Seaweed products are useful products for organic gardeners wishing to avoid synthetic fertilisers or fertilisers made from animal products, providing a useful source of potassium, magnesium and trace elements. Seaweed and seaweed fertilisers are usually a sustainable, renewable resource.
Seaweed is rich in trace elements (nutrients that plants only need small amounts of), which are often lacking in common fertilisers such as Growmore and fish, blood & bone, but which are nevertheless important for plant health. However most soils have adequate levels of micronutrients. Nowadays there are a number of dried and processed seaweed available on the market. Some liquid seaweed fertilisers may be applied as foliar feeds, where the nutrients are sprayed onto the foliage and taken up through the leaves.
Fresh seaweed, or dried and processed seaweed products are not true fertilisers because their plant nutrient content is not guaranteed or standardised. However there are more and more fertiliser products available to gardeners that now contain additonal seaweed extracts.
Fresh seaweed has long been used by coastal gardeners as a soil improver and plant feed, but there is no public right to collect seaweed from the beach unless you own the beach and it is not designated a site of special scientific interest or a special area of conservation. If you plan to gather seaweed, it is always advisable to check with the land owner and the local council first.
If you do have access to fresh seaweed, it is a useful substitute for farmyard manure, and does not need to be rotted down before use. It is best dug in fresh before it has had time to dry. Although seaweed is salty, the salt is not usually present in sufficient amounts to damage crops or soil, and the salt will in any case leach out readily with rainfall as it is highly soluble.
Some seaweed products claim added benefit becaue they are said to contain natural plant growth regulators such as cytokinins.