2.0 THE ORGANIZATION FOR A SOIL INVENTORY

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2.1 Defining the Purpose and Objectives of the Soil Inventory

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As discussed in more detail in other sections of this chapter, it cannot be overstressed that it is essential for the purpose or objective of the planned soil survey to be stated clearly at the outset. Far too often in the past, unclarity of purpose has resulted in confusion and misunderstanding during the conduct and utilization of the survey.

The purpose or objectives of the survey can be simple or complex. They vary according to the uses of soil for which predictions are needed. Essentially all of the decisions which must be made during the planning stage of the survey (e.g. scale, type of map unit) are based on the identification and determination of purpose. Although this may be difficult due to the variety of user groups who wish to use the information from the soil survey, this task must not be omitted or diluted.

2.2 Designing the Soil

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The objectives of a soil survey dictate the design of the inventory and how the map is made. From the objective(s) the following decisions should be made:

1. The smallest size of map polygons to be differentiated.

2. Field inspection density required.

3. Map scale.

4. Differentiating criteria for the soil map units.

5. The type of legend to be formulated.

The size of the soil survey area, along with decisions regarding the above points, will determine the schedule for the project and the cost. The type of user groups will have an influence on the style of legend and map symbols as well as on the base maps used for final publication. For example, regional land use planning may require different forms of presentation than would applications for specific forestry land uses. In addition, it will be necessary to decide on the format of the final map and supporting data. For example, will there be an electronic data base and/or hard copy of the maps, legend and supporting data.

Organizational items requiring decisions are not independent; changes in any one usually affects the others, often in a profound manner. Lack of flexibility in any one factor will also influence the others.

2.2.1 Minimum Size Delineation

The minimum size delineation is the size of the smallest polygon on the soil map. It is not the average or usual size, but the smallest area to be shown as a map polygon. The determination is usually based on cartographic considerations related to the ability to print a symbol inside the polygon, and map legibility. The recommended size is 0.5 cm2. Some projects may require a minimum size delineation that is larger than 0.5 cm2 but this value is generally used by most soil survey organizations. Table 2.1 provides an outline of the relationship between map scale and the size of map delineations. For example, if the objectives of the survey requires that soil areas as small as 2 hectares be differentiated, then the appropriate map scale should not be smaller than 1:20,000. Where specific small features need to be identified, on-site symbols (e.g. for excavations) can be used. These, however, should be standardized symbols for the soil survey area. Examples of standard on-site symbols are provided in Mapping Systems Working Group (1981). The use of Geographic Information Systems (GIS) and their associated cartographic methods provide the means of plotting maps at any scale and are a useful tool for displaying and recording soil data and improving map legibility.

Table 2.1 The Relationship Between Map Scale, Map Unit Area (cm2)
and Area on the Ground (ha) *

Map Unit Area cm2

Map Scale

 

1:5,000

1:20,000

1:50,000

1:100,000

1:250,000

 

(ha)

(ha)

(ha)

(ha)

(ha)

0.25

0.06

1

6.25

25

156

0.5

0.13

2

12.5

50

312

1.0

0.25

4

25

100

625

5.0

1.25

20

125

500

3125

10

2.5

40

250

1000

6250

100

25

400

2500

10000

62500

* From Mapping Systems Working Group, 1981.

Map legibility is important, and is a function of the average size of the map polygons. The use of colour for polygon differentiation aids in map legibility. With todays cartographic tools, legibility is usually not a significant issue unless the average size polygon approaches the minimum size delineation. In addition, the use of digital mapping can lessen concerns regarding legibility since it is possible to specify particular map units and other attributes when making use of the soil map.

2.2.2 Survey Intensity Level and Map

Given that the objective of a soil survey is to provide information on the soil resource to particular user groups for the purpose of predicting soil performance, it is important that the user be provided with an understanding of the accuracy and precision of the mapping and data gathering. This is fundamental, since interpretations of the soil data are only as useful as the basic information utilized for the interpretations.

