Chapter 1: Introduction
History of Soil Classification in Canada
The early years, 1914-1940
Classifying soils in Canada began with the first soil survey in Ontario in 1914. When A.J. Galbraith set out to map the soils of southern Ontario, concepts of soil and methods of soil classification were rudimentary in North America. G.N. Coffey, formerly of the U.S. Bureau of Soils, advised Galbraith during the early stages of the survey. The system of classification used was that of the U.S. Bureau of Soils, which was based largely upon geological material and texture (Ruhnke 1926). Nine "soil series" were mapped in all Ontario south of Kingston by 1920. The broad scope of these "series," which were somewhat analogous to geological formations at that time, has narrowed progressively to the present.
Changes in the system of classifying the soils of Canada have resulted from the combined effects of international developments in concepts of soils and increasing knowledge of Canadian soils. From the time of the first surveys Canadian pedologists were influenced by the concept of soil as a natural body integrating the accumulative effects of climate and vegetation acting on surficial materials. This concept was introduced by Dokuchaev about 1870, developed by other Russian soil scientists, and proclaimed to western Europe by Glinka in 1914 in a book published in German. Marbut's translation of this book made the Russian concept of soil as a natural body easily available to the English-speaking world (Glinka 1927). The concept is of paramount importance in soil science because it makes it possible to classify soils on the basis of properties of the soils themselves rather than on the basis of geology, climate, or other factors. Classification systems based on the inherent properties of the objects classified are called natural or taxonomic systems.
Recognizing the relationships between soil features and factors of climate and vegetation was not limited to Russian scientists. In the United States, Hilgard had noted this association in a book published in 1860 (Jenny 1961), and Coffey had recognized soils as natural bodies by 1912 (Kellogg 1941). However, the Russians can be credited with developing the concept of soils as natural bodies with horizons that reflect the influences of soil-forming factors, particularly climate and vegetation.
Although classification was on the basis of texture in the earliest soil surveys undertaken in the Prairie Provinces during the 1920s, an increased awareness of soil zones and of the soil profile is evident in soil survey reports published during that decade. Preliminary soil zone maps of Alberta and Saskatchewan were presented by Wyatt and Joel at the first International Congress of Soil Science in 1927. They showed the broad belts of brown, black, and gray soils. The Congress and the associated field tours brought Canadian pedologists into close contact with international concepts of soil and systems of soil classification.
Developments in soil classification occurred independently in each province because surveys were carried out by university departments of soils or chemistry. For example, a numbering system indicating the soil zone, nature of parent material, mode of deposition, profile features, and texture was developed in Alberta. J.H. Ellis in Manitoba recognized the impossibility of developing a scientific soil taxonomy based on the limited knowledge of Canadian soils in the 1920s. Influenced by concepts of C.C. Nikiforoff in Minnesota, Ellis developed a field system of soil classification that was useful in soil mapping and endures in various revised forms to this day. The system identified "associations" of soils formed on similar parent materials and "associates" that differed according to topographic position within the association (Ellis 1932, 1971).
During the 1930s soil surveys proceeded in Ontario and the Prairie Provinces and were started in British Columbia in 1931, Quebec and Nova Scotia in 1934, and New Brunswick in 1938. Soil surveys began in Prince Edward Island in 1943, the Northwest Territories and the Yukon in 1944, and Newfoundland in 1949. The few soil surveyors employed permanently by federal and provincial departments of agriculture in the 1930s worked cooperatively with personnel of university soils departments. By 1936 about 15 000 000 ha (1.7% of the land area of Canada) had been surveyed, mainly in Alberta, Saskatchewan, and Ontario. Soil classification was limited by the fragmentary knowledge of the soils of Canada.
Canadian pedologists were influenced in the 1930s by Marbut's developing ideas on soil classification, Ellis's system of field classification (Ellis 1932), and the system of classification used in the U.S. Department of Agriculture (USDA) described by Baldwin et al. (1938). The latter system divided soils at the highest category among three orders:
- Zonal soils, which are those with well-developed characteristics that reflect the influence of active factors of soil genesis such as climate, organisms, and particularly, vegetation (e.g., Podzol).
