Seaweed extract

Comparing fertilisers

I recently began an experiment with fertilisation.  I originally wanted to compare the performance of seaweed extract against  Chilli Focus and tomato feed. My aim was to conduct side by side tests with three sets of African devil seedlings, each consisting of three plants in separate trays.  Each tray was to have received only of each type of fertilizer until it came to the time for the seedlings to be hardened off.   I would then make a judgment on which fertilizer had done best

I thought I had all the bases covered. One set of seedlings, which I considered mycontrol group, was to get Chilli Focus diluted at 2.5 ml per litre of water. The second set was set to be fed at a dosage of 2ml per litre of water.

The only decision I needed to make was how much the seaweed extract needed to be diluted by.

Normally I would have used the NPK of a fertiliser to do this, but I was unable to find out what the NPK for seaweed extra is. The NPK was not on the packaging, nor on the supplier’s website.

What I did find on the website was a recommendation that the extract should be applied at two percent per litre of water for Chillies, vegetables, herbs, tomatoes, potatoes. It recommended feeding these plants at this application rate every ten to fourteen days.

However, that’s all it said. It doesn’t mention whether this dosage applies to young plants, mature plants, or anything else.  I believed it was important to have this information to make the comparison. I had the NPK’s of the Chilli Focus and tomato feed. The only outstanding piece of information was the NPK of seaweed extract

Is it a fertiliser?

Growth enhancer

So, I decided to do some research on the web. What I found made me change my mind in using seaweed extract in the comparison at all.  It seems that trying to compare seaweed extract against Chilli Focus, and tomato feed, was like comparing chalk and cheese. Here’s why

I found that seaweed extract has a very low NPK value.  The value varied from source to source, but seems to be in the region of 0-0-0.5. However, according to what I read, this doesn’t seem that important. Rather, what came through loud and clear was that nutrients should be considered to be  the main reason for using seaweed extract. It should not be seen as a primary source of nutrients, but rather as a growth enhancer (bio stimulant) and tonic for plants.

Seaweed extract contains minerals, vitamins, and enzymes that are natural growth stimulants. . They are nutrients that  facilitate cell growth, which leads to stronger, healthier and more disease resistant plants

Also, seaweed extract makes nutrients found in soil fertilizers more available to plants.  It facilitates higher nutrient uptake. Seaweed stimulates bacteria in the soil to decompose soil particles into simpler nutrients that the plant can more readily absorb. Chelating compounds in seaweed latch onto these nutrients, preventing them from leaching out of the soil, but yet readily release them to plants. Seaweed can be compared in this way with worm castings. Worm castings are not a good source of nutrients, but they can make nutrients more available to plants.

Seaweed also contains chemical compounds that combat viruses, bacteria and fungi. When seaweed extract is sprayed on plants, they absorb these compounds and use them for the same purpose.


Given that seaweed extract is not a fertiliser in itself, I decided that I will no longer use it for that purpose in my experiment. Instead, I will use it as a supplement to Chilli Focus. This means that instead of a batch being tested using seaweed extract alone, I will now decide whether seaweed extract enhances the performance of Chilli Focus. This will be done using the control batch. The comparison objective of the tomato feed remains the same

Seaweed extract contains auxins that can improve the germination rates of seeds. To do this, add a quarter cup of seaweed extract to one cup of water. Soak the seeds for twelve hours, and then plant directly afterwards.


Image credit

Ardfern / CC BY-SA 3.0 / via Wikimedia Commons

By Grubio–1 – Own work/ CC BY-SA 4.0 / via Wikimedia Commons


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