While the objectives of the soil survey will determine the level of detail required, there are several terms that require definition in order to communicate these concepts to the user groups (Mapping Systems Working Group, 1981). They have profound implications in terms of the usefulness of the survey, map scale, schedule and cost.

Accuracy (Also referred to as Reliability)

The closeness with which the information portrayed on the map and in the data base conforms to the actual soil characteristics in the field.

Precision (Also referred to as Purity or Level of Detail)

An expression of the range of a soil or soil properties in a map unit. In other words, how much and what types of variability from a mean value for a particular soil characteristic is there within map units that are identified by the same symbol.

Scale

The unit of measurement on the map in relation to the unit of measurement on the ground. For example, a common scale such as 1:20,000 denotes 1 unit of measurement on the map equals 20,000 of the same unit of measurement on the ground. Soil mapping often makes a distinction between a working or compilation scale (for field work) and a publication scale.

For the purpose of soil mapping in B.C. and Canada, precision required in the mapping is determined by Survey Intensity Level (SIL). This term is often used by soil surveyors when they discuss scale. Because precision and SIL are linked, the soil surveyor, when discussing the scale of the soil map with colleagues, is also implying a certain level of detail or precision. The five (5) usual SIL's are defined in Table 2.2.

In an indirect way, the reliability of soil map units can be evaluated by the density of field inspections. Table 2.3 provides examples of the number of inspections per square centimetre that have been commonly used. This table also shows that the use of aerial photography significantly reduces the density of inspections required to achieve the same level of reliability. This may also be the case with other forms of remote sensing such as data and imagery provided by satellites (e.g. LANDSAT and SPOT - the french acronym for Trial Earth Observation System).

Table 2.2 Criteria for Identifying Survey Intensity Levels3

Survey Intensity Level (SIL)

Common Name4

Definitive Characteristics

  

Associated Features

Appropriate scale (usual)

   

Inspection Intensity

Methods of Investigation

Main Kinds of Soil Components

Map Units5

  

SIL1

Very detailed

At least one inspection in every map delineation (one per 1-5 ha). Boundaries observed throughout entire length (over 50% in wooded area).

Transects and traverses less than 0.5 km apart. Profile descriptions, samples and laboratory analyses for all soils.

Series or phases of series.

Mainly simple units.

1:14,000 or larger (1:5,000).

SIL2

Detailed

At least one inspection in 90% of the delineations (one per 2-20 ha). Boundaries plotted through observations and interpretation of aerial photographs, verified at closely spaced intervals.

Transects and traverses less than 1.5 km apart. Profile descriptions, samples and laboratory analyses for all major soils.

Series or phases of series.

Simple and compound units.

1:5,000 to 1:40,000 (1:20,000).

SIL3

Reconnaissance

At least one inspection in over 60% of delineations (one per 20-200 ha). Boundaries plotted through observation and interpretation of aerial photographs, verified with some observations.

Traverses mainly by vehicle up to 3 km apart. Profile descriptions, samples and laboratory analyses for most major named soils.

Series or phases of subgroups.

Compound and some simple units.

1:20,000 to 1:200,000 (1:50,000).

SIL4

Broad reconnaissance

At least one inspection in over 30% of delineations (one per 100-1000 ha). Representative delineations studied, remainder spot checked or interpreted. Nearly all boundaries inferred from remotely sensed data and/or aerial photos.

Traverse mainly be vehicle up to 8 km apart. Helicopter may be used in some areas. Profile descriptions for major soils, samples and laboratory analyses for some.

Phases of subgroups.

Mainly compound.

1:50,000 to 1:300,000 (1:100,000).

SIL5

Exploratory

Descriptions in less than 30% of delineations. Mapping by widely spaced observations. Boundaries established by remotely sensed data and/or aerial photo interpretation.

Traverses 20 km or more apart. Use of helicopter common. Profile descriptions for major soils, samples and laboratory analyses for a few.

Phases of Soil subgroups, great groups or orders.

Mainly compound.

1:100,000 or smaller (1:250,000).

