The top soil is the essential layer for plant growth.
The texture and composition of the soil generally depend upon the parent rock from which it has very gradually been formed, by the interaction of water, climate and vegetation.
Weathering agents such as frost, rain and sun break the rock down over thousands of years to form the basic mineral structure of the soil.
Plants grow on the rock debris and myriads of micro-organisms that work on the dead roots and fallen leaves to decay them, producing the essential organic constituent of the topsoil, known as humus.
In some parts, the soil bears no relation to the rock beneath it because it has been carried to its place by a natural force.
The depth of topsoil varies.
The average garden should have between 300 mm and 600 mm (1ft and 2ft) of topsoil, but a depth of as little as 150mm (6 inches) is sufficient for growing a large number of plants.
The depth of the topsoil can be tested by using soil anger, a tool like a giant corkscrew, which will bring up a sample of the soil profile.
Another way is to dig a hole with steep sides and make the soil profile visible.
For revitalization of an old garden, it is suggested to remove soured topsoil and replace it with new topsoil.
When buying topsoil care should be taken to see that it is vegetable topsoil with organic content and free of disease and weeds.
A period of deep cultivation and the addition of plenty of organic material will increase the amount of topsoil already in a garden by encouraging bacteria to work within the top layer of subsoil.
Between topsoil and parent bed rock there may be many layers of stone and gravel, but the layer immediately beneath the topsoil is generally the subsoil.
Its depth varies according to the hardness of the underlying rock and the amount of erosion it has suffered.
The colour and texture of subsoil are usually different from those of the topsoil because it is in a transitional stage, without humus or organic material.
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