How do I relieve compacted soil?

Three different depths of compaction and the different tool each one needs — plus the biological route that fixes it permanently while the mechanical one only buys time.

Difficulty: intermediate
Time: 3 hr
Read: 4 min
Safety: caution

Short answer

Match the tool to the depth. Surface capping: a light forking and a mulch. Compaction at 100 to 200mm: drive a fork in fully and lever without turning the soil over, on moist soil in autumn. A deeper machinery pan: a broadfork, or double digging one strip if the area is small. Then keep off it, keep it covered, and sow deep-rooted green manures — that is what makes it stay fixed.

Do this first

How do I tell if my soil is compacted?

Breaking a pan mechanically takes a day and lasts about three years if nothing changes. What actually keeps soil open is roots and worms, working continuously, and they only do that if there is organic matter to feed them and nobody standing on the ground. The mechanical job is the start of the fix, not the fix.

Safety

Check for buried services before driving anything deep into ground you do not know — electricity supply cables, water pipes and gas services often run at 400 to 600mm and are shallower than that on older properties and on garden extensions. Levering a long fork is hard on the lower back; work in short sessions and let the tool do the leverage rather than your spine.

What you'll need

A garden fork, or a broadfork for deeper work
Compost or well-rotted manure
Scaffold boards (optional)
Green manure seed — fodder radish, rye or vetch (optional)
A cable and pipe detector (optional)

Step by step

Find the depth first.Everything below depends on it. A rod pushed in by hand tells you whether you are dealing with 40mm of capping or a 250mm machinery pan, and those need completely different work.
Work on moist soil, in autumn if you can choose.Wet soil smears and you will make it worse. Bone-dry soil will not let a fork in at all. Autumn is ideal because winter weather then works on the cracks you have opened, and nothing is growing.
For surface capping, fork lightly and cover.Break the crust 50mm deep with a fork or a hand cultivator, then mulch 50mm deep with compost. The mulch is the actual fix — capping happens because rain hits bare, low-organic soil directly.
For compaction at 100 to 200mm, lever without inverting.Drive the fork in vertically to its full depth every 200mm across the area, and pull the handle back perhaps 30 degrees to crack the soil, then withdraw. Do not lift and turn. You are fracturing the layer, not digging.
For a deeper pan, use a broadfork or dig one strip.A broadfork reaches 300 to 400mm and levers with two hands and body weight, which is far easier on your back than a garden fork. Where a broadfork will not reach, double digging a single strip and breaking the base of the trench with a fork does the job on a small area.
Work in wide passes, not planting holes.Relieving compaction only under each new plant creates a sump: an open pocket in a sealed layer, which fills with water and drowns the plant. Treat the whole bed or none of it.
Get organic matter onto the surface immediately.Fifty millimetres of compost. It feeds the worms whose channels will keep the soil open, and it stops the surface capping again over winter while nothing is growing.
Sow a deep-rooted green manure over the treated ground.Fodder radish and tillage radish drive a thick taproot straight through compacted layers and rot in place, leaving a channel. Rye and vetch do a shallower version. This is the biological fix and it works while you are not there.
Change what caused it, or it returns.Permanent paths, beds no wider than about 1.2m so you never step on them, a scaffold board when you must cross in winter, and no rotavating. Without this, you will be doing the same job again in three years.

Tips

A broadfork is the single most useful tool for this job and almost nobody owns one. Several allotment sites have one to borrow.
Never rotavate to relieve compaction. It loosens the top 150mm beautifully and creates a fresh smeared pan immediately below the blades, which is worse than what you started with.
On a lawn, hollow-tine aeration removes cores and genuinely relieves compaction; solid spiking mostly compresses the sides of each hole and does much less.
Do not attempt to relieve compaction under an established tree by forking. Use compressed-air soil decompaction through a specialist, or accept it — fork tines through structural roots do real damage.

Common mistakes

Turning the soil over while forking — Inverting the profile buries the topsoil and its biology and brings subsoil and weed seed up. The aim is to crack the pan, which needs leverage and not inversion.
Working wet clay — You compact and smear it further with every footstep and every tine. Wait for it to crumble rather than smear, even if that means waiting a month.
Fixing the soil and then walking on it again — Freshly loosened soil compacts more easily than untouched soil, not less. The traffic change has to happen at the same time or the work is wasted.

Questions people ask

How long does the mechanical fix last?

Two to four years if nothing else changes, and indefinitely if you combine it with annual organic matter, deep-rooted plants and no traffic. Soil re-compacts under its own weight and under rain unless roots and worms are actively keeping it open.

Do gypsum or 'clay breaker' products relieve compaction?

No. Gypsum flocculates sodium-affected clays, which are rare in British gardens, and does nothing mechanical. Compaction is a physical problem and needs a physical or biological answer.

Is it worth hiring a machine?

For a large area with a deep pan, a subsoiler behind a compact tractor is genuinely effective and there is nothing a gardener can do by hand that matches it. For an ordinary garden bed, a fork and a season of green manure is the proportionate answer.