Soil Mapping

There are several thousand different types of soil throughout the world, a fact that is not surprising when bearing in mind the differences there are worldwide in the agents responsible for the building and forming of soil (landscape, climate, geology, vegetation, time and man).

We know that there are at least this number of soil types because they have been mapped. In the past 50 years many countries of the world have been involved in making maps of their soils to determine the range of soil types in their territory, where the soils occur and how they can be used. Soil mapping involves locating and identifying the different soils that occur, collecting information about their location, nature, properties and potential use, and recording this information on maps and in supporting documents to show the spatial distribution of every soil.

IIn order to map and identify different types of soil it is necessary to have a system of soil classification. Just as in the case of plants where we have the Linnaean system, so an equivalent approach is needed for soils. Soil classifications have provided a major challenge, one that has yet to be adequately solved. At present there is no single universally used system (unlike the case for plants). There are two international systems, FAO-UNESCO and US Soil Taxonomy, but a number of other systems have evolved and many countries have produced their own systems suited both to local conditions of soil formation and to local knowledge. However, the one uniform feature in all these systems is that they are based on the soil profile, that is the appearance of a section of the soil from the land surface through to the rock or other parent material that lies below the surface where the investigation is taking place. This examined thickness is usually about 1 metre but in tropical areas with deeper soil development it can be several metres, and in steep rocky landscapes may be much less than 1 metre. Soils are allocated to a type according to their profile characteristics including, abundance of organic matter in the topsoil, the texture (combination of the different particle sizes and their ratio), colours representing aerobic or anaerobic conditions, structure of each layer, etc.

Traditional soil mapping is conducted with an auger and spade at intervals throughout the landscape. The intervals between inspections can be according to a pre-determined grid (grid-survey) or, more often, are based upon the judgement of the surveyor who uses their knowledge of the inter-relationship between soil type and landscape, geology, vegetation, etc to determine where to make inspections.

Auger borings are supplemented by excavated profile pits at determined points in the landscape. These profile pits are used to demonstrate lateral changes in the soil as well as vertical ones, and are important for the full description of type soils and for the taking of soil samples for chemical, physical and less commonly, biological laboratory analysis. In this way a picture is built up of the soil in a region and its relationship to the landscape in which it lies.

Soils can be mapped at a range of scales from very detailed at 1:1,250 to 1:5,000 by which the pattern of soils in individual fields can be identified, through to scales of 1:500,000 to 1:500,000,000 which provide only a very generalised picture of the soils of a country or continent.

Sometimes soils are mapped with a specific aim in mind, such as the suitability of soils for a particular crop, suitability for irrigation, erosion risk and many other specific needs or environmental threats. Most organised soil surveys in the past have been general purpose surveys. These have the advantage that they provide the basis for many different uses, some of which may yet not be known.