- Intrazonal soils, which are soils having more or less well-defined characteristics that reflect the dominant influence of some local factor of relief or parent material over the normal effects of climate and vegetation (e.g., Humic Gleysol).
- Azonal soils, which are soils without well-developed characteristics due either to their youth or to some condition of relief or parent material (e.g., alluvial soils).
Zonal soils were divided at the suborder level on the basis of climatic factors, and suborders were subdivided into great groups that were more or less similar to the great groups of today. Canadian experience showed that the concept of zonal soils was useful in the western plains, but was less applicable in eastern Canada. In eastern Canada parent material and relief factors had a dominant influence on soil properties and development in many areas. However, the 1938 USDA system was used in Canada, and it influenced the subsequent development of the Canadian system.
From 1940 to 1996
The formation of the National Soil Survey Committee of Canada (NSSC) was a milestone in developing soil classification and pedology, generally, in Canada. The initial organizational meeting was held in Winnipeg in 1940 by the Soils Section of the Canadian Society of Technical Agriculturists (Ellis 1971). Subcommittees were established to prepare reports on six major topics, including soil classification. At the suggestion of E.S. Archibald, Director of the Experimental Farms Service, the NSSC became a committee of the National Advisory Committee on Agricultural Services. The first executive committee of the NSSC consisted of: A. Leahey, chairman; P.C. Stobbe, secretary; F.A. Wyatt, western representative; and G.N. Ruhnke, eastern representative. Terms of reference for the NSSC were developed by A. Leahey, G.N. Ruhnke, and C.L. Wrenshall.
They were modified and restated in 1970 by the Canada Soil Survey Committee (CSSC) as follows: To act as a coordinating body among the soil survey organizations in Canada supported by the Canada Department of Agriculture, provincial departments of agriculture, research councils, and departments of soil science at universities. Its stated functions include the following:
- Improving the taxonomic classification system for Canadian soils and revision of this system because of new information.
- Improving the identification of physical features and soil characteristics used in describing and mapping soils.
- Reviewing the methods, techniques, and nomenclature used in soil surveys and the recommendation of changes necessary for either a greater measure of uniformity or for their improvement.
- Recommending investigations of problems affecting soil classification, soil formation, and the interpretation of soil survey information.
- Recommending and supporting investigations on interpretations of soil survey information for soil ratings, crop yield assessments, soil mechanics, and other purposes.
- Cooperating with specialists in soil fertility, agronomy, agrometeorology, and other disciplines in assessing interrelated problems.
Much of the credit for the present degree of realization of these objectives is due to A. Leahey, chairman from 1940 to 1966, and P.C. Stobbe, secretary from 1940 to 1969. W.A. Ehrlich was chairman from 1966 to 1971 and was succeeded by J.S. Clark. In 1969 the name of the committee was changed to the Canada Soil Survey Committee (CSSC).
Developments in soil classification in Canada since 1940 are documented in reports of the meetings of the NSSC held in 1945, 1948, 1955, 1960, 1963, 1965, and 1968 and of the CSSC in 1970, 1973, 1974, and 1976. Soil classification was one of the main items on the agenda of the first meeting, and a report by P.C. Stobbe provoked a prolonged and lively discussion. He and his committee recommended a system of field classification similar to that developed in Manitoba by Ellis (1932). The proposed system was a hierarchical one with seven categories as follows:
Soil Regions-tundra, woodland, and grassland soils.
Soil Zones-broad belts in which a dominant kind of soil occurs, such as Podzol or black soil.
Soil Subzones-major subdivisions of soil zones, such as black and degraded black.
Soil Associations or Catenas-the group of soils that occur together on the same parent material to form a land pattern.
Soil Series, Members, or Associates-the individual kinds of soils that are included in an association.
Soil Type or Soil Class-subdivisions of associations or of series based upon texture.
