Concept, Definition, and Types of Soil Survey

Before soils could be classified, knowledge of individual soils which constitute the group is very essential. A soil survey makes us know the properties of soils in an area with a view to classify them if need be. It involves a systematic examination, description, classification, and mapping of soils in an area.

Soil surveys help the extension and adaptation of research knowledge from one area to another. However, soil surveys emphasize only the less changeable properties of the pedon and site such as the physical and mineralogy of the pedon and its climate. Changes such as fertilizer and lime regimes are not usually indicated in a survey report.

Types of Soil Survey

Soil surveys are of six types; namely, Schematic soil surveys, exploratory soil surveys, Reconnaissance soil surveys, Semi-detailed soil surveys, detailed soil surveys, and Very Detailed soil surveys. The type adopted depends on the level of detail required from the soil map.

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1. Schematic Soil Surveys

These are soil surveys that result in much-generalized soil maps which have essentially taxonomic significance. They are often derived from intuitive extrapolation about the soils in areas where no soil surveys have taken place. Thus, such survey maps are often interpretations of climatic, geologic, and vegetation maps.

Most often, the road traverses linking “mapping units” are used for soil checking. Invariably, the mapping units employed in these surveys are broad; normally the Soil Orders or the Great Soil Groups of the World, and a very general picture of soil distribution pattern is given.

Generally, many national soil maps are compilations obtained through Schematic Soil Surveys and their publishing scales lie in the region of 1:1,000,000 or smaller, just like in the case of atlas maps.

Indeed, a very good example of a schematic soil survey is the one that has resulted in the publication of the FAO/UNESCO Soil Map of the World at a scale of 1:5,000,000 (FAO/UNESCO, 1974).

Schematic soil surveys, also termed Syntheses or Compilations, are useful in promoting public awareness about soils of a broad region and have only some direct practical value at global and international levels of planning.

2. Exploratory Soil Surveys

These are soil surveys often undertaken to identify the forms of development that are physically possible within large regions of a country.

The level and nature of the national effort required to implement such development are assessed in general terms, providing a basis for establishing priorities and a timetable for the use of limited facilities in development.

The surveys are often undertaken at a final mapping scale of 1:250,000 to 1:1,000,000 as general inventory maps of soils as a natural resource. At these small scales, the surveys rely on satellite imageries or extensive air-photo reconnaissance as most boundaries are drawn in the laboratory on the evidence of interpretative methods.

The soil mapping units often consist of land units of various kinds, preferably enclosing identified soil Orders and Great Soil Groups.

3. Reconnaissance Soil Surveys

This type of soil survey fulfills a similar purpose to those described under exploratory surveys. However, they are often commissioned for more specific purposes rather than as a general inventory survey.

Their major purpose is to identify possible areas for further intensive soil survey work as might be done before locating new irrigation or farm settlement schemes.

Aerial photographic interpretation is the basis of soil boundary location, but the soil observations become slightly more numerous than in exploratory surveys and the mapping units comprise individual great groups or associations of great groups.

4. Semi-Detailed Soil Surveys

Semi-detailed soil surveys are also termed Medium Intensity soil surveys. They are carried out to identify specific areas apparently suited to specific forms of agricultural development.

A reliable interpretation is obtained of the overall proportion and general distribution of soils of differing potential for the development purpose (s) in view.

Such information may be sufficient to assess the economic feasibility and even permit the implementation, of the less intense forms of agricultural development.

However, for more intensive land utilization on types such as irrigation, these surveys usually serve only a “pre-investment” purpose – to identify “project areas” within which expenditure on more intense studies for investment feasibility assessment and implementation appears to be justified (FAO, 1979).

Final maps are often published at scales of 1:25,000 to 1:100,000 and mapping units often consist of associations of soil series, and physiographic units (enclosing identified soil series).

5. Detailed Soil Surveys

These are high-intensity soil surveys that are carried out at scales between 1:10,000 and 1:25,000. At such scales, it becomes possible for the cartographer to indicate field boundaries upon the topographical map.

Consequently, in a soil survey produced at these scales, soils can be related directly to the parcels of land which enclose them. The soil surveyor can indicate with considerable accuracy, the location of soils on the landscape and can show the intricacy of their boundaries on the map.

Detailed soil surveys are very useful in areas where soil-related problems are known to exist as one can see at a glance if a particular field is likely to have soil with a specific problem.

Also, extrapolation of information from one area to another is possible and it is also possible to indicate whether or not a development scheme is feasible. Detailed soil surveys are therefore also known as “Feasibility Surveys” (Bridges, 1982).

Soil mapping units in detailed soil surveys often consist of phases of soil series and soil complexes.

6. Very Detailed Soil Surveys

These are very high-intensity soil surveys that are carried out at scales larger than 1:10,000. They are concerned with the precise location of high-cost projects or management problems of specialized crop production.

Surveys in this category have specific objectives and the data to be collected would be contractually agreed on. Boundary lines between different soil series, or even phases of soil series occupy zones less than 2m wide and so the soil surveyor can show the soil boundaries with exactitude.

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