In most cases the results of both soil surveys and studies derived from them are expressed in the form of maps indicating the distribution of different recognized units and their relationships.

Pedagogical and non-pedological soil maps include a vast variety of forms, but these can be distinguished by the density and precision of their detail, their scale, and their legend.

At the end of this article, students should be able to;

·  Different kinds of maps as a function of the density and precision of detail contained in their information

·  Different kinds of maps as a function of scale

·  Different kinds of maps as a function of their objectives

·  Methods of soil mapping

 

Important facts to know

·Maps could be as a function of density and precision of detail of their information

·There are three levels of cartography distinguished on the basis of precision, as the above-mentioned type of map is rarely encountered

· A common classification of soil maps is as a function of their field scale

·Maps may be created as a function of their objectives

 

Types of Soil Maps

A. Maps as a function of the density and precision

In regions, where essentially no valuable surveys of terrestrial resources exist, maps of the “probable” distribution of principal soil types expected as a function of existing information on soil formative factors (geologic deposits, climate, topography, and even vegetation), can be compiled. These maps do not have definite significance except at the scale of synthesis (1:1,000,000 to 1:5,000,000), or at medium scales (1:200,000 to 1:500,000). There are three levels of cartography distinguished on the basis of precision:

1. Reconnaissance maps: These are based on observations and results obtained from traverses conducted throughout the study region and on known elements of factors of formation, as well as relationships which have been established during the course of the investigation between the observed soils and those diverse factors in particular, at the end of the study of the toposequences formed over the principal parent rocks of the area. The soil map of France at the scale of 1: 1,000,000 conforms to this definition, at least for the majority of the country.

2. Semi-detailed maps: Such surveys are carried out using traditional procedures, but the precision of observation, at least theoretically, corresponds to one observation of the map.

3. Detailed maps: These maps are the result of very precise, detailed studies. The level of precision necessary for this category is minimally four observations for eachcm2 of the map. 

Such limits of precision are very theoretical, and are hard to apply to practical situations since calculations of the gain in precision are difficult (the coefficient by which it would be necessary to divide the preceding recommendations or multiply the envisaged surfaces for a known point) due to the use of aerial photography and additional satellite imagery.

The use of these modem techniques permits greater rapidity and more detail in establishing the limits between the map units, it certainly needs to be supported by numerous traverses and observations of the

 

B. Maps as a function of scale

Another common habit is the classification of soil maps as a function of their field scale.

However, it should be remembered that field work is often undertaken at a scale that is at least double, and preferably quadruple to, that at which the map is published. (For example, in France the field scale is 1:25,000for a 1: 100,000published map).  Classification on this basis has different potential possibilities and significance for the use of these documents.

1. Small scale maps: Maps at the scale of 1:1,000,000 or smaller permit general interpretations. As such they are of great didactic value since they permit the performance of interesting geographical studies of soils in either diverse regions or on several continents, and allow useful extrapolations about the consequences of land use, in particular agronomic use.

According to our conception with respect to the French classification system, the legends of such maps can include classification levels as low as subgroups with their associated phases, and can even distinguish those families which have particular importance.

2. Medium scale maps: Scales of 1:50,000 or 1:100,000 are correlated with maps designed for regional planning. These in effect serve as a base for prospective work. In France, those at 1:100,000 have been retained simply because of the time and effort invested; those at 1:50,000 are more interesting in terms of their applications.

In tropical countries a scale of 1:200,000 as in the soil map of Bossangoa (Boulvert, 1974) or 1:500,000 in the map of Upper Volta (Fauck, 1977), is more commonly utilized.

In the legend of these maps the soil families as distinguished by the lithographic nature of the parent material can be indicated, even as can be soil series that generally correspond to significant gradations of soil depth for the land use, especially if it is principally agricultural.

3. Large scale maps: At scales larger than 1:50,000, the soil map permits practical applications for local development planning and area development. Soil series, and even phases of those characterized by different erosion intensities or internal drainage conditions, are distinguished on the legends of these maps. Even if these two general methods of soil map classification are clearly different, their results nevertheless partially overlap.

For example, maps at a scale of 1: 1,000,000or at smaller scale are in general types of reconnaissance maps if they are not really derived from a synthesis of more detailed maps, such as those at the 1:200,000 or1: 100,000 scales.

Similarly, maps at larger scales are not always reconnaissance maps; rather they are more often detailed maps.

C. Maps as a function of their objectives

1. Pedological maps: Theoretically, for pedological maps, the kinds of maps and legends follow the rules of the precision and level of information, as a function of their scale as given in the beginning of the second part of this paper. The legend is linked as narrowly as possible to a soil classification system, as for example, the morphogenetic(soils map of France) or morphological (soil map of the U. S.) classification systems.

2. Regional planning maps: In the last several years, it has become increasingly more apparent that representation of the milieu at a medium scale (1: 100,000 or 1:200,000) is insufficient as a basis for regional planning. The global characterization of the evolution of diverse soil types, their distribution, and even their relationships with various factors will not suffice for a general description of the milieu for expressing the general possibility of its use.