Soil Phase-subdivisions of mapping units based upon external soil characteristics such as stoniness and topography.
This proposal was for a field system of classification, or a system for classifying the units of soils mapped at various scales. The classes at all levels, phase to region, were segments of the landscape that included all the soil variability within the area designated. Thus, a soil zone was a land area in which a "zonal great soil group occurred as a dominant soil." The system was not intended to be a scientific or taxonomic one in which the classes at all levels had clearly defined limits based on a reasonably thorough knowledge of the properties of the entire population of soils in Canada. The proposed system was accepted for trial by the committee, which represented all provinces. Thus, an important step was taken in the development of a national system of classifying the units of soil mapped in soil surveys.
The first Canadian taxonomic system of soil classification was presented by P.C. Stobbe at the NSSC meeting in 1955. This system was a marked departure from the mapping or field classification system proposed in 1945. It probably resulted from the following circumstances:
- the greater knowledge of Canadian soils
- the desire to classify soils, even at the highest categorical level, based on the properties of the soils themselves
- the need for a taxonomic system better than the old USDA system (Baldwin et al. 1938) that focused unduly on "normal" soils. The Soil Conservation Service had begun to develop a new system in 1951, but the fourth approximation of that system was judged to be too complicated and too tentative for Canadian needs (National Soil Survey Committee 1955).
Unfortunately, formal discussion of field systems of classifying soils or soil mapping systems was dropped for several years at NSSC meetings, but the need for such systems was recognized by leaders in pedology. This need can be illustrated by an example of mapping soils at a particular scale and classifying the kinds of soil that occur.
If the map is at a scale of 1:100 000 and the smallest area delineated is a square measuring 1 cm on each side, that area represents 100 ha. Such an area commonly includes upland and lowland positions in the landscape and the associated kinds of soils. The kinds of soils within that area are identified by digging pits at different topographic positions in the landscape. At each of these points the profile exposed usually has a rather narrow range of properties that reflects the influence of soil-forming factors at that point. Therefore, the soils at each point of observation can be classified as a single class in a taxonomic system. The area delineated on the map cannot be classified as a single class in such a system because it includes several kinds of soils. However, the area mapped can be classified as a kind of soil mapping unit such as a soil association in the system of Ellis (1932). Thus, the need was evident for both a taxonomic system to permit the naming and the ordering of information about specific kinds of soils, and a mapping system to permit the ordering of information about the areas delineated on soil maps and the naming of them.
The taxonomic system outlined in 1955, which is the basis of the system used today, had six unnamed categorical levels corresponding to the order, great group, subgroup, family, series, and type. The seven taxa separated at the order level were: Chernozemic, Halomorphic, Podzolic, Forested Brown, Regosolic, Gleisolic, and Organic. Taxa were defined only in general terms down to the subgroup level. Although this was inevitable because of the lack of sufficient information, it led to differences of interpretation of the taxa in various provinces and some lack of uniformity in the use of the system. The need for correlation was clearly recognized by senior Canadian pedologists.
Progress in developing the Canadian system of soil classification since 1955 has been toward more precisely defining the taxa at all categorical levels and increasingly emphasizing soil properties as taxonomic criteria. This is evident from the reports of NSSC meetings held in 1960, 1963, 1965, and 1968, at which the main topic of discussion was soil classification. Some changes in taxa at the order, great group, and subgroup levels were made at these meetings. For example, in 1963 the Meadow and Dark Gray Gleisolic great groups were combined as Humic Gleysol; in 1965 a system of classifying soils of the Organic order was presented and accepted; in 1968 the former Podzolic order was divided into Luvisolic (clay translocation) and Podzolic (accumulation of Al and Fe organic complexes) orders, and the concept and classification of Brunisolic soils were revised. Criteria of classification involving morphological, chemical, and physical properties became increasingly specific through this period. The bases of classifying soils at the family level were outlined and the series and type categories were defined more specifically.
