Seaweed has traditionally been used as a fertilizer by coastal communities throughout the world. In the Channel Islands it is said to help in giving Jersey Royal potatoes their unique taste. For the organic gardener it is a boon for not only is it completely natural but it contains about twice as many trace elements as land plants and also has growth hormones, amino acids and lots of other stuff that make plants thrive. Greenkeepers have used a liquid seaweed fertilizer for years to produce healthy turf in parks, golf courses and even football pitches. So, if it's good enough for the Premier League, it's good enough for me. Having said that, some caution should be exercised when gathering it for a number of reasons. Like all natural products it is a finite resource. Only ever take dead material that has been washed up by high tides and never pull it from its anchoring rocks. Also be careful if you live near an estuary with heavy industry as it can concentrate some of the toxins and pollutants they release. In some places the gathering of even dead seaweed is an offence so don't blame me if the hand of big brother claps you on the shoulder and hauls you off to the workhouse.
My tomatoes were fed entirely on seaweed fertilizer last year and it was the first time they’ve shown no signs of magnesium deficiency. In fact, every crop I grew performed exceptionally, considering the very dry weather we had in spring and early summer. The main reason is that seaweed helps promote healthy root growth and also enables plants to quickly recover from stress. I sometimes think that, as gardeners, we spend too much time worrying about what happens above the ground and not enough about the soil itself. If the soil isn't healthy your plants won't be healthy and that's the main reason to use seaweed. It can turn even the most barren of soils into a fertile growing medium.
So, how do you apply it?
I tend to gather a couple of sackfulls at a time from the shore. One is used to make a liquid feed by simply bunging it in a plastic drum and filling with water then leaving it for a few months. Put a lid on the container because it does get a bit smelly, especially when you stir it up.
The other lot is left for a few weeks in a sack with holes in to allow the rains to wash out any salt and is then added to the compost heaps. The salt isn’t really enough to harm your plants and is beneficial to some such as brassicas. Trouble is, worms don’t like it and you don’t want to upset the worms in your compost, do you?
After a few weeks/months the liquid feed will be a black gloopy and very smelly mess. Using a garden fork take out the weed and put it on the compost heap, or directly into bean trenches, etc. The liquid is best decanted into containers with a stopper for obvious reasons. For dilution I use an old plastic two-pint jug of mixture to each two gallon watering can and top up with water. You can apply as a foliar feed or direct onto the soil.
If you can get enough seaweed it also possible to dry and burn it and use the resulting ash as a base/top dressing at similar application rates to other organic fertilizers. I should add a note of caution. Seaweed is not rich in nitrogen or phosphorous so don't be tempted to use it as your only source of plant food. As part of a balanced organic regime it can give amazing results but you will need to think about everything else that you give to your precious crops. In fact, you should think more in terms of seaweed being a soil conditioner/improver rather than a fertilizer as it helps unlock the nutrients that are already in the soil and makes them available to the plants. That it also contains many nutrients itself is a bonus.
Now, for those of you unfortunate enough not to live near the coast and who don’t fancy bringing back a car load of smelly weed from a day out at the seaside there are alternatives. A number of companies have started producing high quality seaweed-based feeds and conditioners and a quick internet search will give you all the details. Give it a try.