The ideal soil is made up of 22% water, 20% sand, 20% air, 15% silt, 10% clay, 8% ‘unavailable’ water (that is, water trapped within the soil that the plant cannot use) and 5% organic matter.
Soil texture is how the soil feels when you handle it. This is due to the basic rock the soil is made of and cannot be altered. Soil structure is how the particles are held together in the soil. This influences whether the plant can get at the air, water and nutrients in the soil. It can be improved by adding organic matter, ensuring good drainage and digging in fall season to allow the breakdown of clods in heavy soils during winter. It is surprising how much difference adding organic matter can make to almost any soil.
Silty soil. This feels silky to the touch but not sticky and you can mould it in your fingers to some extent. It is moderately fertile and holds less water than clay soils but is easily compacted and can acquire a hard cap, which prevents both water and air from getting through to the plant’s roots.
Clay soil. Clay soils feel cold and heavy and can be molded in the fingers. They are often very fertile but they are also heavy and may become waterlogged. They are slow to warm up in spring and may become very compacted when wet and covered with a cap or crust, which reduces the air available to roots and seeds. Plants from hot, dry areas are very unhappy in clay unless it has been much improved with sand, gravel and organic matter. Plants that grow well in clay include day lilies, roses, astilbes and peonies.
Sandy soil. Light, free-draining and easily worked, sandy soil warms up quickly in spring, giving plants a good start. For this reason it is useful in producing early crops. Its disadvantages are that water drains through too easily and minerals can leach out quickly. Mediterranean plants and many herbs grow well in sandy soil.
Peaty soil. Peat is usually found in low-lying areas. Very dark brown and often acidic, peaty soil is not very fertile and often poorly drained. Rhododendrons and heathers grow well in it.