Soil is a highly complex material formed through the weathering of rock and the breakdown of organic matter and can vary widely in composition and structure from place to place. It contains minute particles of rock, accumulated organic matter (humus), living organic matter, water and air. Healthy soil is a key element for successful gardening: it acts as a physical support for plants and supplies them with the water, air and nutrients necessary for growth.

The structure of soil may be divided into three general layers: topsoil, the topmost layer, where most roots, micro-organisms, and other animal life are located; subsoil, which is deeper and often more dense and less rich in organic matter; and a layer derived from the bedrock.

The proportions of minerals and organic matter determine the structure and other characteristics of a particular soil. It is worth spending some time to identify the type of soil in your garden so that you can select plants that will thrive in the given environment. However, certain soil conditions may be altered or enhanced by the addition of materials such as compost, lime, mulches and top dressing - all of which are discussed fully in our section on soil improvements. Details on soil cultivation may be found in Practical Gardening.