Seaweed Fertilizer: Oceanic Bounty For Plants

Seaweed fertilizer can be a powerful tool for plant growth. We talk about this resource, how it's applied, and even how to make your own!

Seaweed fertilizer


You may have consumed seaweed in the form of crispy snacks or wrapped around sushi, but did you know that seaweed fertilizers can also nourish the plants in your garden? 

Seaweed is a generic term that encompasses many different types of multicellular algae that grow in our oceans. This primitive plant group has over 10,000 species. Seaweed is different from the plants that we grow terrestrially two important ways. Firstly, all cells of a seaweed species can photosynthesize. Secondly, seaweeds do not have a system where roots help take up water and nutrients from the soil; seaweed cells can take up nutrients directly from the surrounding water. 

Seaweeds have important cultural and ecological value. They are prized ingredients used in different traditional cuisines and their production is a major industry for many coastal nations. Seaweeds, including the giant kelp, provide food and shelter for other marine species. Furthermore, due to their ability to turn carbon dioxide (CO2) into biomass through photosynthesis and their rapid growth rate in comparison to terrestrial plants, they can become a key player in carbon sequestration and the fight against climate change. 

Cultivating a demand for more seaweed products through fertilizers, food products and other uses can further the growth of this industry. As you incorporate seaweed fertilizers in your garden, you are choosing a product that supports regenerative farming of both land and sea.

Good Seaweed Fertilizers To Try In Your Garden:

What Is Seaweed Fertilizer?

Seaweed fertilizer
Seaweed fertilizer can be a powerful tool in a gardener’s arsenal. Source: calilily

Seaweed fertilizer are products that are derived from marine algae and are approved for organic use. These are natural soil amendments made from sustainable sources. Specifically, brown alga species of Ascophyllum, Ecklonia (kelp) and Fucus are most frequently found in fertilizers. The two most common seaweed fertilizers are kelp meal and “liquid seaweed fertilizer”. Unlike chemical fertilizers, natural seaweed fertilizers contain high amounts of micronutrients, minerals and other organic matter that promotes healthier plants. Using seaweed in your garden is a great way to promote plant vigor, produce more abundant vegetables and fruits, all while improving soil quality through the support of beneficial microorganisms.  

Benefits of Seaweed Fertilizer

The many benefits of using seaweed products in agriculture have been widely studied and documented. High quality seaweed fertilizers are best known as a source of micronutrients. Out of the seven micronutrients [boron (B), chlorine (Cl), copper(Cu), iron (Fe), manganese (Mn), Molybdenum (Mo) and Zinc (Zn)], seaweed extracts (mainly derived from the brown alga Asophyllum nodousm) can especially boost zinc and manganese supply to plants and shore up plants’ defence against stressors.

Cold soil temperatures in the spring are a big constraint for gardeners in temperate climates around the world. Cold temperatures can stunt the growth of plants by inhibiting their root development. Studies have shown that micronutrients have the potential to help tender seedlings during their early growth period. The addition of micronutrients can promote root growth, both in density and length, to set the plant up for success the rest of the season. Micronutrients can also be deficient in soils that have poor draining, poor structure or have pH that’s too high or low. If this is the case for your soil, you might want to condition the soil by adding some seaweed fertilizer.

Seaweeds also contain natural plant growth hormones auxin, cytokinins and gibberellins. Each of these hormones are essential biostimulants that boost plant health.  Auxin is known for the positive effect on roots and is what’s used in synthetic rooting hormones. Cytokinins trigger rapid cell division and help with new growth including roots, flowers and fruits. Gibberellins can kickstart germination and support stem and flower development. Plants that are grown in soil with added seaweed extract exhibit better germination, increased plant growth, resistance to pathogens and better fruit yield.

Seaweed drying out
Many forms of seaweed are used to make liquid or dry seaweed fertilizers. Source: Kevin Doncaster

To illustrate the value of these fertilizers, commercial olive growers have reported increased fatty acid content in olive oil pressed from olive trees that have received seaweed amendments. Scientists have also reported the beneficial effects of these organic fertilizers on diseases and insect pests for a range of crops. In one study of strawberry and turnips, a weekly foliar application suppressed the spread of powdery mildew and gray mold. Other studies have also reported antifungal properties on plants like tobacco and antibacterial properties on cotton.

Liquid seaweed sprays seem to also reduce the number of aphids and spider mites. Also, a liquid seaweed extract can help reduce root damage by nematodes in tomato plants both as a foliar spray or as a soil drench at the time of transplantation.  All these findings suggest that the benefits of using seaweed in agriculture extend beyond supplying micronutrients to stimulating biochemical changes at the cellular level. 

Drawbacks of Seaweed Fertilizer

Seaweed contains very low NPK values. Whether you purchase a liquid or powder product, you should be able to find the NPK ratio of that product on its packaging. Due to the limited macronutrients in seaweed fertilizers, you may need to supplement using things like liquid fish fertilizer or fish meal.  There are also commercially available fortified liquid seaweed products with pre-added nitrogen, phosphorus, and potassium. These fortified liquid seaweed fertiliser formulas are typically tailor-made for specific crops. 

