How do I prepare soil for planting?

Soil preparation is the part of gardening with the largest effect and the least attention. It is also where the received advice — dig it over every year — has changed considerably.

Safety

Step by step

1. Find out what soil you have.Squeeze a handful of damp soil. Sticky and holding its shape means clay; gritty and falling apart means sand; holding loosely and crumbling means loam. Everything else follows from this.
2. Check the drainage.Dig a hole a spade deep, fill with water and see how long it takes to drain. More than a few hours means a drainage problem that adding compost alone will not fix.
3. Test the pH if things have failed before.A cheap kit. Most plants want 6 to 7. It explains why acid-loving plants such as rhododendrons fail on chalk, and it is worth knowing once rather than guessing repeatedly.
4. Clear the perennial weeds first.Roots and all, before adding anything. Compost over bindweed or couch grass feeds it, and it is far harder to remove once planting is in.
5. Add organic matter — this is the main job.Five centimetres of garden compost, well-rotted manure or leaf mould. It opens up clay, helps sand hold water and feeds soil life. It improves every soil type, which almost nothing else does.
6. Spread it on top rather than digging it in.Worms take it down and the soil structure stays intact. Digging breaks up the structure, brings weed seed to the surface and releases carbon. This is the biggest change from older advice.
7. Do not dig unless you must.Breaking new ground or relieving serious compaction justifies it. Annual digging of an established bed does more harm than good.
8. Never work wet clay.It compacts into something close to brick and takes a season or more to recover. Wait until it crumbles rather than smears.
9. Use a board to spread your weight.Standing on a plank rather than the soil avoids compacting it. Ideally arrange beds so you never need to stand on them at all.
10. Add grit only for specific plants.Mediterranean herbs and alpines need sharp drainage. Adding a little sand to clay makes it worse, not better — it needs a lot of grit or none.
11. Warm the soil before sowing.Cover with black plastic or a cloche for a couple of weeks in spring. Seed sown into cold wet soil rots rather than germinates, and this is the usual cause.
12. Rake to a fine tilth for seeds.Small seeds need a crumbly surface — break the clods and level it. Larger plants and transplants are far less fussy.
13. Let it settle before planting.A week or two after preparing, so the soil settles and any remaining weed seed germinates and can be hoed off before you plant.
14. Keep adding organic matter every year.It is a continuing process rather than a one-off. A mulch each autumn or spring maintains what a single application starts.

Tips

• Organic matter is the answer for nearly every soil problem — heavy clay, thin sand, poor drainage, low fertility. Spread it on top and let the worms work.
• The squeeze test takes ten seconds and determines most of what you should do afterwards.
• Never walk on wet clay. Damage done in one afternoon takes a season to undo.

Common mistakes

• Digging over an established bed every year — It breaks up soil structure, brings weed seed to the surface and releases carbon. Spreading compost on top and letting worms incorporate it works better with less effort.
• Adding sand to clay to improve it — A small amount of sand in clay produces something closer to concrete. It needs a large volume of coarse grit, or organic matter instead.

If it doesn't work

Questions people ask