Know Your Soil Before You Spend a Penny

Start with the ground, not the wish list

Almost every garden we are called into begins the same way: a list of plants the owner loves, and a piece of ground that has already decided which of them will live. The commonest reason a plant fails here is not neglect and not cold. It was never suited to the soil it went into.

Soil sets the shortlist. Taste chooses from it.

Almost any soil can grow a beautiful garden. But you cannot improve a soil you have not read.

The squeeze test

Do this when the soil is damp but not sodden — a day or two after rain, which here means most of the year. Take a trowelful from about 15cm down, below the loose surface layer. Pick out stones and roots, wet it slightly if it is dry, then work it between your fingers for twenty seconds and squeeze it into a ball.

What it does next is the answer.

Sandy soil

It feels gritty, will not hold a ball, and falls apart the moment you open your hand. Sand drains fast and warms early, so you can work it weeks before your neighbour on clay. Water and nutrients run straight through, so it needs organic matter regularly and dries out badly in an east-coast summer.

Clay soil

It feels smooth and slightly sticky, rolls into a ball easily, and takes a shine if you rub it with your thumb. Bend that ball into a ring — if it holds without cracking, you have a heavy clay. Clay holds water and nutrients well, which makes it fertile, but it is slow to warm, sets hard in drought and turns to porridge underfoot in winter. Never dig or walk on it wet.

Loam

A balance of all three, rather than a material in its own right. It holds a ball but crumbles when prodded, feels neither gritty nor greasy, and smells sweet. It drains yet holds moisture, warms early enough, and grows almost anything. If you have it, protect it — the fastest way to ruin loam is compaction.

Chalky or shallow limestone soil

Pale, often stony and shallow over rock, and it may fizz faintly if you drop vinegar on it. Common on the thin soils over limestone in the midlands and the Burren. It drains freely and is reliably alkaline, which locks up iron and turns acid-loving plants yellow between the veins.

Peaty soil

Dark brown to black, spongy and light in the hand. It holds water yet is very hard to re-wet once dry. Widespread over old bogland and cutaway, acidic and low in some nutrients — but wonderful for ericaceous plants.

Two follow-up tests worth doing

The squeeze test tells you texture. Two cheap follow-ups tell you the rest.

• The jam-jar test. Half-fill a clear straight-sided jar with soil, top up with water, add a drop of washing-up liquid, shake hard and leave it on a windowsill for a day. The particles settle in bands: sand at the bottom, then silt, then clay, with organic matter on top. Measure them for your rough percentages, and a truer idea of how much clay is in that "heavy" soil.
• A pH test. A kit costs very little and takes five minutes. Anything from 6.5 to 7 suits the widest range of plants. Below 6 you are on acid ground and rhododendrons will thank you. Above 7.5, stop trying to grow them. pH can be nudged, never reversed.

What each soil opens up

• Sand and free-draining ground: Lavandula angustifolia 'Hidcote', Cistus, Salvia yangii and Eryngium earn their place because they hate winter wet more than they mind drought.
• Clay: roses are at their best on it, and Hydrangea, Viburnum opulus, Alchemilla mollis, Astrantia major and Persicaria amplexicaulis root happily into moisture-retentive ground.
• Acid and peaty ground: Rhododendron, Camellia, Pieris and Vaccinium corymbosum thrive where the pH stays low, and none will ever be happy on limestone.
• Chalk and limestone: Taxus baccata, Buddleja davidii, Cotoneaster and Dianthus tolerate alkalinity without sulking.
• Loam: almost anything, which is why it is worth looking after.