Best seaweed fertilizer: reviews and buyer’s guide (updated for 2023)

I highly recommend seaweed fertilizer, whether you’re growing in containers or in the ground. It’s packed with micronutrients, completely natural and is a sustainable, harm-free way of adding plant-beneficial goodies to your soil. Alys Fowler says that if she could only choose one type of fertilizer, she would pick seaweed!

It’s part of my essential kit for gardening in containers and I wouldn’t go without it. I prefer the calcified version that can be mixed into the top few inches of a pot, and which breaks down over time, but there are lots of other alternatives. Many gardeners favour the liquid version.

In this article I want to offer some general information about using seaweed fertilizer as well as tell you about my favourite picks from around the web. Let’s dig in.

The short version: best seaweed fertilizer

Click here to jump to our reviews of some of the best seaweed fertilizers from around the web.

Seaweed Fertilizer	Image	Price	Rating
Down to Earth Kelp Meal		$$	10/10
Neptune's Harvest Liquid Seaweed Fertilizer		$$	10/10
Sea Magic Dry Soluble Seaweed Extract		$$	10/10

What is seaweed fertilizer?

Seaweed fertilizer is usually a kelp-derived soil-additive that is rich in micronutrients (it also has NPK content). It has been used for a very long time, especially by farmers that live near the coast, and can be prepared in a number of ways. You can even add kelp directly to the soil (but it might take a while for it to break down).

Generally speaking, there are four main ways you can buy seaweed fertilizer:

• Liquid seaweed – Liquid seaweed is probably one of the best options for container growers because applying liquid feed will mean that plants can access nutrients quickly. It is produced by soaking and straining kelp in water.
• Seaweed/kelp meal – Seaweed meal is made by drying the seaweed and, in the case of powder, crushing it.

Liquid plant food is a good option for container growers.

• Seaweed extract powder – Powder is usually made by heating seaweed so as to get rid of all the water content. What is left is a solid, highly potent form of feed.
• Calcified seaweed – Calcified seaweed isn’t made from kelp. It consists of various types of calcified coral and certain types of algae that are harvested from the sea. It consists of around 50% calcium along with the usual minerals so beneficial to plant growth. A lot of the nutrients are directly available to plants (they don’t have to be broken down by bacteria).

The case for container gardening

As I’ve already mentioned, I like seaweed fertilizer. It’s not synthetic and it doesn’t include any animal products. It’s a real godsend for container growers too. A lot of the popular commercial feeds don’t contain a significant amount of trace elements. Because container growers often need to rely on liquid feed, it’s important that they find a way of adding micronutrients.

Here are some general tips to make sure that your container plant feeding is successful:

• Use seaweed fertilizer in addition to a normal balanced fertilizer. It doesn’t contain hefty amounts of the main plant nutrients (nitrogen, phosphorous and potassium) so needs to be supplemented.
• I don’t follow any hard and fast rules when it comes to the amounts of granular fertilizer I apply. Usually a small handful suffices, sprinkled around the plant and forked in.
• Apply liquid fertilizer weekly through the growing season. For herbs and perennials tone it down to bi-monthly. Feed monthly or not even at all through winter and at half-solution through early spring/late autumn.
• You can buy fortified products, where extra NPK is added. If using a feed of this type, you don’t need to supplement with anything extra.
• Always check that your fertilizer comes from a sustainable source if that’s something you’re concerned about. It usually does but there’s no harm in double checking.
• Personally, I don’t bother with foliar application or misting. The only exception to this is in an emergency when one of my plants is showing signs of nutrient deficiency.
• I also tend to advise the use of liquid seaweed for container growers because kelp meal needs to be broken down by bacteria in the soil, which often aren’t present to the same extent in container media.

What about “making” your own?

Seaweed isn’t just good for plants. It seems that it’s healthy for anybody (or anything) able to get their hands on it! But seeing as the focus of this site is plants, I’d better stick to that application…still send in the recipes if you have any!

Seaweed breaks down easily in the soil, supporting microorganisms that assist in its decomposition in the process. I have to confess that I don’t know that much when it comes to ethically and sustainably collecting seaweed for use in your garden. If you know of anywhere that grows it for commercial use, I’d be tempted to use them. My understanding is that it’s OK to collect seaweed from the shore that has washed up. Storms apparently tend to dredge up huge quantities of the stuff.

You can of course always add it to your compost heap too! You can add partially rotted-down seaweed to the top of pots for a quick boost, where it will continue to break down.

Best seaweed fertilizers: Some picks from around the web

***Full Disclosure – Where appropriate, I’ve linked product images to commercial sites and Amazon (see Amazon Disclosure). If you buy from these sites, I earn a small affiliate fee, which helps me keep Urban Turnip going.***

Down to Earth Kelp Meal (1-0.1-2)

Down t0 Earth’s kelp meal, which is harvested from the North Atlantic Ocean, is hand-dried before being crushed into powder. As you can see from the nutrient ratio in the title above, it is rich in both nitrogen and potassium. The product is also completely organic.

This fertilizer is available in three different weights  – 5 pounds, 20 pounds and 50 pounds – and should be applied to soil at an amount of 1 to 2 pounds per 100 square feet. If you are using it for container plants you can always soak  a handful or so of the dried seaweed in water and apply it as a liquid feed.

Neptune’s Harvest Liquid Seaweed

Neptune’s Harvest is a well-known and respected manufacturer of organic fertilizer and this option is a good choice for container growers. Nutrients in liquid feed are more readily available to potted plants. It has a 0-0-1 NPK ratio, so is best used as a micronutrient feed in conjunction with a balanced fertilizer.

You can buy it in three sizes – 18 ounces, 32 ounces and 128 ounces – and one tablespoon should be added to a gallon of water. The bottle should last for a reasonable amount of time depending on how much you’re watering. All in all this is a great product and I would highly recommend it as an inexpensive option.

Sea Magic Dry Soluble Seaweed Extract 

This extracted soluble seaweed fertilizer from Sea Magic is a good option for both container and soil plants. The entire pack results in about 66 gallons/25o litres of liquid feed (which is exactly the same as the liquid feed above) so the choice is up to you. If your uncertain, my advice would be to experiment with both types of fertilizers and see which works best for you. Reviews have been positive all round so it’s definitely worth giving a go!

Let us know your thoughts! Leave a comment below!

What are your thoughts? Have you tried any of the fertilizers above? Leave a comment below and let me know!

Image credit: Kelp by Mariell Jüssi.