Seaweed as Fertiliser: What Cast Seaweed Gives Your Soil, Which Species to Use, and the Law on Collecting It

By Jayson Byles, forager & chef · September 2026

Seaweed fertiliser is seaweed used to feed soil and plants: fresh from the strandline as a mulch, rotted in the compost, dug into a trench, or steeped in water as a liquid feed. It is low in nitrogen, useful in potash, rich in trace elements, and carries the growth regulators that help seedlings root. We cut living seaweed on the Fife shore for the kitchen and for trade; what goes on our own beds is the cast weed the tide leaves behind. This is how we use it, what the trials say it does, and what Scottish law says about carrying it home.

What seaweed actually gives a garden

Start with numbers, because seaweed is usually sold on feeling. SAC Consulting, the advisory arm of Scotland’s Rural College, publishes typical values for fresh seaweed spread on Scottish grassland: about 15 per cent dry matter, and in every tonne 3.8 kg of nitrogen, 1.4 kg of phosphate and 13.5 kg of potash. Only 5 to 15 per cent of the nitrogen reaches the crop in the first year, but around 80 per cent of the potash does, with sulphur, magnesium and traces of boron, cobalt, copper, iodine, manganese, selenium and zinc. So it is a potash and trace-element manure, not a nitrogen feed. The RHS says the same from the other side: fresh seaweed and simple extracts are not true fertilisers, because their nutrient content is not standardised.

The rest of the effect is not about nutrients. The FAO’s review of traditional seaweed use puts its value on the land less in N, P and K than in soil conditioning and growth promotion: better germination, better fruit set, some resistance to pests and disease. Alginate, the gel that makes a kelp frond slippery, binds soil crumbs and holds water, and brown seaweeds carry the cytokinins, auxins and gibberellins a plant uses to decide where to grow roots.

Can I put seaweed straight on the garden?

Yes. It is the question we are asked most, and the RHS answer is plain: although seaweed is salty, the salt is not usually present in sufficient amounts to damage crops or soil, and it leaches out readily with rain. We do not rinse ours; on the east coast of Scotland the weather does it within a fortnight. If you garden somewhere very dry, or are about to put a trailer-load on one small bed, hose it down on the path first. Sand does no harm.

Timing matters more than salt. Cast weed is at its most useful in the first day or two, before it turns to slime, and it shrinks fast once spread. We use it on the vegetable beds through autumn and winter, in the bottom of the potato trench in spring, and round fruit bushes. Digging in, SAC’s farm rate of 20 to 35 tonnes a hectare is 2 to 3.5 kg per square metre. The mulching method itself, depth, timing, topping up and sowing through it, is on our practical page seaweed in the garden; this page is about what the weed is worth as a fertiliser and what the law says about taking it.

Seaweed in the compost heap

Seaweed rots quickly and carries no land-weed seeds, which makes it a good compost ingredient with one rule: mix it. The RHS advises that fresh seaweed on the heap should be mixed with woody or fibrous material, prunings or paper, because alone or with grass clippings it collapses into a wet, airless mass. We chop kelp stipes with shears, layer the fronds with brown material and add whatever is left from mulching or tea-making.

Liquid seaweed feed: what a bucket brew is worth

The home method is a bucket of chopped cast kelp or wrack steeped in water for a week or more, strained, diluted and watered round young plants. The step-by-step, dilution and the version you can drink are on our seaweed tea page, which is where we keep the recipe.

Be honest about what a bucket brew is. Commercial extracts break the cell walls with alkali or heat, and a 2026 meta-analysis found alkaline extracts watered into the soil worked hardest and simple water extracts sprayed on leaves least. Your bucket is a cold-water extract that varies by batch: potash, trace elements and some growth regulators for nothing, a tonic rather than a measured dose.

Seaweed for tomatoes, and which plants respond

Tomatoes are the classic case because they want potash once they fruit. The RHS advises a high-potassium liquid feed every 10 to 14 days once the first fruits begin to swell. Seaweed fits that job, though a home brew is weaker than a bottled tomato feed and you may want both. The tomato’s relatives respond too: a two-year open-field trial published in 2025 using a commercial Ascophyllum nodosum extract raised pepper yields by 28 and 46 per cent and aubergine by 81 and 108 per cent over two seasons, mostly by setting more fruit per plant. That was a standardised product on a research farm, not a bucket.