Following the publication of The System of Soil Classification for Canada in 1970, topics other than soil taxonomy were emphasized at CSSC meetings. However, in 1973 a Cryosolic order was proposed to classify the soils with permafrost close to the surface, and some refinements were made in several orders. These changes, including the newly developed Cryosolic order, were incorporated into The Canadian System of Soil Classification published in 1978.
Between 1945 and 1970 little consideration was given at NSSC meetings to systems of naming and classifying soil mapping units. Between 1970 and 1978 a satisfactory taxonomic system was developed. In 1978 the Expert Committee on Soil Survey was formed, replacing the Canada Soil Survey Committee. Although the work of the Canada Soil Survey Committee focused on interpretation, mapping systems, and soil degradation, soil classification also formed an important part of its activities.
In 1980, after C. Tarnocai became chairperson of the Soil Classification Working Group, work began on a number of problems in soil classification that had been identified by soil scientists from various regions. The main items covered included the classification of Gleysols (McKeague et al. 1986), Folisols (Fox 1985, Fox et al. 1987, Trowbridge 1981, and Trowbridge et al. 1985), Podzols and the definition of contrasting horizons. At a meeting of the working group held in 1984, proposals were presented on these topics and solutions were formulated and presented to the Expert Committee on Soil Survey (Tarnocai 1985). The Second Edition of The Canadian System of Soil Classification published in 1987 incorporated the recommendations accepted by the committee.
In the late 1980s, as a result of an international soil correlation tour organized by the USDA Soil Conservation Service, attention was drawn to Vertisolic soils; soils of high clay content whose properties result from shrinking and swelling, caused by wetting and drying. Previously, Vertisolic soils were thought to occur only in temperate and tropical climates. However, when international experts attending this meeting examined high clay content soils in southern Saskatchewan, they found them to have the same properties as Vertisols occurring elsewhere and considered these soils to be Vertisols. This changed viewpoint initiated activities aimed at incorporating Vertisols in The Canadian System of Soil Classification. The Canadian Soil Classification Working Group formulated a number of proposals for classifying these soils. Between 1991 and 1994 field tours were held in the Prairie Provinces, British Columbia, and eastern Canada to test these proposals in the field. As a result of these activities, the working group recommended establishing a Vertisolic order to classify these soils and set up criteria for this new soil order.
This Third Edition of The Canadian System of Soil Classification includes minor revisions throughout the system and introduces the Vertisolic order. This edition also introduces the Vertic great group for Solonetzic soils and Vertic subgroups for the Chernozemic, Gleysolic, Luvisolic, and Solonetzic soils. It also provides a major revision of the Cryosolic order resulting from international activities, both in the United States and Russia.
Rationale of Soil Taxonomy in Canada
During some 80 years of pedological work in Canada, concepts of soil and systems of classification have progressed as a result of new knowledge and new concepts developed in Canada and elsewhere. Here an attempt is made to enunciate the current rationale of soil taxonomy based on the historical material outlined in the previous section and on recent publications on pedology in Canada.
The nature of soil
The concept of soil in Canada and elsewhere (Cline 1961; Knox 1965; Simonson 1968) has changed greatly since 1914 when the first soil survey was started in Ontario. No specific definition is available from that early work, but clearly soil was thought of as the uppermost geological material. Texture was apparently considered to be its most important attribute. Currently, soil is defined in general terms by pedologists as the naturally occurring, unconsolidated, mineral or organic material at the earth's surface that is capable of supporting plant growth. Its properties usually vary with depth. They are determined by climatic factors and organisms, as conditioned by relief and, hence, water regime acting on geological materials and producing genetic horizons that differ from the parent material. In the landscape, soil merges into nonsoil entities such as exposed, consolidated rock or permanent bodies of water at arbitrarily defined boundaries. Specific definitions of soil and nonsoil are given in Chapter 2.
