About your soil

Soil varies from place to place depending on the local geology and natural forces that have shaped the land. Knowing what lies beneath the surface will help you garden more successfully.

Your soil profile

Take a spade and dig a hole about half a metre square. Keep at least one side of the hole as straight and even as possible and dig down 50-60cm, if you can. If it's easy digging and you can go quite deep, then the soil is probably light and free draining. However, if it's tough going you've probably got a heavy soil. Or some parts might be relatively easy with the odd layer that's hard to break through (known as a "pan").

Take a breather and look at the straight side of your hole. If your soil has been undisturbed by construction or gardening you may see several layers of material (the soil profile), perhaps with different colours or textures. Closest to the surface is topsoil with one or more layers of subsoil below. There may be layers of pebbles or stones left behind by ancient rivers, or volcanic ash, clay or silt deposited by eruptions, wind or floods in days gone by. The soil profile can vary from one part of your garden to another, so checking out what's down there can give you a better idea of what to expect from your plants.

There's little you can do to change subsoil. You can, with some effort, break hard pans to improve natural drainage or install drains. But there's a lot you can do to improve the upper 20-30cm of soil, where the majority of the real growing action takes place.

What is soil?

Soil is a mixture of minerals, microrganisms, organic matter, water and air.

One way the minerals (derived from broken-down rocks) are classified is according to their size into sand, silt and clay.

Clay particles are roughly 1000 times smaller than the largest sand particles, with silt mid-way between the two. Because clay particles are so tiny, they occupy the spaces between the larger sand and silt ones.

So the more clay in a soil the fewer spaces there are to contain air and the natural downward flow of water is impeded. Even when dry it weighs more than soils with less clay, so is often called a heavy soil. Soils with a high proportion of clay (more than 40-50%) can make gardening a real challenge.

However, clay has good points too. It helps retain water and holds nutrients, unlike sand particles that allow them to leach away. A soil with around 40% sand, 40% silt and 20% clay would be the ultimate in dream gardening.

To determine the level of clay in your soil, try the Horseshoe test.