Thus, various authors have attempted to accomplish the objective at this level by presenting maps that are both pedological and morphogenetic.

Without remaining at the initial stage of French soil maps where geomorphological descriptions do not appear except in the form of accessory maps at a smaller scale, nor proceeding to the morphogenetic maps in which soil characteristics appear only in a secondary form, the methodology of Beaudou and Chatelin (1976) can be followed:  

A description of the pedological regions, followed by pedological soil landscapes, and finally functional segments or elements of toposequences and catenas of soils. Eschenbrenner and Badare (1975) used schematic drawings to describe and explain morphogenetic landscapes of the northern Ivory Coast.

In this method, the landscapes are defined by the presence of characteristic morphological elements: inselbergs, residual relief, buttes (which are generally cupped with ironstone), the remains of plateaus, derived forms more or less flattened or convex with, an appearance of the slopes of lower bottoms, and nick-point values. Landscapes are also defined by the relative importance of soils at the level of the subgroup and their associated phases, and even of the families.

Maps of grouped morpho-pedological landscapes have been constructed at the scale of 1:200,000; but each of them is supplemented by a pedological detailed map, at the scale of 1:50,000, which is representative of a typical landscape, and by corresponding air photos.

3. Maps of agronomic application: As has been previously stated, maps of agronomic applications can be very different, both in their detail and scale, but they must be based on a soils map established on an identical or larger scale.

i. Maps of soil resources are established at smaller scales (such as the 1:500,000map of Upper Volta), and they are analytical in nature. They include delineation of agro climatic zones and emphasize texture, primarily that of the surface horizon, but also that of the lower horizon to the extent that it affects plant performance.

Taxonomic units are indicated with respect to the principal kinds of improvements proposed for various characteristic features: drainage conditions, actual water consumption, organic matter content, exchangeable bases, physical properties (particularly unfavorable ones), and the presence of toxic elements.

Some subunits are defined by the association of different component units in a zone or “spot” of the soils map, as this had been indicated in the units of the pedological map.

In northern climatic zones, cultivatable lands have been separated into areas suitable for dry land and irrigated agriculture and rangelands. On the map itself, a table was compiled that indicated the order of the units and subunits as assembled on the pedological map, and these units were given values characteristic of the various land uses for each of the retained fertility factors.

ii. At medium and detailed scales (1:100,000) or larger, synthesized maps of optimum agricultural utilization or suitability for cultivation are assembled.

The legend includes units of “universal agricultural value” and the principal possible uses as a function of the soil characteristics themselves (their type of evolution, parent material. depth, etc.) and also as a function of their environment, slope, degree of erosion, etc.

The most interesting system, as previously mentioned, indicates for each unit of land the relative fertility for each of the principal kinds of use or possible cultivation groupings, and the principal for seen improvements.

It is of course indispensable that these documents be prepared with collaboration of an agronomist. 

An example is given by the management maps compiled for the high-plateau steppes of Algeria which were prepared by Pouget (1977) in collaboration with geomorphologists, botanists, and agronomists.

The maps include recommendations and for seen management and- the potential yield of forages.

iii. Maps of cultivation constraints have been rarely established by French pedologists, as many of the previous map types include in their taxonomic description’s constraints such as “utilizable depth” or various other unfavorable physical properties.

However, maps have been made for the northern Cameroons by P. Brabant that analyze depth, texture, profile differentiation, insufficiency or excess of available water, and degree and danger of erosion. 

They have also been made in France by the “Organization for the Management of the Hills of Gascogne.” The limiting factors are primarily the slope and the depth of usable land, and extreme textures, the excess of calcareous materials and any fertility or chemical insufficiencies.

iv. In France, purely thematic maps are also established at very detailed scales with regard to drainage operations (various working groups of INRA), or for particular irrigated cultivations (Organization for the Management of Lower-Rhône Languedoc).

The maps compiled at very small scales (1:1,000,000 or 1:5,000,000) concerning the dangers of desert formation and the degradation of soils.

  

Soil Mapping and the Scientific Method

Soil mapping uses the scientific method, in which the scientist must:

(1) Develop questions

(2) Generate hypotheses that answer those questions

(3) Test the hypotheses

(4) Confirm or reject the hypotheses.

After a tentative delineation of a soil body is drawn on an aerial photo or digital image, the soil mapper (step 1) questions what type of soil exists within that delineation.

Typically, the delineation follows a landscape feature, such as a large flood plain or a ridge summit. Based on previous knowledge about the soils of the region, the mapper (step 2) develops hypotheses, such as the Alpha and/or Beta series occurs within the delineation.

The mapper (step 3) tests those hypotheses by auguring, backhoe trenching, or observing natural exposures and (step 4) confirms or rejects each hypothesis.

