Understanding Soil and Soil Structure

Soil is a surprisingly complex material, made up from a collection of fine mineral particles and varying amounts organic matter, containing quantities of air and water along with a surprising array of worms, insects and other living things.

The precise mix of any soil defines its character while its physical and chemical properties govern everything from its ease of cultivation, to what kinds of plants it will grow.

The study of this enormously important substance forms a branch of science all of its own – and while it certainly isn’t necessary to have an extensive knowledge of the geology and chemistry of soil to create a good garden, an insight into its nature can only be of benefit.

Topsoil and Subsoil

If you dig down, going progressively deeper through the soil, you will notice a succession of roughly horizontal layers each with its own distinct appearance and colour. To the soil scientist, these “horizons” form the basis for the international system of classification, but the gardener need only think in terms of “topsoil” and “subsoil” since these broader distinctions cover the most relevant aspects.

Topsoil holds most of the organic matter in the soil – and most of the soil organisms – and consequently most of the nutrients, while subsoil is usually lighter in colour and lower in both humus and plant nutrients. This can be a useful practical guide when digging; little difference in colour between these two layers can indicate that the topsoil is low in organic matter and in need of the addition of some well-rotted farmyard manure or compost.

Soil Composition

The composition of any soil principally comes down to the proportions of three types of particles – clay, silt and sand – sometimes called the “fine earth fraction” because they are all less than 2mm in diameter. Clay particles are very small, even the largest ones being a thousand times smaller than coarse sand and the way these particles are arranged plays an important part in determining the soil’s overall structure.

Analysing the relative proportions of these three basic building blocks allows a very accurate description of the soil to be made, but for practical purposes, it is usually more helpful to talk in terms of broad “families” of soils – clays, sandy soils, silts and loams.

  • Clays – fertile but heavy to work and slow draining. They warm up slowly in the spring, can become compacted easily and may “bake” in hot weather.
  • Sands – fast draining, easy to work but often poor in nutrients. Quick to warm up in the spring but often benefit from improvement with added organic material.
  • Silt – fertile and holds water well, but can become compacted.
  • Loams – the ideal balance of soil particles. The perfect combination of fertility, ease of cultivation, good water retention and proper drainage makes them highly prized soils.

With a little experience most people can get pretty good at finding out much of what they need to know from simply rolling some soil between their fingers and seeing how it feels. Sandy soils, for instance, feel quite gritty and don’t stick together very well, while clays can be rolled out very successfully and silts feel slightly soapy to the touch.

There are many other aspects to any soil which can affect its character, including the proportion of gravel and stones it contains, its acidity or alkalinity and the chemistry of the underlying rock. All of these have a bearing on the sorts of plants that will thrive in your particular plot, but in the end, the best way to understand the properties of your own garden’s soil is by working with it. After all, they do say that experience is a great teacher!