There is something quietly satisfying about turning the tide's leftovers into next year's runner beans. Seaweed is one of the coast's most generous free resources: it arrives in drifts after every autumn gale, it is packed with trace elements, and it conditions heavy or sandy soil beautifully. The catch is that it is wet, salty and prone to turning into a black, whiffy sludge if you simply tip it in a heap. Get the balance right with dry garden material, though, and you will have a first-class soil improver that neither smells nor annoys the neighbours.

Why seaweed earns its place in the heap

Fresh seaweed is roughly 80 to 90 per cent water, so it behaves more like a soil conditioner than a concentrated fertiliser. It brings potassium, magnesium and a long list of trace elements, feeds the worms and soil microbes, and helps sandy coastal soils hold on to moisture. It also breaks down quickly once mixed, which makes it a useful activator for a slow, autumn-heavy compost heap.

Before you fill a sack, check the rules where you live. Loose seaweed washed up on the strandline can usually be collected in small quantities for personal garden use, but byelaws vary from one council to the next, and nature reserves, Sites of Special Scientific Interest and other protected coastal sites almost always prohibit removal. Ask your local authority or the landowner if you plan to take more than a bucketful or two, and never pull living seaweed off rocks or cut it below the waterline. Take only what has already been washed ashore, and leave plenty behind for the creatures that depend on it.

Getting the ratio right: seaweed versus garden waste

Think of seaweed as a very wet "green". It is rich in nitrogen, the same as grass clippings or kitchen peelings, but it comes with a great deal of water attached. That means it must be partnered generously with dry, carbon-rich "browns" to keep the heap aerated and sweet.

As a rough guide, use one part seaweed to two or three parts browns by volume. Good browns for a coastal garden include:

• Shredded cardboard, egg boxes and plain newspaper
• Twiggy prunings and hedge clippings, chopped small
• Dry autumn leaves, saved in bags for summer use
• Straw, woodchip or sawdust from untreated timber
• Dead, woody stems from the flower border

If your grass clippings are also going in, hold some back. A heap of clippings and seaweed together has almost no structure and will go anaerobic within days.

Collecting and preparing your seaweed

Quality matters more than quantity. Lift fresh, springy weed from the top of the strandline rather than the dark, matted sludge at the bottom, which is already rotting and often full of sand and grit. Shake each clump, pick out fishing line, plastic fragments and the odd crisp packet, and be selective about what you take home.

Salt is the main concern for garden use, though it leaches out far more readily than most people fear. You have two simple options:

• Rinse the seaweed in a bucket of water, then tip the salty water on paths or gravel rather than treasured borders.
• Spread it thinly on a paved area or tarpaulin and let a fortnight of British rain do the job for you.

Chop thick kelp stipes and holdfasts with garden shears or a spade so they break down at the same pace as the leafy fronds. Leaving the seaweed to wilt for a few days before it goes on the heap also cuts down the slime factor considerably. Shells and small stones are harmless — they simply add lime.

Building a heap that works

Site your bin or heap on bare soil or paving, well away from the house, a neighbour's fence or a boundary wall. Then build in layers:

• A 15cm base of twiggy prunings or woodchip for drainage and airflow
• A 10cm layer of wilted seaweed
• A 20 to 30cm layer of mixed browns
• A thin scatter of garden soil or finished compost to introduce microbes

Repeat these layers as material accumulates, finishing with browns on top. Aim for the texture of a wrung-out sponge: if you squeeze a handful and water drips, add more cardboard; if it crumbles dry, add a little water or more seaweed. Turn the heap every two to three weeks, which mixes the salty, nitrogen-rich weed through the drier material and keeps air moving. A well-built heap will reach 55 to 60°C in the middle, and seaweed compost is usually ready in three to six months.

Keeping it sweet and neighbour-friendly

Almost every composting complaint comes down to one of three things: too wet, too much green, or food waste on the surface. Seaweed can trigger all three if you are careless, so a few habits are worth adopting.

• Bury seaweed in the middle of the heap; never leave it draped on top where gulls and flies will find it.
• Use a lidded bin, and keep the lid on in heavy rain so the heap does not become a bog.
• Add a handful of shredded cardboard whenever you notice a sour or ammonia smell, then turn the heap.
• Keep cooked food, meat, fish scraps and dairy out altogether — they attract rats.
• Position the bin at least a few metres from boundaries, patios and open windows.

If the heap does start to pong, it is usually a sign of compaction rather than a lost cause. Fork it over, mix in dry browns and give it a week. A faint earthy smell is exactly what you want; a sharp, sour or rotten-egg smell means more air is needed.

Putting your seaweed compost to work

Once dark, crumbly and smelling of woodland floor, your compost can be spread as a 5cm mulch around shrubs, fruit bushes and perennials, keeping it clear of stems. It is especially welcome on light coastal soils that drain too fast, and on hungry crops such as potatoes, courgettes and brassicas. For a mid-season boost, steep a sack of fresh seaweed in a bucket of water for two to three weeks, then dilute the resulting dark liquid one part to ten with water and use it on tomatoes and containers.

Seaweed is not a complete fertiliser, so treat it as a soil conditioner and a source of trace elements rather than a replacement for your usual feeding regime. Used in moderation, balanced with plenty of dry garden waste and sited thoughtfully, it will quietly improve your soil year after year — proof that the sea really does give something back to those who live beside it.