Seaweed fertilizers | Gardening Tips and Guides
Seaweed has been used as a manure and fertilizer for centuries in seaside areas and now branded products derived from seaweed have become generally available all over the world.
Like all plant materials, seaweed contains nutrients. But since seaweeds are rather unusual plants – they’re actually algae -and since they grow in a rather unusual environment, it is not surprising that they contain a rather unusual blend of nutrients.
Seaweed contains all three of the major plant foods – nitrogen, phosphate and potash. It contains about 1 per cent of potash, rather more than most composts and for this reason can be valuable on flowering and fruiting plants. One popular brand of seaweed-derived fertilizer is recommended especially for tomatoes, which have a high potash requirement.
On the other hand, seaweed has precious little phosphate, the nutrient that promotes healthy roots.
Seaweed also contains fairly high levels of trace elements such as iron and magnesium. Many gardeners feel that this confers special advantages, but. since most soils aren’t short of trace elements anyway, there is little evidence for the belief. On balance, therefore, seaweed makes a useful fertilizer, but by no means an exceptional one. You’ll have to decide for yourself whether its potash and its trace elements are worth paying more for.
Not surprisingly, considering its origin, fresh seaweed also has fairly high levels of sodium and chlorine, the chemical raw materials of common salt. That’s fine if you want to grow the very few plants which need plenty of sodium – sugar beet, for example -but it can be a problem in gardens. The solution is to compost any fresh seaweed you buy or collect: partly to allow the salt to be washed out by rain: partly to ensure (as with any fresh organic material) that the soil is not depleted of nitrogen while it rots down: and partly to avoid the swarms of flies that always seem to congregate whenever piles of seaweed are left uncovered.