Seedlings are the other place we see it work. A 2025 study in the Journal of Applied Phycology made water extracts from cast UK Ascophyllum nodosum, Fucus serratus and Fucus vesiculosus, our wracks, every month for a year and tested them on lettuce, beetroot and a model plant. Diluted 1:50 they gave significantly heavier seedlings than water, and the strongest extracts came from weed collected between May and August. That is greenhouse work, promising rather than proven, but done with the species that lie on the Fife strandline.

Which seaweed for which job

Everything below is cast weed; we never cut living seaweed for the garden. The kelps, oarweed and sugar kelp, come in after autumn and winter storms and are richest in alginate and potash. The wracks make up most of the wrack line all year and rot slowest. Sea lettuce and gutweed pile up green in summer and vanish into the heap in days. Sea spaghetti comes ashore in autumn when the year’s fronds break away. The reds, dulse and carrageen, are rarely cast in amounts worth carrying.

The law on collecting seaweed from a Scottish beach

Modest amounts of cast seaweed for your own garden are a centuries-old Scottish habit, resting on custom and tolerance rather than a written right. Our plain guide to the law on picking seaweed in Scotland has the detail; for a gardener these are the points.

• The foreshore has an owner. The strip between high and low water belongs to someone, most often Crown Estate Scotland, sometimes an estate or council. Access rights let you walk it; the Scottish Outdoor Access Code is clear that taking anything away for commercial purposes or profit is excluded from access rights, and that customary picking for your own use is simply not covered by the legislation either way.
• Personal use is tolerated; trade is licensed. Crown Estate Scotland licenses seaweed harvesting on Crown foreshore and seabed, with the licence type set by the volume you expect to take, once NatureScot confirms there is no unacceptable environmental risk. That is the side we work under. A few sacks for the compost heap is not what the licence is for.
• Crofters have a specific right. NatureScot’s seaweed harvesting advice notes that traditional gathering of beach-cast seaweed by crofters needs no landowner permission, under the Crofters (Scotland) Act 1993.
• Protected sites and vehicles. Removing seaweed inside a Site of Special Scientific Interest may need NatureScot’s consent; signage wins. NatureScot also says vehicles do not belong on the foreshore.
• South of the border it is stricter on paper. The RHS states there is no public right to collect seaweed from the beach unless you own it.

Then the ecology. The Scottish Government’s 2016 environmental assessment of wild seaweed harvesting records that beach-cast seaweed feeds dune habitats, which stabilise sediment and protect the coast; it also feeds the sandhoppers that feed the shore birds. So we never clear a beach: take a third of a patch and move on, pick out the plastic, carry it in onion sacks or buckets. The respect we bring to cutting living seaweed applies to the dead.

A very old habit

None of this is new. Coastal farmers along Europe’s Atlantic edge have spread seaweed on their fields for as long as there are records, the FAO notes, and in Scotland it shaped the landscape. The lazy beds, feannagan, that still ridge the hillsides of the west Highlands and Islands were built by turning sods over a layer of manure, and near the sea that manure was seaweed; the potato that made them essential reached the Highlands in the 1740s. On the Fife coast the same weed was burned for soda ash rather than spread, a story told in full in the history of seaweed in Scotland. The Scottish Parliament’s 2018 briefing on kelp records that storm-cast kelp is still gathered for the machair soils of the Uists today.

The bottle came later. A British chemist, Reginald Milton, began experimenting with liquid seaweed extract in the late 1940s and set up the first manufacturing plant in Slough in 1950, the start of Maxicrop; by the mid-1960s, the FAO reported, the UK was making around 250,000 gallons of liquid seaweed a year.

