Seaweed Fertiliser - Gardening Australia (2024)

Seaweed is a wonderful fertiliser, a great soil builder and an excellent compost activator. All in all, seaweed is terrific stuff for the garden. There's a long tradition of seaweed being used as a fertiliser to improve crop production. For example Celtic and Scandinavian farmers have put it onto their fields for centuries.

Kelp is one of many different types of seaweed. One type is powdered kelp. It is convenient for adding to the garden. And what is it about seaweed that makes it such a good fertiliser? Seaweed contains complex carbohydrates and these really get the soil humming with life. This has two really important functions for the garden. Firstly, it stimulates the microbial fungi in the soil and these assist plants in their uptake of nutrients. They also assist in defending plants from soil borne diseases. So adding seaweed fertiliser helps crop protection, and plant nutrition

Of all the fertilisers, seaweed has the broadest and most balanced range of nutrients, to promote early flowering and cropping and increases the sugar content of fruit. All in all, it's very good stuff.

An extract from seaweed is Algin. It's sold in the shops as agar agar. Add it to water and make a liquid paste. Pour it onto the soil and it acts like a natural wetting agent. Excellent for sandy soils.

Land plants have cellulose, which thickens their cell walls and allows them to resist gravity, and they can grow upright. Seaweed is supported by water and has no need for cellulose. In a compost heap, that means seaweed breaks down really quickly. It activates the compost heap. But powdered kelp works just as well.

Seaweed also comes in a liquid form, so spray it on plants and they take in the food directly through leaves. Just ensure it's diluted as recommended, because it's really strong stuff and can burn seedlings and roots.

Another benefit of using seaweed fertiliser over time is it acidifies and adds iron to the soil, which is great news if you are growing acid and iron hungry plants like gardenias, camellias, azaleas and rhododendrons. It's great on native plants as well. If you use seaweed fertiliser on native proteaceae, like banksias and grevilleas, just make sure that it hasn't been fortified with phosphorous, because that can do some harm.

Seaweed contains natural plant hormones, so it's really useful in preventing transplant shock whenever you move a plant around the garden. It's also useful for improving the germination of seeds. For example peanuts, which have a large seed, should be soaked for 24 hours in seaweed fertiliser for a good germination rate. Seaweed also helps to improve the thickness of plant cell walls. This makes them much more resistant to pest and disease attack and also improves frost resistance.

Seaweed is easy to use. Apart from the powdered form and the liquid concentrate buy it in an odourless granular form, ideal for house plants. It's even available in a form that will clip onto the hose and will feed the entire garden. You may be wondering whether it's possible to make your own seaweed fertiliser. The answer is that you can, but it's important to first check with your local authority, because collecting seaweed in many parts of the country is illegal and removing it can damage sensitive ecosystems. But using the normal commercially available products can produce pretty good results.