Soil is rock which over thousands of years has beets crushed and mixed with decayed organic matter, known as humus. The three main types of soil are light, sandy soil, which is made up of' large particles that do not adhere to each other and through which water and plant food pass easily; medium silt, which is made up of medium- sized, smoother particles; and heavy clay, which is made up of minute particles which pack together, retain moisture and form sticky lumps that dry solid. Most garden sod is a mixture, with one of the above predominating.
Topsoil is the name given to the relatively thin, darker top layer of soil which has built up over the years as it has been enriched with humus through the natural decay of organic matter such as dead leaves, animals and plants, and through cultivation. The subsoil lying below this gives the sod its general character and to a large extent determines whether it is rich or poor in nutrients, and whether it is warm or cold and has good or poor drainage, as well as whether it is acid or alkaline. In a new house and garden there may be no topsoil at all, but subsoil can be converted into topsoil by good cultivation.
Fertile soil, loam, is an approximately equal mix of sand, silt, clay and humus. Humus is a vital, organic, ingredient which introduces a wide range of chemicals and bacteria into the soil. It improves soil texture in its own right, and encourages the activity of earthworms, .whose work not only breaks it down and releases its vital ingredients as food for plants, but also aerates the sod.
Poorly drained soils are cold and inhospitable, and their compacted structure deprives the plants' roots of the air they need. Very wet soils cause roots to rot and plants to die. If rain does not drain away but continues to lie in your garden, if only moisture-loving plants thrive, and if the lawn is full of moss, it may he that you need to install drainage
Dig trenches in the garden for the drains leading into a soak away. Make these at least 30 cm/1 ft deep, with a slight fall towards the soak away (following any natural slope). Place a layer of coarse grit or fine gravel along the bottom
Lay the drains (which may be made of clay or perforated plastic) on the bed of gravel. Use a T-shaped connector if you are adding side drains
