Why does my soil go hard and crusty after rain?
Capping is raindrops smashing weak aggregates apart on bare soil — why it stops seedlings emerging, and the three changes that end it.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 3 min
Short answer
Bare soil with little organic matter has weak crumbs. Raindrops hit them, break them into fine silt, and the silt seals the surface into a crust that sets like a biscuit as it dries. Seedlings cannot push through it and water runs off rather than in. The fix is never to leave the surface bare: mulch, or cover crops, or sow into a fine tilth topped with compost.
Capping is one of those problems that looks like the soil going wrong and is actually the soil doing exactly what physics says it should. It matters most in a seed bed, where a crust forming three days after sowing is the difference between a full row and a row with six plants in it.
What you'll need
- Compost or fine leaf mould
- A watering can with a fine rose
- Fleece or netting to break rainfall (optional)
Step by step
- Confirm it is capping and not compaction.Capping is a thin crust, 3 to 20mm, that you can break with a finger, sitting on loose soil below. Compaction is a dense layer deeper down that a screwdriver will not go through. They are different problems with different fixes.
- Understand the mechanism so the fix makes sense.A raindrop hitting bare soil at terminal velocity carries real energy. On soil whose aggregates are weakly held together — low organic matter, high silt or fine sand — the crumbs shatter, the fine particles wash into the pores, and the surface seals. Drying then bakes that seal into a crust.
- Break the existing crust gently.A hand cultivator or the corner of a rake drawn across at 20mm depth. Do it when the soil is just moist, not when it is wet, and be careful where seedlings are already emerging.
- Cover the surface — this is the actual fix.Fifty millimetres of compost, leaf mould or fine bark on a bed. On a seed bed, sow into the soil and cover the drill with sieved compost or vermiculite rather than the soil you dug out. The covering absorbs the impact of the rain.
- Water with a rose, never a bare hose.An unrestricted hose is worse than heavy rain. A fine rose on a can, or a soft spray setting, breaks up the droplets and does not damage the surface.
- Build organic matter over seasons.Capping is a symptom of weak aggregate stability, and aggregate stability comes from organic matter and the fungal threads and bacterial gums that go with it. An annual compost mulch is the permanent answer and it takes two or three years to be obvious.
- Keep something growing on it.Bare soil over winter is when most capping damage happens. A green manure, or simply leaving the previous crop's roots in place, protects the surface and keeps aggregates being built.
- Reconsider your seedbed preparation.Raking soil to a fine, dust-like tilth maximises capping risk, because you have already broken the aggregates yourself. A slightly coarser seedbed with compost over the drill germinates better in a wet spring.
Tips
- Silty and fine sandy soils cap most readily, clay caps moderately, and a well-structured loam with plenty of organic matter barely caps at all.
- Fleece laid over a seedbed until germination breaks the force of rain as well as keeping birds off. It is the cheapest capping insurance there is on a spring sowing.
- Carrots, parsnips and lettuce suffer worst, because they are small-seeded, slow to germinate and weak at emergence. Sowing them slightly shallower and covering with sieved compost transforms the strike rate.
- A crust that has already formed over a sown row can be broken carefully with a hand fork and watered gently — the seedlings underneath are often perfectly alive and simply trapped.
If it doesn't work
Sowed a row, watered, then heavy rain, and almost nothing came up
Cause: A crust formed over the drill before the seedlings could emerge — Fix: Scratch the crust open along the row and water gently. Next time cover the drill with sieved compost or vermiculite rather than the excavated soil.
Water runs off the bed and pools at the edge instead of soaking in
Cause: A sealed surface with almost no infiltration — Fix: Break the crust and mulch. Until the surface is covered, this will recur after every heavy shower.
The crust is pale and almost white when dry
Cause: Fine silt washed to the surface, and on hard water areas sometimes calcium deposits too — Fix: Same answer: cover the soil. Also check that watering is not being done with a bare hose.