Because soil occurs at the surface of the earth as a continuum with variable properties, it is necessary to decide on a basic unit of soil to be described, sampled, analyzed, and classified. Such a unit was defined by United States' pedologists (Soil Survey Staff 1960) and is accepted in Canada. Called a pedon, it is the smallest, three-dimensional body at the surface of the earth that is considered to be a soil. Its lateral dimensions are 1-3.5 m and its depth is 1-2 m. The pedon is defined more specifically in Chapter 2.
Nature and purpose of soil classification
Soil classification systems are not truths that can be discovered but rather are methods of organizing information and ideas in ways that seem logical and useful (Soil Survey Staff 1960). Thus no classification system is either true or false; some systems are more logical and useful for certain objectives than others. A classification system reflects the existing knowledge and concepts concerning the population of soils being classified (Cline 1949). It must be modified as knowledge grows and new concepts develop.
Both the theoretical and practical purposes of soil classification have been discussed in the literature (Cline 1949, 1963; De Bakker 1970). The general purpose of soil classification in Canada may be stated as follows:
- to organize the knowledge of soils so that it can be recalled systematically and communicated, and that relationships may be seen among kinds of soils, among soil properties and environmental factors, and among soil properties and suitabilities of soils for various uses.
The related purposes of soil classification are to provide a framework for formulating hypotheses about soil genesis and the response of soil to management, to aid in extending knowledge of soils gained in one area to other areas having similar soils, and to provide a basis for indicating the kinds of soils within mapping units. Soil classification is essential to soil surveys, to the teaching of soils as a part of natural science, and to meeting the practical needs related to land use and management.
The overall philosophy of the Canadian system is pragmatic; the aim is to organize the knowledge of soils in a reasonable and usable way. The system is a natural, or taxonomic, one in which the classes (taxa) are based upon properties of the soils themselves and not upon interpretations of the soils for various uses. Interpretations involve a second step that is essential if the information is to be used effectively. If the taxa are defined on the basis of soil properties, and if the boundaries of these classes, or of combinations of them, are shown on a map, interpretations can be made on the basis of properties implied in the class definitions.
Misconceptions about soil taxonomy
Misconceptions about the functions of a system of soil taxonomy are evident periodically. Some of these are listed to warn users of the Canadian system against unrealistic expectations.
- It is a misconception that a good system results in the assignment of soils occurring close together to the same taxon, at least the higher categorical levels. This is neither possible, nor desireable, in some areas. Pedons a few metres apart may differ as greatly as pedons hundreds of kilometres apart within a climatic region.
- Another common misconception is that a good national system provides the most suitable groupings of soils in all areas. This is not possible because criteria based upon properties of the whole population of soils in the country are bound to differ from those developed on the basis of properties of soils in any one region. Criteria developed for a national system will inevitably result in areas where most of the soils have properties that straddle the boundary line between two taxa.
- The idea that if the system was soundly based there would be no need for changes every few years is erroneous. As new areas are surveyed, as more research is done, and as concepts of soil develop, changes in the system become inevitable to maintain a workable taxonomy.
- Another unfortunate hope is that a good system will ensure that taxa at the order level at least can be assigned unambiguously and easily in the field. Actually, in a hierarchical system the divisions between orders must be defined just as precisely as those between series. With pedons having properties close to class boundaries at any taxonomic level, classification is difficult and laboratory data may be necessary.
- The assumption is made by some that a good system permits soils occurring within mapped areas to be classified as members of not more than three series. Clearly, this is not reasonable because the number of taxononic classes occurring within a mapping unit depends upon the complexity of the pattern of soils in the landscape, on the scale of the map, and on the narrowness of class limits.
- The idea that a good system is simple enough to be clear to any layman is erroneous. Unfortunately, soil is complex and, although the general ideas of the taxonomy should be explainable in simple terms, the definitions of taxa must be complex in some instances.
- Another misconception is that a good system makes soil mapping easy. Ease of mapping depends more upon the complexity of the landscape, the access, and the predictability of the pattern of soils within segments of the landscape than upon taxonomy.
Attributes of the Canadian system
The development of soil taxonomy in Canada has been toward a system with the following attributes:
- It provides taxa for all known soils in Canada.