Table 2.3 Examples of Soil Survey Inspection Densities (From Vink, 1963)

Scale of map

1 cm2 on map (ha)

Inspection/cm2 on map

  

Hectares/Inspection

   

Grid Sampling

No Aerial Photos

Aerial
Photos
Used

FAO8

Grid

No Photo

Photo

FAO*

1:10,000

1

5

1-5

1-5

0.5

0.2

0.2-1

0.2-1

2

1:20,000

4

5

4-20

0.5-2

0.5

1

0.2-1

2-10

8

1:50,000

25

5

3-6

0.25-1

0.5

5

4-8

3-100

50

1:100,000

100

5

2-20

1

0.5

20

5-50

100

200

* Food and Agriculture Organization of the United Nations, Rome.

Level of detail, precision, or purity is also associated with the density of field inspections. It is usually taken for granted that greater detail requires a greater number of field inspections. This is usually the case since the need for more detail assumes that the soil map units should have greater purity or less variability. Putting large effort (and therefore cost) into excessive ground-truthing for reconnaissance level mapping may be unwarranted, while not providing enough inspection density for detailed mapping is untruthful and unprofessional. However, it is important to understand that increased precision does not necessarily improve map reliability and therefore inspection density is not always a direct reflection of accuracy. For example, having a large number of field inspections does not necessarily mean that the soil map is more accurate.

Mapping scale is determined as a function of the objectives of the project (implicit in this is Survey Intensity Level), the need for field control, and cartography related to map legibility. Figure 2.1 provides a number of examples of map scale. It is important to remember that a soil map is just that, and not a map of cultural, legal or political features or boundaries. The scale of mapping is a function of the intricacy of the soil pattern in relation to the intensity of soil use for which the survey is undertaken. In other words, where complex soil patterns exist, the potential for a unique soil use (as defined by the survey objectives) may be so low that it is not justified to differentiate the individual soils in detail. In such cases, the ability of the soil survey to define the individual rather than map its real extent may have to suffice. Table 2.4 provides, as a guide only, an outline of the interdependencies of SIL, scale and purpose of the survey. Characteristics such as rate-of-mapping (which are really schedule and cost variables) are very dependent on landscape complexity, prior knowledge about the study area, and experience of the survey team.

For the purposes of soil mapping in B.C., one inspection per one square centimetre (relative to the intended publication scale of the map) is a recommended guide and the acceptable range, depending on landscape complexity and surveyor experience, should be 0.2 to 2 inspections per cm2 (Agriculture Canada Expert Committee on Soil Survey, 1987a). All soil surveys should be planned with this concept in mind and any significant deviation that affects the quality control inherent in a particular SIL must be fully explained and justified.

2.2.3 Differentiating Criteria for the Map Units and Legend Formulation

During the initial design of a soil survey, it is important to establish the criteria, or soil properties and parameters, that will form the basis for recognizing soil map units and differentiating among them. These decisions greatly influence the structure of the map legend and in turn will affect the ease of application of the map by user groups. While this subject is dealt with more specifically in Chapters 3 and 4, some concepts are necessary to apply during the organization of the survey.

Perhaps the most important concept is that soil map units should be designed specifically to meet the needs of the user(s). A soil map can be called such as long as it depicts soils (i.e. the map units are based in a significant way on pedons). Ecological or Biophysical maps containing other differentiating criteria such as climate, physiography or vegetation may still be referred to as soil maps as long as the map unit differentiation recognizes pedons as central or differentiating mapping criteria. For the purpose of clarity, the following definitions should be used for map terminology (from Agriculture Canada, 1987b):

Soil Map

A soil map shows the geographical distribution of various soil classes. The definition does not include maps of single soil attributes such as texture, soil colour, or slope, though these may be derived from the former. They would be called single factor maps such as "soil texture maps".

Soil Survey Map

A soil map made from information collected by field procedures.

Interpretive Map

A map compiled for a particular purpose from information contained in the soils map (e.g. "Suitability For Vegetable Production").

Single Factor Map

A map depicting any one feature derived from the soil map. Interpretive and single factor maps may collectively be called derivative maps or extractive maps.

Generalized Soils Map

A map developed from the recompilation of more detailed published soils information.

