1.0 THE ESSENCE OF SOIL

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1.1 Objectives and Purpose of the Manual

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At the outset, it is important for the user to know that this is a "how-to" manual. The objective of the manual is to provide a discussion of soil inventory methodology and to define what soil inventory is, how it is conducted, and how it can be used for land management. The purpose is to provide a thorough description of the techniques used for field mapping, map unit definition and data interpretation. The manual is intended for use by those who are planning to undertake a soil inventory. It also provides those who wish to utilize existing soil maps and data the means of applying and understanding the consistent and scientifically valid methods used in soil survey. This manual is not a fully comprehensive document in the sense of duplicating other manuals and reports. Rather, it references and incorporates work done by others, and restricts itself to the basic "how-to" purpose.

There are seven publications that are most relevant for understanding how to conduct a soil inventory and which were referenced in the preparation of this document. These are listed below and the reader is encouraged to refer to them should further detail be required1.

Soil inventories have been conducted in British Columbia for almost seventy years (Agriculture Canada, 1978). The first, in 1926, was undertaken by the B.C. Forest Service in the Central Interior for the purpose of establishing forest reserves. Soil inventories have progressed from this initial work in terms of both mapping scales and the taxonomy used to describe soils, as well as in the definition of map units and the provision of interpretations for land use planning and management. Today we find in B.C. a variety of maps, reports and data that collectively are referred to as soil surveys or soil inventories.

The terms "Soil Inventory" and "Soil Survey" are used interchangeably in this manual. They refer to the systematic examination, description, classification, mapping and specified use interpretations of the various soils in an area. The kind and frequency of field examinations defines the level (intensity) of soil survey. Five intensity levels are outlined in Mapping Systems Working Group (1981) and are discussed in following sections of this manual. In all cases, for a map to be called a soil map, the map units must be based largely on soil characteristics (pedons) (Figure 1.1). To say this another way, as long as the map legend (i.e. the map units) is defined in terms of soil parameters, then the map is a soils map. This does not diminish the role of soil scientists and the appreciation of soils information in the preparation of other forms of maps. Biophysical, Ecological and Vegetation maps, to name a few, generally all use some soils-related data in the definition of a map unit. While these maps should not be confused with or described as soils maps, they serve a specific purpose to which the soil scientist can offer input and guidance.

A brief discussion is provided in the following chapters with regard to soil survey intensity and purpose. Of all the conundrums facing the soil scientist who is responsible for the survey, these factors are perhaps the most difficult to resolve. In large measure, these result from the multitude of users who desire to use the information as well as the time and budget available for the work. Often, these are more or less mutually exclusive. It is up to the individual(s) designing the survey to make the appropriate decisions at the outset of the program and to ensure these decisions are adhered to throughout the completion of the workplan. The decisions do not reduce or alter the value of a general purpose survey or a special purpose survey, but instead point to the need for the soil inventory to have an objective. In all cases, the usefulness of a soil survey depends on the accuracy (regardless of level of intensity) of the mapping of soil properties and the relevance of the properties for the purpose or objective of the survey. A general purpose survey may satisfy many needs but fail to provide the specific requirements that only a special purpose survey can supply (e.g. land suitable for irrigation). On the other hand, special surveys will often concentrate on only a few specific soil parameters and thus may require the survey area to be re-mapped for other applications. Spending the time to fully define the purpose and objective of the soil survey at the planning stage is very important to the utility of the product provided to the user groups. It must also be recognized that land resources consist of a wide variety of natural elements such as hydrology, climate, vegetation, and geology as well as socio-economic factors such as land use, institutional and legislative factors, land ownership and political constraints. Soils constitute a part of this complex array and as such are not usually the sole basis for making decisions on land use and management.

Figure 1.1 A Land unit and the soil profile that characterizes it

1.2 Definition of Soil Classification and Soil Inventory

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Soil surveyors have a story to tell. Their science is the link between the animate and the inanimate world; that is, between the world of living things (man, plants, animals) and the physical world of geology, geomorphology, climate and time. The story deals with the character of the soil and soil-like materials that occur in the survey area, where they are located, how they function as part of the landscape and how they can be used. In essence, there are two main objectives of a soil survey:

1. the identification, description and classification of the different kinds of soils in a given area.

