How do I store opened compost and fertiliser?

Compost degrades in the bag and fertiliser does not — the storage that keeps each of them usable, and the realistic shelf life of both.

Difficulty: beginner
Time: 15 min
Read: 4 min
Safety: warning

Short answer

Fold compost bags over, keep them under cover, off the ground and out of the sun, and use them within a season. Compost genuinely deteriorates — peat-free faster than peat. Fertilisers keep for years if they stay dry and sealed: granular in an airtight tub with the label, liquid in its original bottle. Damp is what ruins fertiliser, not age.

The two materials behave in opposite ways and get stored the same way, which is why people end up with rock-hard fertiliser and useless year-old compost. Compost is a living, decomposing material with a real shelf life. Fertiliser is a stable chemical that keeps almost indefinitely as long as it stays dry.

Safety

Fertiliser must stay in its original labelled container, sealed, out of reach of children and pets. Bone and blood products are attractive to dogs and a dog that eats a quantity of bone meal needs a vet. Keep fertilisers away from heat and from anything they might contaminate, never store them alongside food, and never decant them into a drinks bottle or an unlabelled tub. Bagged compost carries Legionella risk which increases with damp storage — dampen before handling and keep your face out of the dust.

What you'll need

Airtight tubs or buckets with lids
A dry, frost-free, lockable store if you have one (optional)
A permanent marker for labelling

Step by step

Fold compost bags shut and keep them under cover.Roll the top down and weight it, or tape it. Store under a bench, in a shed or under a tarpaulin — never open to the rain and never in full sun, which cooks the bag and drives the moisture out unevenly.
Keep compost bags off bare ground.On a pallet, a slab or a shelf. A bag sitting on soil wicks moisture in through any puncture and grows roots up into it from below.
Use compost within the season.The organic components continue to break down in the bag. Structure collapses, the built-in feed depletes, and wood-fibre products get worse for nitrogen drawdown. Peat-free deteriorates faster than peat did — a year-old bag is a genuinely inferior product, particularly for sowing.
Revive dried-out compost rather than binning it.Compost that has gone dusty and water-repellent is not spoiled. Tip it onto a tarpaulin, add water gradually while turning with a fork, and let it stand overnight. It takes up water again and is perfectly usable.
Decant granular fertiliser into an airtight container.A bucket or tub with a proper seal. Damp is what ruins granular fertiliser — it cakes into a solid lump that will not spread evenly. Add a silica gel sachet if the store is damp.
Keep the label with it, always.Cut the application rate and product name off the bag and tape it inside the lid, or write it on the tub with a marker. An unlabelled tub of granules is a guess, and guessing is how gardens get over-fertilised.
Keep liquid feeds in their original bottle, frost-free.Original bottle, cap tight, upright, out of sunlight. Most liquid concentrates keep for years. Freezing can separate some formulations permanently, so a frost-free store matters more than a warm one.
Store it all out of reach and out of temptation.High shelf or locked cupboard. Bone and blood products in particular attract dogs, and a dog that gets into a bag of bone meal is a genuine veterinary emergency. Never decant anything into a container that has held food or drink.
Break up caked granular fertiliser rather than discarding it.It has not gone off chemically. Break the lumps up with a trowel or in a bucket, and use it — but check it will still spread evenly, because a partly caked product applies unevenly and scorches where it lands thick.

Tips

Buy compost for the season rather than stockpiling. It is one of the few garden materials where buying in advance costs you quality.
A dustbin with a clip-on lid is the ideal compost store for opened bags — dry, rodent-proof and easy to scoop from.
Fertiliser genuinely does not have a meaningful expiry date if it stays dry and sealed. Ignore the 'best before' marketing on the pack, and judge it by whether it is still free-flowing.
Organic fertilisers such as blood, fish and bone and chicken pellets are the exception — they can go mouldy and attract vermin if stored damp. Keep those particularly dry and sealed.
Compost stored in a hot conservatory or greenhouse dries out and degrades faster than compost in a cold shed. Cold is fine; hot is not.

If it doesn't work

Granular fertiliser has set into one solid lump

Cause: Moisture got in — a torn bag, a damp shed, or an unsealed tub — Fix: Break it up and use it; it is chemically unchanged. Check it spreads evenly, and store the rest in a sealed tub with the label.

A year-old bag of compost gives poor results

Cause: Structure has collapsed and the built-in feed is gone, plus nitrogen drawdown in wood-fibre products — Fix: Use it on beds as a soil improver rather than in pots, and buy fresh for sowing and potting. Mix it half and half with fresh if you must use it in containers.

Liquid feed has separated or crystallised in the bottle

Cause: Frost, or long storage — Fix: Shake it hard and warm it to room temperature. If it will not redisperse, dispose of it responsibly rather than pouring it down a drain — check your local authority's guidance on chemical disposal.