Schematic Map

Essentially an interpretive "bestguess" map based on a combination of ancillary information such as aerial photo interpretation and vegetation maps.

Exploratory Soil Map

Differs only slightly from a schematic map and contains about the same level of information but is based on some actual field observations.

Soil/Landform Map

A soils map that combines the genesis of surficial materials, their relief, and form with soil (pedon) factors.

Biophysical or Ecological Map

A map that utilizes regional climate, physiography and plant ecology as differentiating characteristics for map units at broad scales; where soils are used to establish more detailed map units, then these maps may be referred to loosely as soil maps.

Table 2.4 Characteristics and Uses of Different Survey Intensity Levels (SIL)6

SIL

Range in Scale

Minimum-size Delineation7

Approximate Rates of Progress

Typical Objectives

 

(usual)

(usual)

Mapping8

Map Support9

  

SIL1

1:14,000 or larger (1:5,000)

0.4 ha (0.1 ha)

1250 ha/mo

50%

Very intensive planning that requires appraisal of the soil resource of very small areas. The map units are very refined and can provide soils data for such uses as defining experimental plots or predicting characteristics of septic fields.

SIL2

1:5,000 to 1:40,000 (1:20,000)

0.1 ha to 10 ha (2 ha)

5000 ha/mo

75%

Information for many purposes down to the level of local planning for groups of farms, stream catchments, small parks or irrigation management. It will not likely be used for specific site selection.

SIL3

1:20,000 to 1:130,000 (1:50,000)

2 ha to 80 ha (12.5 ha)

25 km2/mo

100%

Information for a limited number of uses to the level of farming areas, county planning, major stream catchments, large parks, irrigation districts, or wildlife habitat differentiation.

SIL4

1:50,000 to 1:300,000 (1:100,000)

12.5 ha to 450 ha (50 ha)

200 km2/mo

75%

Information for relatively generalized purposes to the level of large agricultural areas, regional plans, and wildlife habitat and ecological characteristics.

SIL5

1:100,000 to 1:1,000,000 (1:250,000)

50 ha to 5000 ha (300 ha)

700 km2/mo

30%

Information for a few general purposes for broad regional or provincial plans, and regional wildlife habitat assessment.

Figure 2.1 Maps of the Same Area at Different Scales

One other concept that is sometimes misunderstood is the relationship between soil taxonomy and soil map units. This is perhaps to be expected, since names like Chernozemic and Luvisolic tend to be foreign to most users. Map units represent real divisions of the land and as such will often contain more than one taxonomic group. Map units are established to meet the objectives of the soil survey and not necessarily the categories of the soil taxonomy. In other words, a change in soil taxa across the landscape does not necessarily result in a change in the map unit. The Canadian System of Soil Classification has five categorical levels within which numerous taxonomic groups occur (the Soil Order, Soil Great Group, Soil Subgroup, Soil Family and Soil Series). These taxonomic groups allow the soil surveyor to organize an understanding of the soils so that soil properties can be more easily defined and presented.

Each of the five categorical levels in the Canadian System of Soil Classification are briefly defined as follows. They are each subdivisions of the next higher category.

Soil Order Differentiation on the basis of properties that reflect the nature of the soil environment and the effects of the dominant soil-forming process (e.g. time). There are nine Soil Orders defined in the Canadian System of Soil Classification (1987a).

Soil Great Group Differentiation within the Soil Order on the basis of properties that reflect differences in the strengths of a dominant process or a major contribution of a process in addition to the dominant one. For example a Luvic Gleysol is differentiated on the basis of gleying (water saturation) but clay translocation is also a major process. There are 28 Great Groups defined in the Canadian System of Soil Classification (1987a).

Soil Subgroup Differentiation within the Soil Great Group on the basis of the kind, arrangement and relative prominence of soil horizons; for example, the presence of an ortstein horizon in a Humo-Ferric Podzol soil. There are 193 Subgroups defined in the Canadian System of Soil Classification (1987a).

Soil Family Differentiation within the Soil Subgroups on the basis of parent material characteristics such as texture and mineralogy, soil climate factors and soil reaction.