2. the identification, prediction and delineation of the different kinds or combinations of soils on a map in a consistent manner.

There are two central products that result from achieving these objectives:

1. a soil map and legend.

2. a soil report or similar related data base (used to describe, define, classify, and interpret for use, the different kinds of mapped soils).

In order for the soil surveyor to meet these objectives and provide these products to the user groups he2 must have a number of specific definitions and means (or tools) available to him. First and foremost, he requires a definition of what soil is and also a taxonomy for classifying the different kinds of soils he encounters in the survey area. Perhaps the most comprehensive and useful definition of soil is that provided by the Soil Conservation Service of the U.S. Department of Agriculture (U.S.D.A, 1975b) which parallels that used in Canada and B.C. (Agriculture Canada, 1987a). That is:

"Soil is the collection of natural bodies on the earth's surface, in places modified or even made by man of earthy materials, containing living matter, and supporting or capable of supporting plants out-of-doors."

In this context, the upper limit of soil is air or shallow water, the margins grade to deep water or barren areas (bedrock, ice, etc.) and the lower limit is the extent of main biologic activity (usually the common rooting depth of native perennial plants). The vertical dimension of soil is defined by what is termed the control section. For mineral soils this extends to a maximum depth of 2 metres and for organic soils to a maximum depth of 1.6 metres. The soil usually (but not always) contains soil horizons (more or less horizontal layers) that differ from each other and from the underlying soil parent material due to the independent and combined interactions among climate, living organisms, parent material and relief acting over a period of time. For the most part, soil is the natural medium for the growth of land plants.

Part of the definition of soil is that it is a "natural body" much like a plant or animal. Similar to these other natural populations, soils are identified by their shape and form (morphology) and are classified by a taxonomic system based on their observed and measured characteristics. In B.C., the Canadian System of Soil Classification (Agriculture Canada, 1987a) is used for this purpose. This system recognizes that soils with variable properties occur as a continuum on the earths surface and that it is necessary to define a basic unit of soil that can be described, sampled, analyzed and classified.

The basic soil unit is called a Pedon and it represents a 3-dimensional soil body that usually measures approximately 1 to 3 metres laterally and 1 to 2 metres vertically. It is the Pedon that is used to describe the 3-dimensional nature of soil. To adequately determine the characteristics of a soil, it is not enough to examine only the soil surface but, more importantly, the horizons or layers that make up the soil must be examined and studied as well.

To map soil distribution, it is necessary to classify a unit or area of soil that differs from adjoining units in the landscape due to the interactions of soil forming factors, singularly or collectively, (climate, parent materials, living organisms, time and relief). This recognition leads to the definition of a Polypedon which is a group of contiguous, similar pedons that are bounded by either nonsoil or by pedons of differing character (Figure 1.2). While the pedon defines a soil taxonomic unit, a soil map unit may contain more than one kind of pedon.

The Canadian System of Soil Classification provides the taxonomy for classifying soils and, like any classification system, it is an artificial arrangement of things, developed for the purpose of organizing knowledge. The system recognizes sets of soil properties and provides names for specific soils that are similar, enabling recall of scientific facts and interpretive values about the soil. During the conduct of a soil inventory, the classification system helps to identify map units, to understand and define relationships among soils in the map area, and helps the surveyor remember soil properties that are used in the makeup of the soil map units. However, map units are not necessarily the same as soil taxa since the limits imposed by a soil taxonomic class rarely coincide precisely with a mappable area. The polypedon provides the means for expanding the concept of an individual taxa to the recognition of a map unit.

