How do I tell if my soil is healthy?
Four things you can read from a single spadeful — structure, colour, roots and life — and what each of them is actually telling you to change.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 4 min
- Safety
- caution
Short answer
Lift one spade-depth block intact and drop it onto a board from waist height. Healthy soil breaks into rounded crumbs, has a distinctly darker top layer, has fine roots running in every direction, and holds visible worms and worm channels. It smells sweet and earthy. Slabs, plates, pale colour throughout, sideways roots and no worms all point the same way: more organic matter, less traffic.
Soil health is not a mystical property and it does not need a laboratory. Everything that matters to a plant — air, water, roots getting where they want to go — shows up in one block of soil if you know what you are looking at. This is a version of the assessment agronomists use, cut down to what a gardener can do with a spade in ten minutes.
Safety
What you'll need
- A spade
- A board or a tray to drop the block onto
- Gloves
Step by step
- Lift a block intact rather than digging a hole.Cut three sides of a block roughly 200mm square to the full depth of the blade, then lever the fourth side and lift the whole thing out without breaking it. Do this when the soil is moist, not baked or saturated.
- Drop it once from waist height onto a board.Healthy soil falls apart into rounded crumbs between about 2 and 10mm across. Poor soil lands as angular clods, flat plates or one intact lump.
- Read the colour change.A clear dark layer at the top with paler soil below is organic matter doing its job. The depth at which the colour changes is the working depth you actually have. Pale to the surface means the organic matter has run out.
- Read the roots.Fine roots running in all directions, including down and outward, mean nothing is stopping them. Roots that all run flat along one horizontal line have hit something they cannot get through — that is a compacted layer, and it is the most useful single observation in the whole test.
- Count the worms.In a spade-depth block in spring or autumn, look for several. None at all in moist soil at the right time of year is a real signal. Look for the channels too — open vertical burrows lined with dark casts mean the population is working even if you missed the worms themselves.
- Smell it.Sweet and earthy is right — that smell is actinomycete bacteria and it means aerobic soil life. Sour, or like rotten eggs, means waterlogging and no oxygen. No smell at all usually means almost no organic matter.
- Check how it holds together when wet.Drop a dry crumb into a glass of water. Healthy aggregated soil holds its shape for a while; poor soil collapses into a cloud of silt within seconds. That collapse is what causes capping and surface run-off.
- Act on what you found.Slabs and plates: reduce traffic, add organic matter, and relieve compaction. Pale throughout: an annual compost mulch, every year, for several years. Sideways roots: find the depth of the pan and break it. Sour smell: it is a drainage problem, not a soil problem.
Tips
- Repeat the test in the same spot each spring. Change over three or four years is far more informative than an absolute judgement in year one.
- Worm numbers swing hugely with season and moisture. Test in spring or autumn in moist soil, not in a dry July when they have all gone deep.
- A soil that fails on every measure is not a lost cause. Organic matter on the surface, every year, plus staying off it, fixes almost all of it — it just takes three to five years rather than a weekend.
- Lawn is not a good test site. Grass roots hold soil together and make the structure look better than the beds nearby.
If it doesn't work
The block comes out as one solid lump and will not break
Cause: Severe compaction, or clay tested when too wet — Fix: Retest when the soil is genuinely moist rather than saturated. If it still holds as a slab, treat it as compaction: relieve it with a fork and stop walking on it.
No worms anywhere, in moist soil in spring
Cause: Almost no organic matter to eat, heavy compaction, or persistent waterlogging — Fix: Spread 50mm of compost on the surface and leave it. Worms respond to food faster than to anything else — expect a visible change within one to two seasons.
Grey or blue-grey mottling in the block
Cause: Gleying — soil that spends long periods with no oxygen because it is saturated — Fix: This is a drainage diagnosis, not a fertility one. Find out why water sits there before adding anything.