Organic Gardening

There are five main soil types - clay, sand, silt, limestone and peat - and it is the nature of the original rock and the size of the mineral fragments that determine the soil type. It is important to know what kind of soil you are dealing with in your garden because the way in which you manage it, the timing of cultivations and the plants you grow will depend to a large extent on the nature of the soil.

However, having said this, most soils contain a mixture of minerals. If a soil is referred to as, for example, "clay", then this indicates its major constituent. Soil mixtures are known as loams; for example, a soil made up of 50 per cent clay and silt and 50 per cent sand is a "medium loam". Similarly, a soil which contains a high proportion of sand might be described as a "sandy loam", while one which contains a relatively large amount of clay might be described as a "heavy loam".

Soils can also be heavy or light. A heavy soil contains a much higher proportion of clay. This type of soil has very small particles, which tend to pack together, preventing free passage of water. Heavy soil is often very difficult to work initially because it tends to be either very wet and sticky or very dry and hard. Eventually though, when it has been ameliorated by the natural drainage afforded by plant roots and the addition of organic matter, heavy soil becomes an excellent moisture- and nutrient-retaining medium. Light soils, on the other hand, are easy to dig, and warm up quickly in the spring but allow very free drainage, which has its own problems. Water and nutrients disappear through the topsoil and into the subsoil and eventually the drainage system. Light soils require constant additions of organic matter to form a topsoil that retains moisture, and generally need more applications of fertilizers than heavy soils.

Soil may also contain lime, which will cause it to be either "acid" or "alkaline", depending on the amount. The lime content will make a considerable difference to the fertility of the soil, and will govern the range of plants you can grow, as it has the ability to make some nutrients unavailable to plants.

The proportion of stones or gravel in your soil does not influence its texture classification, but may affect its fertility and drainage. Stony soil has the advantages and disadvantages of a free-draining soil and it may need regular applications of bulky organic matter to improve water retention. If you are lucky enough to have a heavy topsoil and a very stony subsoil, you have the best of both worlds, with surface moisture and nutrient retention, plus good drainage of excess water.