Soil Structure

 

If you could look at a cross-section of your soil you would notice that it is divided into three layers: topsoil, subsoil and a layer derived from the bedrock (parent rock).

The upper topsoil layer (approximately 15-20 cm/6-8 in deep) tends to be a dark colour, due to its high levels of organic matter. This includes vegetation and leaves that have rotted down, artificially added organic materials and a host of useful micro-organisms.

Subsoil is usually lighter in colour and its content varies according to the nature of the soil and its parent material. It does not contain as many nutrients or as much organic matter as topsoil, and is much less fertile as a result. If you notice that your subsoil and your topsoil are similarly coloured, this may indicate that the topsoil lacks sufficient quantities of organic matter.

When cultivating soil, always remember that topsoil and subsoil should NEVER be mixed.

Sometimes a dense layer of soil forms on or beneath the surface of the topsoil. This impermeable layer is called soil pan or hardpan, and prevents air and water moving freely to the soil below. Hardpan is most often formed in acidic or clay soils and may be broken up by double digging. Alternatively, soil amendments such as manure, compost or peat can both improve local drainage and promote the proliferation of earth worms that can, over time, break relatively thin hardpan layers. More difficult hardpans may be treated by adjusting the soil pH with lime if the soil is acidic, along with the addition of gypsum.