After documenting the results, the mapper returns to step 1 (develops questions) and repeats the process for a neighboring area. This process allows the soil scientist to map soils efficiently.

Rather than making a large number of observations on a regular grid pattern to discover the kind of soil present, the mapper selects a limited number of strategically located points in the landscape to make observations.

The observations confirm or reject the previously developed model.

The mapper essentially is predicting the soil beforehand and only making an observation to confirm the prediction, rather than discovering the soil only after each observation is made.

As long as the model is accurate, relatively few observations are required to make an accurate map (Hudson, 1992).

The scientific method is also used when investigating soil genesis. Although soil mapping and soil taxonomic classes are based on quantifiable properties rather than soil genesis (Smith, 1963), it is nevertheless useful for the soil mapper to develop conceptual models about soil genesis throughout the mapping process (Arnold, 1965).

The most useful is the “multiple working hypotheses” method, which is based on the premise that when a scientist creates multiple hypotheses for an observed feature rather than one hypothesis, they are less likely to develop a parental attachment to “their” hypothesis (Chamberlin, 1897).

Instead, the scientist becomes engaged in finding evidence that disproves each of the competing hypotheses.

The “working hypothesis” is the one that survives. This method of testing multiple hypotheses simultaneously not only enhances the quality of conceptual models but also lessens antagonistic debates between scientific colleagues (Platt, 1964).

 

Soil map

Historical Approach

Aerial photographs were used as the mapping base in most soil survey areas in the United States during the 20th century. Conventional panchromatic (black and white) photography, color photography, and infrared photography were used for remote sensing and as base maps for the soil survey.

Information on the applicability of each type of base map and how the older map products were used is covered in the 1993 Soil Survey Manual (Soil Survey Division Staff, 1993).

Aerial Photographs

Even in the current digital age, the use of aerial photographs remains an effective means of mapping soils in areas where suitable digital imagery and data layers or the required skills, resources, or support for digital mapping techniques are not available. 

Aerial photographs are still a viable mapping base in soil survey. They provide important clues about kinds of soil from the shape and color of the surface and the vegetation. 

The relationships between patterns of soil and patterns of images on photographs for an area can be determined. These relationships can be used to predict the location of soil boundaries and the kinds of soil within them.

Aerial photographs using spectral bands not visible to the eye, such as color infrared, enable subtle differences in plant communities to be observed. Other spectral bands in the infrared are useful in distinguishing differences in mineralogy and moisture on the soil surface and also have better cloud penetration. These data must be interpreted by relating the visual pattern on the photographs to soil characteristics found by inspection on the ground.

Features, such as roads, railroads, buildings, lakes, rivers, and field boundaries, and many kinds of vegetation can be recognized on aerial photographs and serve as location aids.

Cultural features commonly are the easiest features to recognize on aerial photos, but they generally do not coincide precisely with differences in soils, except in areas with significant anthropogenic alteration.

Relief can be perceived by stereoscopic study. Relief features are helpful in locating many soil boundaries on the map.

Topographic maps also provide insight to relief, slope, and aspect. Relief also identifies many kinds of landforms commonly related to kinds of soil.

Many landforms (e.g., terraces, flood plains, sand dunes, kames, and eskers) can be identified and delineated reliably according to their shapes, relative heights, and slopes. Their relationship to streams and other landforms provides additional clues. The soil scientist must understand geomorphology to take full advantage of photo interpretation.

Accurate soil maps cannot be produced solely by interpretation of aerial photographs. Time and place influence the clues visible on the photographs. Human activities have changed patterns of vegetation and confounded their relationships to soil patterns. The clues must be correlated with soil attributes and verified in the field.

Contemporary Approach:

Digital imagery has replaced photographs as the mapping base in 21st century soil survey. The ability to overlay multiple imagery resources for comparisons, the ability to quickly adjust scale, and the use of raster-based soil maps have increased the speed of delivering soil survey products as well as the variety of products available.

Customized soil survey products are enhanced by the choice of background imagery (e.g., color imagery and topographic imagery) used to display soil survey information.

Conclusion on Soil Mapping: Meaning, Types and Methods

In regions, where essentially no valuable surveys of terrestrial resources exist, maps of the “probable” distribution of principal soil types expected as a function of existing information on soil formative factors (geologic deposits, climate, topography, and even vegetation), can be compiled.

Another common habit is the classification of soil maps as a function of their field scale.

 However, it should be remembered that field work is often undertaken at a scale that is at least double, and preferably quadruple to, that at which the map is published. (For example, in France the field scale is 1:25,000 for a 1: 100,000 published map).

Maps of agronomic applications can be very different, both in their detail and scale, but they must be based on a soils map established on an identical or larger scale.

Maps could be as a function of density and precision of detail of their information. There are three levels of cartography distinguished on the basis of precision, as the above-mentioned type of map is rarely encountered. A common classification of soil maps is as a function of their field scale and maps may also be created as a function of their objectives.