But what is soil?
‘Soil’ is a word which has several different meanings. To the engineer, soil is usually thought of as the finely ground, loose rock material at the Earth’s surface (often termed ‘overburden’ and frequently regarded as an inconvenience because it may have to be stripped, stored carefully and replaced as part of an engineering project). The geologist calls this layer the ‘regolith’ (essentially meaning the same as the engineer’s overburden) and frequently begins investigations below it. The farmer and gardener think of the soil as the top few centimetres – the depth of plough or cultivation for the former and a spade or garden fork depth for the latter (the ‘topsoil’). They tend to ignore what’s underneath. Yet this deeper material (or ‘subsoil’) is very important for plant growth, storing and supplying nutrients and water. Also, a number of chemical substances (whether naturally produced within the soil or added by human activity) pass from the topsoil through to the subsoil. These may eventually reach underground water stores and rivers, burns or lochs.
Good quality soil must be capable of carrying out all the uses for which it is needed, without long-term deterioration. Scientists often talk about sustainable use of soils – current uses of the soil should not affect its range of other uses, either now or in the future. If we do not maintain the soil in a reasonable condition, certain aspects of modern life could start to become unsustainable. It may be considered as ‘dirt’ by some, but to those who are concerned with its ability to support the world’s population, it is perhaps the most valuable non-renewable resource on Earth.