- It involves a hierarchical organization of several categories to permit soils at various levels of generality to be considered. Classes at high categorical levels reflect, to the extent possible, broad differences in soil environments that are related to differences in soil genesis.
- The taxa are defined specifically so as to convey the same meaning to all users.
- The taxa are concepts based upon generalizations of properties of real bodies of soils rather than idealized concepts of the kinds of soils that would result from the action of presumed genetic processes. The criteria chosen define taxa in accordance with desired groupings of soils. The groupings are not decided upon initially on the basis of arbitrary criteria.
- Differentiae among the taxa are based upon soil properties that can be observed and measured objectively in the field or, if necessary, in the laboratory.
- The system can be modified on the basis of new information and concepts without destroying the overall framework. Periodically, however, the entire framework of the system will be reevaluated.
Although taxa in the Canadian system are defined on the basis of soil properties, the system has a genetic bias in that properties or combinations of properties that reflect genesis are favored as differentiae in the higher categories. For example, the use of the terms chernozemic A and podzolic B imply genesis. The reason for the genetic bias is that it seems reasonable to combine, at high categorical levels, soils in which particular horizons developed under similar dominant processes resulting from broadly similar climatic conditions. Classification is not based directly on presumed genesis because soil genesis is incompletely understood, is subject to a wide variety of opinion, and cannot be measured simply.
Bases of criteria for defining taxa at various categorical levels
The bases for differentiating taxa at the various categorical levels are not clear cut. In a hierarchical system of soil classification, logical groupings of soils that reflect environmental factors cannot be obtained by following any rigid systematic framework in which all taxa at the same categorical level are differentiated based on a uniform specific criterion, such as acidity or texture. The fact that criteria must be based on soil properties, rather than directly on environmental factors or use evaluation was recognized by some pedologists almost three-quarters of a century ago (Joel 1926). The general bases of the different categorical levels, which are presented below, can be inferred from a study of the system. They apply better to some taxa than to others; for example, the statement for order applies more clearly to Chernozemic and Podzolic soils than to Regosolic and Brunisolic soils.
Taxa at the order level are based on properties of the pedon that reflect the nature of the soil environment and the effects of the dominant, soil-forming processes.
Great groups are soil taxa formed by subdividing each order. Thus, each great group carries with it the differentiating criteria of the order to which it belongs. In addition, taxa at the great group level are based on properties that reflect differences in the strengths of dominant processes, or a major contribution of a process in addition to the dominant one. For example, in Luvic Gleysols the dominant process is considered to be gleying, but. clay translocation is also a major process.
Subgroups are formed by subdividing each great group. Therefore, they carry the differentiating criteria of the order and the great group to which they belong. Subgroups are also differentiated on the basis of the kind and arrangement of horizons that indicate conformity to the central concept of the great group (e.g., Orthic), intergrading toward soils of another order (e.g., Gleyed, Brunisolic), or additional special features within the control section (e.g., Ortstein, Vertic).
Taxa at the family level are formed by subdividing subgroups. Thus, they carry the differentiating criteria of the order, great group, and subgroup to which they belong. Families within a subgroup are differentiated based on parent material characteristics, such as particle size, mineralogy, calcareousness, reaction, and depth, and on soil climatic factors.
Series are formed by subdividing families. Therefore, they carry all the differentiating criteria of the order, great group, subgroup, and family to which they belong. Series within a family are differentiated on the basis of detailed features of the pedon. Pedons belonging to a series have similar kinds and arrangements of horizons whose color, texture, structure, consistence, thickness, reaction, and composition fall within a narrow range. A series is a category in the taxonomic system; thus, it is a conceptual class in the same sense as an order.
A pedon is a real unit of soil in the landscape; a series is a conceptual class with defined limits based on the generalization of properties of many pedons. A particular pedon may be classified as a series if its attributes fall within the limits of those of an established series. However, it is not, strictly speaking, a series because the attributes of any one pedon do not encompass the complete range of attributes allowable within a series. Thus, it is not correct to study part of a pedon and to declare, "this is X series." Rather it should be stated, "this pedon has properties that fall within the limits of the X series," or "this pedon is classified in the X series."
