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How to improve garden soil: sand, clay and everything between

Feed the soil, and the soil feeds the plant. Almost every soil problem, from sand that dries out in a day to clay that sets like brick, is improved by the same two things: adding organic matter, and not compacting the ground.

Find out what you have

The feel test

1. Take a small handful of soil from about 10 cm down.
2. Add a few drops of water and knead it until it is like soft putty.
3. Gritty means sand. Silky means silt. Sticky means clay.
4. Squeeze it into a ball. Sand will not hold a ball. Loam holds a ball that crumbles when poked. Clay holds a firm ball.
5. Push it out between thumb and finger. Clay makes a long, smooth ribbon of 5 cm or more.

The jar test

Half-fill a straight-sided jar with soil, top it up with water and a teaspoon of dishwashing liquid, and shake hard for two minutes. Sand settles within a minute, silt within two hours and clay within a day or two. Measure the layers.

You cannot change the texture, but you can change the behaviour

Turning sand into clay would take tonnes of material. What you can change is the organic matter and the structure, and that matters far more.

• In sand, organic matter holds water and nutrients that would otherwise wash straight through.
• In clay, it glues the tiny particles into crumbs, so that water drains, air enters and the soil is easier to dig.

In both, it feeds the soil life that builds structure. Several earthworms in a spadeful is a sign of reasonable health.

What damages soil

• Compaction from feet, wheelbarrows and vehicles, worst when the soil is wet.
• Working wet clay. It smears, then sets like brick.
• Bare soil. Rain seals the surface into a crust, and the sun bakes it.
• Too much digging.

What builds soil

• Add compost, manure and mulch.
• Keep the soil covered with mulch or plants.
• Make beds about 1.2 m wide, so that you can reach the middle from the path, and never walk on them.
• Work the soil only when it is moist, not wet. Squeeze a handful: if water oozes out, wait.

Test the drainage

Dig a hole 30 cm deep and wide. Fill it, let it drain, fill it again, and measure how far the water drops in an hour.

• More than 10 cm: fast, typical of sand. Add organic matter and mulch.
• 2.5 to 10 cm: good.
• Less than 2.5 cm: slow, typical of clay or compacted soil. Improve the structure, or plant in raised beds.

On much of the Highveld and in the Free State, a hard layer of clay or gravel (ouklip) lies below the topsoil. Break through it where trees and shrubs will grow.

Soil pH

Most vegetables, lawns and flowers want a pH of 6 to 7. High-rainfall areas tend to have acid soils, and dry areas tend to have neutral to alkaline soils.

• Test with a kit from a nursery, or send a sample to a soil laboratory before starting a large food garden.
• To make soil less acid, apply agricultural lime well before planting. It works over months.
• Making soil less alkaline is harder. Dig in plenty of compost, or choose plants that like alkaline soil.

Test before you treat. Yellow leaves can come from the pH, not from a shortage of food.

Quick fixes by soil type

• Sandy: lots of compost, mulch, and feeding little and often.
• Clay: compost, mulch and raised beds. Never work it wet.
• Compacted: fork it deeply without turning it over, and keep traffic off.
• Building rubble: remove the rubble, bring in topsoil and compost, and test the pH.

This guide is based on Chapter 2 of Introduction to Gardening in South Africa.