1.3 The Nature and Purpose of Soil

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Soil scientists (pedologists) conduct a soil survey in order to learn what kinds of soils and soil-like material are in the survey area, where they are located, their ecological relationships and how they can be used. The purpose is to produce a soil map that is a 2-dimensional representation of a 3-dimensional landscape (Figure 1.3). This is perhaps the most profound attribute of a soil map in that it is not simply a series of points for which data has been collected but rather the extrapolation of this data to provide the areal extent of a landscape element that is represented by similar points. Hence, the "art" of soil surveying.

The principal intention of a soil survey is one of prediction. Data about soils is gathered and their distribution mapped, in order to predict or forecast their behaviour for various uses and their response to certain management practices. This aim is universal for all soil surveys, regardless of the landscape involved or the scale and intensity of mapping. By its very nature, a soil survey requires that many excavations be made in order to expose the soil profile (the sequence of horizons or layers that make up the soil) and examine it. The soil scientist makes comparisons among the profiles in the map area as well as with others that have been classified in surrounding areas. As well as gathering information on the steepness, length and shape of slopes, drainage patterns, the kinds of native plants, and the kind of bedrock, the soil surveyor groups those soils that have profiles that are similar or alike. These soil groupings are then used to differentiate map units. Depending on the scale and purpose of the survey, these may be as detailed as soil series or phases of soil series, or they may be soil associations where unique natural landscape units are mapped that have a distinctive pattern of soils, relief and drainage pattern. Once the soils of the area are basically understood, the pedologist prepares a legend that acts as a guide to naming and describing the soils that are to be mapped. On aerial photographs and/or base maps, the soil surveyor then draws the boundaries of the individual soil map units. During the progress of a soil survey, samples of soils are taken as needed for laboratory analysis of chemical and physical properties. All of this information, including observed, inferred and measured ratings of suitability and limitations, is then clearly organized in an understandable manner in a soil report and/or digital data base.

1.4 The Use of Soil Inventories

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Soil inventories help to increase our knowledge about the natural world we live in but are usually undertaken for a more practical purpose. All soil inventories have objectives which can either be broad or site specific. Historically in B.C., soil inventories have usually been done to satisfy more than a single purpose. They often covered large areas that had more than one kind of important land use and the user groups often had varied interests and needs.

In B.C., soil surveys have been and continue to be used for four basic purposes (a number of examples are provided under each heading):

1. Land Use Planning

2. Site Specific Land Evaluations

3. Land Appraisal

4. Environmental Protection

All of these uses of soil inventory information have one feature in common. The soil inventory provides scientific information regarding the potential of each soil for a specified land use, as well as limitations to this use. In addition, locations can be selected where the soil properties are favourable or practices can be planned that will overcome the limitations.

Figure 1.2 Polypedons as units of soil and landscape classification

Figure 1.3 The 3-dimensional relationship of named soil units and landscape features

1 Agriculture Canada, Expert Committee on Soil Survey. 1987a. The Canadian System of Soil Classification. 2nd ed. Agriculture Canada. 1646. 164 pp.

Mapping Systems Working Group. 1981. A Soil Mapping System for Canada: Revised. Land Resource Research Institute, Contribution No. 142. Agriculture Canada, Ottawa. 94 pp.

Agriculture Canada Expert Committee on Soil Survey. 1987b. Soil Survey Handbook - Volume 1. Land Resource Research Centre, Contribution No. 85-30. Technical Bulletin 1987-9E. Agriculture Canada, Ottawa.

U.S. Department of Agriculture. 1975a. Revised Soil Survey Handbook. Agricultural Handbook No. 18, Revised. Soil Conservation Service. Washington, D.C.

U.S. Department of Agriculture. 1975b. Soil Taxonomy. Agricultural Handbook No. 436. Soil Conservation Service. Washington, D.C.

Luttmerding, H.A., D.A. Demarchi, E.C. Lea, D.V. Meidinger and T.Vold (eds.). 1990. Describing Ecosystems in the Field - 2nd. Edition. MOE Manual 11, Ministry of Environment and Ministry of Forests. Victoria, B.C. 213pp.

Canadian Society of Soil Science. 1993. M.R. Carton (ed). Soil Sampling and Methods of Analysis. Lewis Publisher. 823pp.

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