Seaweed has been used as a soil improver for centuries, particularly in coastal areas. Seaweed contains several useful plant nutrients, including nitrogen, potassium, phosphate and magnesium. There are dried and liquidised forms available from garden centres and seaweed is a common additive to fertilisers, both organic and non-organic.
- Seaweed is a good source of plant nutrients
- Processed seaweed extracts are sold in dried or liquid form
- Fresh seaweed can be dug into the soil like manure
- Calcified seaweed makes the soil more alkaline
Why use seaweed products?
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) including iron, manganese, zince, copper and boron, 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.
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 because they are said to contain natural plant growth regulators such as cytokinins. These act as a growth stimulant.
When to use seaweed products
Dried or processed seaweed products can be used much as you would any other plant tonic, when the plants are actively growing (usually between March and September). These products are available in liquid or solid form, so always read the label to find out the best way of application.
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 in winter or spring before it has had time to dry.
If dug in fresh, then seaweed is used like any other soil conditioner, digging it in to one or two spades’ depth below the surface and using up to a barrow load per square metre (if you have this much available).
If placed on the compost heap, fresh seaweed should be mixed in with woody or fibrous material (prunings or paper for example). It can become rather slimy and gelatinous on its own or when mixed only with kitchen waste or lawn clippings.
Dried seaweed products include seaweed meal (crushed and dried fresh seaweed), powdered seaweed extract (produced by boiling the seaweed and evaporating off its liquid content, leaving a concentrated solid extract that is powdered), and liquid seaweed extract (produced from fresh seaweed by water extraction).
Fortified seaweed extracts (usually liquid) contain added nitrogen, phosphorus and potassium and quote an NPK ratio on the product label. See our page on fertilisers for an explanation of what this ratio means.
Many popular fertilisers (such as Tomorite) are now available with added seaweed extract, so you can add the purported benefits of seaweed at the same time as a fertiliser application.
Calcified seaweed is produced from naturally occurring beds of calcified and coralline algae. Some sources are more sustainable and renewable than others, so it is worth reading the product information or phoning the manufacturer for details of provenance. When used in the garden, calcified seaweed is an alternative to garden lime, with added trace elements and plant nutrients. It is usually more costly than garden lime.
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