Despite considerable research into their use over time as effective fertilisers, the mechanisms through which seaweed provides beneficial effects are still uncertain.
Seaweeds have very high potassium concentrations in comparison to other plants; nitrogen levels are similar, and phosphate levels are lower. This obviously makes them suitable for use as fertilisers in low potassium areas such as the South Downs where the (alkaline) chalk soils can lead to potassium deficiency in plants e.g. Lime-induced chlorosis in strawberry plants. They also contain important trace elements of iron, manganese, zinc, copper and boron. Seaweed improves the structure of clay soils, because the alginates in the seaweed help break up the clay and prevents it from clumping.
It’s not against the law to collect seaweed that has been washed up with the tide onto the beach, but it is against the law to harvest it from the rocks where it grows: you should NEVER cut seaweed from rocks. The best time to collect seaweed is after a storm as the newly washed up seaweed has less salt in it than the stuff that has been floating around in a harbour, or lying at the high tide mark for weeks.
Washing seaweed before use is a criminal waste of water and completely unnecessary. Despite received opinion, there is not a great amount of salt in seaweed if it’s freshly washed up on the beach; it’s the dried stuff that has been around for a while which contains extra salt as a result of exposure to sea spray from the waves.
How to use Seaweed:
You can apply it fresh directly to the soil, as a 2 to 4-inch mulch layer. You don’t introduce weed seeds with seaweed mulch. However, some members of the Chenopodiaceae (such as Chenopodium album, ‘Fat Hen’) are coastal plants and their seeds could be transferred to your allotment in the seaweed, if they’re growing near your collection site. I’d argue that this was a minor problem compared to the benefits to the soil. Don’t use on rootcrop beds as it could result in forked roots (carrots especially).
If you have ridged beds, cover them with a good layer of seaweed. This will prevent nutrients and soil being washed away by rain, and act as a weed suppressant because a thick layer of seaweed prevents light from reaching the soil and so prevents seed germination.
In your brassica beds use seaweed to mulch around the plants – apparently, as the seaweed rots it gives off boron, which can help to produce good heads on brassica crops.
Use fresh seaweed on rhubarb crowns and around fruit bushes as a mulch; this acts as both a weed suppressant and nutrient. We use it on our potato beds as both fertiliser and mulch. You could also use it on your asparagus bed, adding it in the winter, along with leaf mould; this will protect the crowns through a cold winter & feed the crowns as the spears emerge in the spring.
Seaweed is an excellent compost activator, so adding it to your compost heap will add nutrients and more organic matter. It decays quickly because it contains little cellulose – this winter, I have added a large amount to our leaf mould bin, which has rotted down faster than it normally would, and is now teaming with worms, illustrating how fresh seaweed does NOT kill your worm population.
Some gardeners use dilute seaweed solution to soak their seeds before sowing and report better germination rates.
In the summer dry some seaweed by spreading it out in the sun in a thin layer. When it’s dry, put in a large plastic bag or old baby bath and stamp on it a few times to make seaweed meal and store in an old potato bag for use later.