutritious to the soil.
  • Apply over your garden beds, under the mulch, and in your compost.
  • If you buy it, it comes available in powder and liquid extracts, and different concentrations. Choose the best suitable for your project. Liquid extract for hydroponic systems seems like a great idea. Seaweed powder is more suitable for grow beds, pots, and as a compost improver.


For more useful tips and info regarding seaweeds as fertilizers, I recommend the article Seaweed Fertilizer — Using and Composting Seaweed for Your Garden.


Is kelp the same as seaweed fertilizer?

No, kelp is not seaweed fertilizer. Kelp is a type of alga (one of the plant kingdom) that can be found in salt water. 

It is used for various purposes including fertilizer and food. In fact, it’s one of the most popular seafood supplements out there. Kelp is also commonly known as seaweed because of its similarity to the plant life that grows on land. However, kelp has more in common with plants than seaweed. 

It is often referred to as “kelp” or “rockweed” since it is so similar to the plant life on land.  There are different types of kelp. One of the most common varieties is Alaria esculenta (macroalgae) which is a type of red algae. This variety is the main source of nutrition for people who consume kelp supplements. Another variety of kelp is Laminaria digitata, which is a type of brown algae. These two types of kelp are found in coastal areas. One of the reasons that people use kelp is because it is a natural source of vitamins and minerals. It’s also a great source of iron and iodine. Kelp contains an abundance of vitamins, minerals and proteins.

Is kelp a good fertilizer?

Yes it is a good fertilizer. It is a very good source of nitrogen (N) which is needed by plants. The nitrogen is released as ammonia.

This is then used by the plant for photosynthesis. It is also a good source of potassium (K), magnesium (Mg), calcium (Ca), and phosphorous (P). There are a few problems with kelp though. The biggest problem is that kelp takes a long time to decompose. It can take anywhere from 2 to 10 years. This is because it contains cellulose, which is a natural organic compound.

The cellulose breaks down in soil over time, releasing nutrients. Another problem with kelp is that it tends to build up in rivers and streams. 

If you want to use kelp as a fertilizer for your lawn, then it may work well. Kelp contains a lot of nitrogen. However, this nitrogen is locked away inside the plant. You would have to apply the kelp to the soil at least 3 times before the nitrogen gets released into the soil. The other problem is that the kelp is not easy to spread out on the lawn. It needs to be shredded first before it can be applied to the lawn. DKelp is just one more ingredient to add to your fertilizer bag. You should look at what else you can add to your fertilizer to get the most nutrients into your lawn.

Is seaweed the best fertilizer?

Seaweed is the most widely used organic fertilizer. It is used to fertilize the soil, but not the plants. It is used as a source of nitrogen and potassium in the soil. The application rate of seaweed is 1-3% in the soil depending on the type of seaweed. It is very effective for the growth of vegetables because it is rich in potassium and nitrogen, which are very important for plant growth. It is also very effective in controlling the weeds.


If you use Kelp or any seaweed as fertilizer, there are a few things to consider:

  • You may forage it in small quantities, always trimming the submerged parts of the seaweed, avoid collecting it from the beach. 
  • Notice that seaweed foraging may be an illegal practice in your region. 

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