Topsoil is not actually the top-level of your garden soil, it’s below a thin layer of humus that’s made up of decomposing materials, particularly plants and leaves.
You then have the topsoil which is above subsoil and bedrock. Subsoil is important for drainage but contains few nutrients, and then the bedrock is the original rock from which the soil was created.
Topsoil is therefore the most important growing medium and roots rarely penetrate any deeper. It can vary in-depth from between a few inches to six feet and contains a multitude of goodness including organic matter, bacteria, various micro-organisms together with insects, worms and fungi.
There are five main types of topsoil, clay, sand, silt, chalk and peat. The type of plant you grow and the success you have, will depend to a great degree on your soil type, and how you manage and improve it.
Clay
The main problem with clay is drainage, as a result it should be nutrient rich but hard to work. Dig over in the autumn, introducing organic matter, and let the weather break up the earth. You can add grit and lime will encourage the tiny particles of clay to bind and improve drainage. Also consider using raised beds to help the soil both dry out and warm up.
Sand
Easy to work, but any nutrients are easily drained, so large amounts of organic matter can increase its water retentiveness and additional fertilizer can be used to replace nutrient deficiency. Try to mulch as much as you can and cover the soil with lots of vegetation all year round to minimize nutrient loss. If you are growing vegetables, sow green manure, when this is dug in it will add nutrients and improve the soil structure.
Silt
With a structure similar to clay, the biggest problem is drainage. It’s unwise to walk on this type of soil when it is wet as you will only compact it even more. Dig it over in the autumn so that frost can help to break it down, add coarse grit and organic matter and use raised beds to warm up and dry out the soil.
Chalk
This is a free draining soil from which nutrients can easily be leached and the topsoil depth may also be limited. Add organic matter to improve water retention and mulch with peat and manure to increase acidity. Make sure that the ground has a good covering of vegetation throughout the year, including green manure in the winter.
Peat
This type of soil has a tendency to be acid so use lime to balance the pH especially for vegetables and fruit. Peat is made up from decomposed vegetation so there is no need to dig or add any organic matter, but you may have to add an organic fertilizer to combat any nutrient deficiency.