Types of garden soil: how to tell which one you have and how to improve it

Before you plant anything, it's worth spending ten minutes getting to know your soil type. The texture of the soil — the proportion of sand, silt and clay — determines how much water it holds, how often you need to feed, which plants will thrive and which ones will struggle no matter how much you pamper them. Half of all "I'm just not good at gardening" is really "I planted a hydrangea in sandy soil in full sun" or "I put lavender in a clay that waterlogs in winter".

The good news is that almost any soil can be improved with the right amendment and some patience: a heavy clay soil you can't even dig today, and a sandy soil that holds onto nothing, can both become a loam that is a pleasure to work in two or three seasons. In this guide you'll see the characteristics of the four basic soil types and which plants suit them; three home tests to identify yours without spending anything; how to improve each type with what actually works; why organic matter is the universal amendment; what N, P and K mean and how to read a fertiliser; how to look after soil life; and what growing medium to use for pots. At the end, twelve frequently asked questions.

The four basic soil types

Most real soils are mixtures: "sandy loam", "clay loam", "silty clay". What matters is knowing which way yours leans so you can correct it in the right direction.

Three home tests to identify your soil

The ball test (2 minutes)

Take a handful of slightly moist soil (not soaked) and squeeze it in your hand into a ball.

• If it falls apart as soon as you open your hand: sandy.
• If it forms a ball you can mould like plasticine and you can even roll a "sausage" or a ribbon that shines when you rub it with your thumb: clay.
• If it forms a ball that cracks and breaks when you handle it, with a smooth, slightly floury feel: silty or loam.

The jar test (2 minutes + 48 hours' wait)

Fill a clear glass jar one third with garden soil (remove stones and roots) and two thirds with water. Add a pinch of salt or dishwashing liquid to separate the particles, put the lid on and shake hard for a minute. Let it stand undisturbed for 24-48 hours. Visible layers will form:

• The sand settles to the bottom within seconds.
• The silt forms a layer on top within a few minutes.
• The clay (the finest particle) stays in the top layer and takes hours or even days to settle; the water may stay cloudy.

Measure the thickness of each layer with a ruler: the proportion gives you your texture. A loam soil is roughly 40% sand, 40% silt and 20% clay.

The drainage test (two days)

Dig a hole about 30 cm deep and 20-30 wide, fill it with water and let it drain completely (this saturates the surrounding soil and gives a more realistic reading). Fill it again and time how fast the level drops:

• Less than 2 cm per hour: poor drainage. Plant on raised mounds, improve the structure with organic matter and grit, or choose plants that tolerate wet soil.
• Between 2 and 15 cm per hour: good drainage.
• More than 15 cm per hour: excessive drainage. You'll need to water and feed more often, and add plenty of organic matter so it holds moisture.

Affiliate link: if you buy through it, TuJardínPerfecto may earn a small commission at no extra cost to you. More information in the affiliate disclosure.

How to improve each type of soil

The universal amendment is organic matter: compost, well-rotted manure, worm castings or leaf mould. It improves sandy soil (holds more water and nutrients, acts like a sponge) and clay soil at the same time (opens it up, separates the clay particles and improves drainage and aeration). A 2-4 cm layer per year spread over the surface — without digging it in — transforms a difficult soil in two or three seasons, because the worms and microfauna incorporate it for you, from the top down, without breaking the structure.

Nutrients: what N, P and K mean

The three numbers on a bag of fertiliser (for example, 7-7-7 or 5-8-10) are the proportions of nitrogen (N), phosphorus (P₂O₅) and potassium (K₂O).

As well as these three, plants need calcium (cell walls; a lack of it causes blossom-end rot in tomatoes and peppers), magnesium (part of chlorophyll; a lack of it turns leaves yellow between the veins, which stay green), sulphur and micronutrients such as iron (a lack of it, iron chlorosis: young leaves yellow with green veins, very common in chalky soil, not because there is no iron in the soil but because the pH blocks it).

A good compost supplies N, P, K and micronutrients in a balanced, slow-release form, with no risk of scorching. Mineral fertilisers act faster but it's easy to overdo it and wash the excess into the groundwater. For flowers and fruit, choose a fertiliser with more P and K than N (for example 5-8-10); for lawn and foliage, one with more N (for example 20-5-8); for establishing transplants, one rich in phosphorus.

Soil life: the invisible factor

A healthy soil is full of life: worms, mycorrhizal fungi that partner with roots and bring them water and phosphorus, nitrogen-fixing bacteria, springtails, mites, beneficial nematodes, beetles… a whole network that breaks down organic matter, forms stable crumbs (the "structure" of the soil), releases nutrients in the form plants can absorb, and competes with pathogens. A "dead" soil — heavily tilled, compacted, without organic matter, treated with broad-spectrum products — is just an inert support that needs constant feeding and in which plants get by but don't thrive.