Soil Series Differentiation within the Soil Families on the basis of detailed features of the pedons. Soil series have similar kinds and arrangements of horizons whose colour, texture, structure, consistence, thickness, reaction and composition all fall within a narrowly defined range.

In mapping, the soil surveyor distinguishes a limited number of soil groupings which are repetitive on similar landscapes and have attributes that vary within specific limits. In order to accomplish this, the surveyor makes use of the soil taxonomy as an aid. While map units are named for and correlated to the taxa, they are not the same thing. In other words, a pedon is a real unit of soil in the landscape, whereas a taxonomic category (e.g. Soil Series) is a conceptual class with defined limits. A pedon is not necessarily a soil series since the attributes of only one pedon may not encompass the complete range of attributes allowable within a series. However, a polypedon can accommodate this concept of the landscape and therefore forms the link between the science of the pedon and the art or conceived nature of the taxonomic class.

2.3 Planning the Soil Inventory and the Preparation of a Workplan

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Soil surveys consist of much more than a field mapping exercise. As illustrated in Figure 2.2, there are several component parts that make up a project and its work plan. These can be categorized as:

1. Initial Request for the Soil Survey and its Evaluation.

2. Preparation of Technical Project Plan.

3. Preparation of Management Plan.

4. Mapping Program.

5. Publication Plan.

Each of these component parts is essential for the soil survey to culminate in providing the desired end result. Failure to fully address them at the outset of the survey can greatly affect the outcome. Of all these components, the preparation of a clear and comprehensive statement of objectives and purpose is of utmost importance. This must be accomplished as a joint initiative of those who will be responsible for the survey (and their staff who will conduct the survey), the group or agency who is requesting the survey (as well as the group(s) or agency(ies) who are funding the project if they are separate), and where possible, the users of the soil survey information. As illustrated in Figure 2.2, if the objectives for the survey that are finalized as part of the Technical Project Plan cannot mesh with the schedule, funding and staffing criteria then either the objectives must change (e.g. area of survey; survey intensity level) or the project should not proceed.

Project management is an essential element in the project plan. It is the responsibility of the manager to ensure that the project plan is followed, and that technical, fiscal, staffing and scheduling responsibilities are assured. In most instances, it is not appropriate for the project leader (often referred to as the Party Chief) to also be the project manager since the duties and skills required are often quite different.

To assist in the preparation of a project plan, it is often useful to use a form or checklist to ensure that essential elements are not missed, and to aid in communication with other agencies or individuals involved in the survey. Forms of this type are also useful in formulating the management plan and in providing a record for periodic reference during the conduct of the survey for quality control purposes. Appendix A provides an example of forms for project planning that were developed by Agriculture Canada. While it may not be necessary to utilize such a comprehensive form for all soil inventory programs, any soil survey should include the preparation of similar forms, perhaps taking the essential elements from the one provided in Appendix A, in order to help ensure the success of the project.

2.4 Qualifications of Soil Surveyors

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As in any field of science or technology, it is necessary for the practitioner to have a basic set of skills, education, training, and accreditation.

For the purpose of conducting a soil survey in B.C. as the pedologist in-charge (party leader or party chief) and for undertaking interpretations of the information, the following qualifications are suggested as a minimum.

Figure 2.2

3 Adapted from a Soil Mapping System for Canada: Revised (Mapping Systems Working Group, 1981).

4 The term "small scale" is often used to refer to reconnaissance or exploratory surveys and "large scale" to detailed and very detailed surveys.

5 Simple units consist of over 80% of a single component or a non limiting inclusion. Compound units are complexes or associations of 2 or more components, each less than 80%.

6 Adapted from Agriculture Canada Expert Committee on Soil Survey, 1987a.

7 Based on a minimum of 0.5 cm2 on map (see Table 2.1). In practice the minimum sizes are generally larger than this.

8 Based on experienced surveyors in areas of good access. Considering a 20 day month for 1 crew. (Usually 2 people).

9 Values are given as a percentage of mapping rates and includes legend establishment, checking, correlation and sampling.

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