Kinds of Soil Surveys
Soil survey can be classified based on; purpose of the survey, regularity of observation and scale of mapping. Soil survey can be special purpose or general purpose when it is classified based on purpose of survey. When soil survey is classified based on the regularity of observation, three kinds of soil survey are recognized namely; free survey, rigid grid and flexible grid surveys. When soil survey classification is based on scale of published map, we have seven kinds of soil survey namely; compilation, integrated survey, exploratory survey, reconnaissance survey, semi-detailed survey, detailed and intensive surveys.
Important facts to know
· Soil survey are generally classified based on purpose of soil survey, regularity of observation and scale of mapping
· Three kinds of soil survey based on regularity of observation include free survey, rigid grid and flexible grid surveys.
· Based on scale of published map, we have seven kinds of soil survey namely; compilation, integrated survey, exploratory survey, reconnaissance survey, semi-detailed survey, detailed and intensive surveys.
Kinds of Soil Survey
1. Purpose: This could be special purpose or general purpose. Whereas special purpose soil survey is done for specific purpose in mind e.g. is survey for irrigation or survey for guava plantation, general purpose soil survey is done mainly to add to the already existing inventory of soil information.
2. Regularity of observation: in regularity of observation, the following kinds of soil survey are obtained;
i. Free survey: in this kind, there is no rigid pattern of observation
ii. Rigid grid survey: examinations of soil properties are done at regular or predetermined interval.
iii. Flexible grid survey: this is a combination of both free survey and rigid grid survey.
3. Scale of mapping: we have seven kinds of soil survey in this category namely;
i. Exploratory Survey: these are the kind of soil survey meant to identify forms of development that are physically possible within large regions of the country. Scale is usually 1: 2,000,000 – 1:1,500,000
ii. Compilation: these are soil maps produced in abstraction from other soil surveys with a scale of usually 1:1,000,000 or smaller.The National Soil Map of Nigeria belongs to this category
iii. Reconnaissance Survey: these are mostly based on remote sensing especially aerial photo imagery. Scale is usually 1: 250,000 although smaller scales have been used.
iv. Integrated survey: this is known as land system survey which is based on mapping the total physical environments. Landforms are mapping units. Scale is 1:250,000 or smaller.
v. Semi-detailed survey: in this type of survey, we have a combination of remote sensing and field work with scales varying between 1:50,000 to 1:100,000.
vi. Detailed survey: this is carried out through field examination with pre-determined numbers of observations points or spacing. Scale varies between 1: 10,000 to 1: 25,000.
vii. Intensive survey: intensive survey usually involves rigid grid approach i.e. number of observations points and spacing of observation are pre-determined. Scale of mapping varies from 1:1000 to 1: 10,000 (Soretire et al., 2012).
Procedure for a Detailed Soil Survey
1. Soils are grouped into units possessing certain common physical properties and also morphological properties, which can be readily recognized in the field.
2. Physical and morphological properties are texture, structure, colour, pH, carbonate, natural vegetation, slope, erosion, depth of soil, natural vegetation etc.
3. The soil units are usually defined on the basis of the above-mentioned characteristics of the surface soil, soil depth, slope and erosion and the soil profile. The soil units are the soil series. Cadastral (village) maps are the base maps (Scale 1: 10,000 to 1: 5,000).
4. Select a convenient starting point like a bench mark, or building or pond, or anything in the field and identify the same on the cadastral map (1: 10,000 to 1:5000).
5. The Soil Surveyor moves up or down the slope because usually the soil properties change in that direction.
6. Start walking down the slope from the starting point and continue observing the natural vegetation, slope of the land/soil erosion, soil depth, soil colour, texture, by feeling the Soil; pH with the help of universal indicator; and carbonate with the help of dilute acid at an interval of about 100 to 200 metres.
7. As you are walking down the slope and studying these properties, you suddenly find that a number of soil characteristics change; For example, slope erosion, soil depth, colour, texture etc. Then, follow the line of change in the soil propertiesand demarcate it. This is the boundary between the soil units A and B.
8. Resume walking down (Traversing) the slope till you again find that number of soil characteristics change. Follow the line of change of soil properties between the soil units B and C and demarcate it on the field. In a similar way, find out and demarcate the boundaries between soil units C and D, D, and E and so on.
9. Then dig profiles in each of the soil units A, B, C, D and E the number of profiles depend on the relative area of the soil units.
10. Study profiles, the characteristics of which become the basis of identifying soil series. If the characteristics of the profile in soil units A, B, C, D and Eare different then the soil units A, B, C, D and E become the soil series A, B, C, D and E. Then a soil survey report is written, describing the soil series and providing other useful information about the area.
Conclusion
Purpose of soil survey has to do with special purpose or general purpose. Whereas special purpose soil survey is done for specific purpose in mind general purpose soil survey is done mainly to add to the already existing inventory of soil information. Three kinds of soil survey based on regularity of observation include free survey, rigid grid and flexible grid surveys. When soil survey classification is based on scale of published map, we have seven kinds of soil survey namely; compilation, integrated survey, exploratory survey, reconnaissance survey, semi-detailed survey, detailed and intensive surveys.
