Soil Types

Many natural bodies, such as plants and animals, are discrete entities which can be classified and guidelines for their identification followed. Soils are much more difficult to identify and classify than these discrete bodies for two main reasons: (i) soil is more or less a continuum covering the land surface of the earth, not a set of discrete entities; and (ii) most of the soil is below ground and therefore not readily visible. Soils grade into one another across the landscape usually without sharp boundaries between one type of soil and another. Soil surveyors who make maps of soils have to use their skills in reading changes in the landscape coupled with auger borings in the soil to identify the nature of the soil.

There are several ways of classifying a soil, from the simple to the complex. A soil type may be as simple as ‘a sandy soil’ or ‘a clayey soil’ and this is often the perception of many land users, such as farmers or civil engineers, who see it as material they have to deal with to achieve an end result, such as the growing of a crop of wheat, or the building a road. Simple classifications tend to be of local and restricted relevance only. At the other end of the spectrum is the soil scientist who needs to understand how soils have formed, which types occur where, and for what the different types of soil can be used. The soil scientist seeks a much broader understanding, with the aim of underpinning the use and preservation of this important natural resource, and this has manifested itself in a number of detailed soil classification systems worldwide.

There is an enormous diversity of soils across the world. This is hardly surprising given the fact that soil formation and soil type are influenced by several key factors: the parent material, usually rock or sediment, but occasionally organic materials such as peat; climate, particularly temperature and rainfall; vegetation and other biota; topography/ relief; time; and, increasingly, the influence of humans. All these factors will have an influence on soil development and hence soil type. The potential for different combinations of these factors across the world is immense and hence it is not surprising that there are many thousands of different types of soil in the world, with different properties and potential.

Just as Linnaeus produced a classification of plants that is in regular use today, over the centuries attempts have been made to develop classifications of soils on the basis of their properties, their genesis and in some cases their use potential. Since the late 19th century there have been several national classifications developed and used. Most countries now have a system of nomenclature for their soil types. These classifications have grouped together similar soils and given names to the soils in the various classes. The classifications are mostly hierarchical in the sense that they consist of a number of levels, starting at the top with a number of major divisions, which can be subdivided into further subclasses and degrees of complexity.

There have also been several major attempts at a world classification of soils, each of which have differed in the emphasis they have given to different soil properties. Such classification systems enable soil types to be classified and identified within a national and international framework. The two most widely used international classifications are those of Soil Taxonomy (developed by the US Soil Conservation Service) and based on soil properties that can be objectively measured and observed and the FAO/UNESCO legend/classification which is along broadly similar lines but not as precisely defined. It is regrettable that to date it has not been possible to produce an international classification that has been adopted worldwide but soil science is a young science with an important future and a unified classification system should eventually be developed. Until that is available, soil types should be named using both national nomenclature and have this linked to one or more of the international systems.

Examples of some soil types from different parts of the world are given in the section on Soil Profiles.