What the research says now

Seaweed biostimulants now have a proper evidence base, most of it aimed at farms. What matters to a gardener, with the evidence level stated plainly:

• Our own wracks. The 2025 UK wrack-extract study above: heavier lettuce and beetroot seedlings from cast Fucus and Ascophyllum, strongest from summer weed. Evidence: greenhouse and growth chamber.
• Fruiting vegetables. The 2025 pepper and aubergine trial: two seasons in the open field, large gains from a commercial extract. Evidence: field trial, one site.
• The farm-scale evidence. The 2026 meta-analysis of 198 studies, the James Hutton sugar-kelp project, the WWF-UK barley trial and SAC Consulting’s grassland guidance are set out, with their evidence levels, on seaweed on the farm.

The pattern is consistent: real, moderate gains, most reliable from standardised extracts, with plenty of variation. A gardener with a free strandline can count on the soil-building and the potash, and on the growth-regulator bonus some of the time. For the wider story, from wound dressings to cattle feed, start with the other uses of seaweed; the farm-scale version of this page is seaweed on the farm, and the whole shore is in our complete guide to seaweed foraging in Scotland.

Sources

• Use of seaweed as a fertiliser for grassland (Farm Advisory Service / SAC Consulting, PDF)
• Seaweed: the free fertiliser (SAC Consulting, SRUC, 2026)
• Seaweed products for gardening (Royal Horticultural Society)
• Tomatoes: grow your own (Royal Horticultural Society)
• Traditional uses of seaweeds: fertiliser and animal feed (FAO)
• Crop yield responses to seaweed extract-based biostimulants: a meta-analysis (Frontiers in Plant Science, 2026)
• Plant seedling growth and soil respiration responses to seasonal United Kingdom seaweed wrack extracts (Journal of Applied Phycology, 2025)
• Sustainable agriculture through seaweed biostimulants: a two-year study in pepper and eggplant (Frontiers in Plant Science, 2025)
• New project considers the role of seaweed to improve crop fertiliser use (James Hutton Institute, 2025)
• Insights from the WWF-UK seaweed biostimulant project (FAIRR, 2024)
• Licensing of wild seaweed harvesting (Crown Estate Scotland)
• Collecting wild food (Scottish Outdoor Access Code)
• Advice for the sustainable harvesting of seaweed (NatureScot)
• Wild seaweed harvesting: strategic environmental assessment (Scottish Government, 2016)
• Kelp harvesting, SPICe briefing (Scottish Parliament, 2018)
• Rediscovering lazy beds (High Life Highland Ranger Service)
• The history of Maxicrop (Maxicrop UK)

Questions we get asked

Can I put seaweed straight on the garden?

Yes. Fresh cast seaweed can go straight on as a mulch or into a trench; the RHS notes the salt is not usually enough to harm crops or soil and washes out with rain. Spread it 10 to 15 cm deep away from stems, let rain work on it for a fortnight before sowing into that ground, and top it up as it shrinks.

Do I need to wash the salt off seaweed before using it?

Usually not. Salt is highly soluble and leaches away in the first rain, and rinsing also washes out some of the potash and soluble compounds you wanted. Rinse only if you garden in a very dry climate or are spreading a very large quantity on one small bed; in Scotland the weather does it for you.

Is it legal to take seaweed from the beach in Scotland?

Modest amounts of cast seaweed for your own garden are a long Scottish custom and are generally tolerated, but the foreshore always has an owner, usually Crown Estate Scotland, and access rights do not include taking things. Commercial harvesting needs a Crown Estate Scotland licence. Avoid protected sites, never take vehicles onto the beach, and never strip a strandline bare.

How do I make seaweed fertiliser at home?

Chop fresh cast kelp or wrack, put it in a lidded bucket, cover with water and leave it for at least a week. Strain and bottle, keep it cool and dark, and use within three months. Dilute a part of water for each week brewed, up to one part in ten, test on one plant, then water round roots or spray on leaves.

Is seaweed good for tomatoes?

Yes, as a potash feed once the fruit starts to swell, which is when the RHS recommends a high-potassium liquid feed every 10 to 14 days. Around 80 per cent of seaweed’s potash is available at once. A home brew is weaker and less consistent than a bottled tomato feed, so many gardeners use both, and a seaweed mulch under outdoor plants holds moisture through dry spells.

This is practical shore and garden lore, not legal advice. Foreshore ownership, protected sites and local byelaws vary; if you are collecting in quantity, or anywhere with signage, ask the landowner or Crown Estate Scotland first.