How To Use Seaweed Fertilizers

Fresh seaweed
Freshly harvested seaweed should be thoroughly soaked and rinsed to remove salt. Source: siaronj

The powder form of seaweed fertilizer can be sprinkled on top of soil, mixed into potting soil or diluted to create a nutrient-rich tea. Typically, these powder fertilizers are often sold as natural kelp meal or kelp fertilizer and can be found online or at local gardening stores. They are easy to store and have a longer shelf life than liquid fertilizers. 

Liquid fertilizers are likely to be more expensive than the powder counterpart but can be used throughout the growing season. These are typically concentrated liquid kelp and will need to be diluted before use. Follow the recommended dilution ratio on the back of the product packaging; it will likely call for only a few tablespoons of fertilizer per one gallon of water.   Diluted liquid seaweed fertilizer can be used to drench the roots of transplants. Leave the seedlings or plants in their original pots and place the pots in a larger container with the seaweed solution. Let the plants soak up the fertilizer for 1-2 hours or even overnight. This action is akin to bottom watering plants and is one of the best ways to deliver nutrients directly to the roots. 

Liquid fertilizers can also be watered into the soil or sprayed on to leaves. Make sure not to apply before rain. Plants will take up some of the nutrients of the fertilizer through their leaves. Liquid fertilizer can be used on a variety of vegetables, flowers, fruit trees and even potted plants. Carefully follow the manufacturer’s instructions. If you are using the fertilizer during the growing season, do a test application before applying widely throughout the garden. 

DIY Seaweed Fertilizer Tips

Collecting seaweed
Collecting seaweed for DIY fertilizer is an option if you’re near the beach. Source: Rob

There are many benefits to using seaweed fertilizers but the cost can be steep. Luckily, it’s easy to make your own. If you are lucky enough to live near the ocean, make sure to double-check with your local government or beach management office regarding seaweed harvesting for home use.  You can make a day of it by going fishing and gathering future fertilizer at the same time!

Common varieties of red, green and brown species can all be used in DIY fertilizers. While collecting seaweed in the wild, go during high tide to ensure that you are only taking seaweed that has been washed up. Check to see that the seaweed is not attached to any rock substrates so as to not disturb any potential habitat or food source for local wildlife. The timing of your collection also matters. Different species of marine life, like wild herring, may spawn on seaweed. Even if the seaweed breaks off and gets washed ashore, herring eggs can still survive so it is crucial not to pick seaweed during spawning season. For herrings, the spawning season is February and March; this timing might be different for other wildlife. 

If you do not live near an ocean or do not wish to go collect your own seaweed, you can easily purchase dried seaweed online or at Asian grocery stores, although it will be much more expensive that way. Read the packaging carefully. May stores will sell processed or roasted seaweed for consumption, but you will need to find 100% natural and dried organic seaweed for this project. Avoid any that have been roasted or fried, as they will be coated with oils that you would not want to add to the garden.

The first step for all of the methods below is to soak and wash the seaweed to get rid of salt deposits, stones, and shells on their surface!

Mulch & Compost

Seaweed as mulch
A seaweed mulch adds lots of valuable organic matter to the soil. Source: Rob ‘n’ Rae

Fresh seaweed can be used as a mulch at the end of the growing season. As an organic material, seaweed will break down over time and will condition the soil ahead of the following growing season. Spread a layer of soaked and rinsed fresh seaweed 1-2 inches deep across the soil surface of garden beds. Some potato growers also have found success with putting down a layer of seaweed in their potato trench followed by a layer of compost before placing the seed potato. This method ensures that as the seaweed breaks down into the soil, the nutrients are readily available near the plant’s roots.

Seaweed can also be composted and is considered a “green” material in your compost pile or compost tumbler. You may want to chop the seaweed to 1-2 inch-long pieces to speed up the composting process to a few weeks. Layer in the seaweed in your compost pile with other natural plant materials. Seaweed is a compost accelerator because it contains a lower amount of cellulose found in other plant matter. 


Liquid seaweed fertilizer is a fast-acting type of plant food and helps to make nutrients immediately available.  There are several ways of making liquid fertilizers, but some are more smelly than others. The conventional method is to fill a large bucket, such as a five-gallon bucket, with seaweed and water. Place a lid over the mixture and leave for several weeks to produce the fertilizer liquid. This is a set it and forget it method, but the smell can be very pungent. Another quicker method is to use a food processor to finely chop seaweed. Then, steep the seaweed pieces in water overnight to use immediately on indoor and outdoor plants. This solution will start to smell if left for too long. 


After the seaweed has been rinsed of the salt and other deposits, hang the seaweed to dry in the sun, or use a dehydrator on a low heat setting to completely dry it to a crisp. Dried seaweed can then be pulverized into a meal or powder by using a high power blender. Sprinkle the meal or powder directly into the soil or dilute with water. Accompany it with a dried fish fertilizer for a better plant boost.

Gardener using homemade Fertilizer on Garden Plants


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