To look after soil life:

1. Never leave the soil bare. Mulch or use ground-cover plants. Bare soil erodes in rain and wind, compacts, overheats in the sun (above 40 °C the microfauna dies) and loses life.
2. Till as little as possible. Every deep turn breaks the fungal network, buries the aerobic organisms and "burns" (suddenly mineralises) the accumulated organic matter, which is lost as CO₂. Modern gardening tends towards no-dig: add organic matter on the surface and let soil life do the work.
3. Add organic matter on the surface and let the worms bury it at their own pace. Digging it in with a spade is more work and a worse result.
4. Avoid stepping on growing beds: use planks or fixed paths. Trodden soil compacts and roots can't get through; water waterlogs or runs off the surface.
5. Rotate crops in the veg garden and put in green manures (vetch, clover, mustard, phacelia, lupin) during fallow periods: they fix nitrogen, protect the soil, add biomass when cut and break the pest and disease cycle.
6. Keep broad-spectrum products to a minimum (total herbicides, synthetic insecticides, systemic fungicides): they also affect soil life.

Potting mix: it's not the same as garden soil

Garden soil doesn't work as it is for pots: outside its ecosystem, without the network of roots and life that keeps it open, it compacts with watering, cakes and suffocates the roots for lack of air. For containers:

• Use a quality specific growing medium (peat or coir + compost + perlite) and adjust it to suit the plant.
• Cacti and succulents, Mediterranean herbs: add 30-40% coarse sand or perlite so it drains freely and the root doesn't rot.
• Plants that want moisture (ferns, hydrangeas, calla lilies): more coir and compost, and some vermiculite to hold water.
• Acid-lovers (camellia, azalea, blueberry, gardenia): a specific acidic growing medium, and water with rainwater.
• Refresh the top 3-5 cm of growing medium every spring and repot into a slightly larger pot every 2-3 years, or if you see roots coming out of the drainage holes.
• Feed more often than in the ground (every 2-4 weeks in the growing season, or with slow-release fertiliser), because watering washes nutrients out through the drainage hole.
• Drainage: enough holes, and a saucer you can empty; standing water in the saucer rots the root and breeds mosquitoes.

How to amend a soil before planting (step by step)

1. Do the texture and drainage tests and, if you're going to invest a lot, a pH test.
2. Remove perennial weeds by the root (couch grass, bindweed) by hand or with opaque mulch over several months; don't dig them in.
3. Spread 3-5 cm of compost over the whole area.
4. In very heavy clay soil, also work in grit and agricultural gypsum with a fork, without turning it deeply.
5. Let it rest for a few weeks if you can (better to plant in autumn on a soil amended in summer).
6. When planting, mix the soil from the hole with a handful of compost or worm castings, but don't make a hole much richer than the rest (the root "circles" there and doesn't explore).
7. Mulch with 5-7 cm of organic material, without covering the plants' collars.

Frequently asked questions

How long does it take to improve a clay soil?

You'll notice a difference the first season if you add several centimetres of compost, but the real transformation — a soil that drains and that you can dig without breaking a sweat — takes 2-4 years of annual organic-matter additions, never walking on it when wet and keeping it covered at all times with mulch or plants.

Can I plant directly on builder's soil or fill?

Better not to. Remove the top 20-30 cm if there is rubble, plastic and plaster, and replace it with a mix of quality topsoil and compost. Builder's plaster and cement make the soil very alkaline and compact it, and the rubble stops root development.

Is a laboratory analysis worth it?

Yes, if you're going to make a significant investment: planting fruit trees, a long hedge, a large veg plot, a blueberry patch or an ornamental lawn. For €30-60 you find out pH, texture, organic matter, active lime and nutrients all at once, and save yourself years of trial and error. For a flower bed, the home tests are enough.

Which is better, manure or compost?

Both, if they are well rotted. Fresh manure scorches roots with its excess ammonia and brings weed seeds and antibiotics if the livestock received them; let it compost for 6-12 months or buy an already-cured bag. Home compost is more balanced, doesn't smell and brings soil life. Worm castings are the most concentrated and gentlest, ideal for transplants and seedlings.

Can you have too much organic matter?

In an ornamental garden, it's very rare to overdo it. In a veg garden, a continued excess of manure year after year can build up salts, phosphorus and nitrates that leach; alternate with plant compost, worm castings and green manures to balance it out.

Do eggshells, coffee grounds and banana skins work as fertiliser straight away?

Not much, if you put them straight on the soil: they take a long time to break down and their immediate contribution is minimal. Where they do count is in the compost, mixed and chopped. Coffee grounds are slightly acidic and worms love them, but in excess they cake; eggshell supplies calcium very slowly.

What do I do if my soil is just stones and there's no soil?

On very stony ground, you have two options: create raised beds with imported quality soil (at least 30-40 cm deep), or choose scrubland and rock-garden plants that thrive precisely in poor, free-draining, stony soil (rosemary, thyme, sedum, lavender, rock roses, grasses). See drought-resistant plants.

Does beach sand work to lighten a clay soil?

No: it contains salt, which damages plants, and it's usually fine, which with clay forms a cement. To lighten clay, use washed coarse river sand or grit, and above all plenty of organic matter.

Do I need to turn over the veg garden soil every year?

The current thinking is not to, or to do it very shallowly. A soil that is always covered, mulched and not trodden stays open on its own, with the worms doing the work. Annual deep digging destroys structure and life. It is worth loosening trodden areas with a fork (without turning them over).

How do I know if my soil has little life?

Signs of a soil poor in life: few or no worms when you dig, the organic matter you add doesn't disappear for months, the soil forms a hard crust on the surface, it smells of little or of stale (a healthy soil smells of woodland). It recovers with compost additions, permanent mulch and stopping tilling and using broad-spectrum products.