Relationship of taxonomic classes to environments
A general relationship exists between kinds of environments and taxa at various levels in the system. This follows from the basis of selecting diagnostic criteria for the taxa; the primary basis at the higher levels is to select properties that reflect the environment and properties resulting from processes of soil genesis. Although the system may look like a key with classes defined precisely but arbitrarily on the basis of specific properties, it is one in which the taxa reflect, to as great an extent as possible, genetic or environmental factors.
The Podzolic order, for example, is defined based on morphological and chemical properties of the B horizon. However, these properties are associated with humid conditions, sandy to loamy parent materials, and forest or heath vegetation. Although the great groups within the order are defined on the basis of the amounts of organic C and extractable Fe and Al in the B horizons, they have broad environmental significance. Humic Podzols are associated with very wet environments, high water tables, periodic or continuous reducing conditions, hydrophytic vegetation, and, commonly, a peaty surface. Ferro-Humic Podzols occur in areas of high effective precipitation, but they are not under reducing conditions for prolonged periods. Humo-Ferric Podzols generally occur in less humid environments than the other great groups in the order. An interrelation of climatic and vegetative factors, parent material, and relief determines the occurrence of the different classes of Podzolic soils. Similarly, general relationships exist between other orders, great groups, and soil environmental factors. However, these relationships are much less clear for some Regosolic and Brunisolic soils than they are for most soils of other orders. At lower categorical levels, in general, relationships between soil taxa and factors of the soil environment become increasingly close.
Relationship of the Canadian system to other systems of soil taxonomy
The numerous national systems of soil taxonomy might be looked upon as indications of the youthfulness of soil science. Knowledge of the properties of the soils of the world is far from complete, therefore it is not possible to develop an international system of classification for the whole population of known and unknown soils. Probably even after such a system has been developed, national systems will remain in use because they are familiar and are thought to be more useful for the restricted population of soils within the country. The soil units defined for the FAO-UNESCO world soil map project are useful in international soil correlation, but they do not constitute a complete system of soil taxonomy (FAO 1985). The closest approach to a comprehensive system of soil taxonomy is that produced by the Soil Survey Staff of USDA (1994), which has been under development since 1951. Like previous U.S. systems, it has had a major influence on soil taxonomy in Canada and elsewhere.
The Canadian system of soil taxonomy is more closely related to the U.S. system than to any other. Both are hierarchical, and the taxa are defined on the basis of measurable soil properties. However, they differ in several respects. The Canadian system is designed to classify only soils that occur in Canada and is not a comprehensive system. The U.S. system has a suborder, which is a category that the Canadian system does not have. In the Canadian system Solonetzic and Gleysolic soils are differentiated at the highest categorical level as in the Russian and some other European systems. These soils are differentiated at the suborder or great group level in the U.S. system. Perhaps the main difference between the two systems is that all horizons to the surface may be diagnostic in the Canadian system, whereas horizons below the depth of plowing are emphasized in the U.S. system. This may be because 90% of the area of Canada is not likely to be cultivated.
The Canadian system is a hierarchical one in which the classes are conceptual, based upon the generalization of properties of real bodies of soil. Taxa are defined on the basis of observable and measurable soil properties that reflect processes of soil genesis and environmental factors. The development of the system has progressed with the increasing knowledge of the soils of Canada obtained through pedological surveys carried out over an 80-year period. The system has been influenced strongly by concepts developed in other countries, but some aspects are uniquely Canadian. The system is imperfect because it is based on a limited knowledge of the vast population of soils in the country. However, the system does make it possible to assign soils throughout Canada to taxa at various levels of generalization and to organize the knowledge of soils in such a way that relationships between factors of the environment and soil development can be seen. It is possible to define the kinds of soils that occur within units on soil maps, and to provide a basis for evaluating mapped areas of soil for a variety of potential uses.