An "expert system" approach was used based on available data and the collective knowledge and experience of people involved with land science and the rating of land suitability for crop production throughout the country. The CLI system (ARDA, 1965) was used as a general framework as it has proven useful and it is familiar to people involved with all aspects of land evaluation. Thus, a seven class system was chosen with Class I having the highest suitability (least limitations) and Class 7 having the lowest suitability (greatest limitations). The component breakdown and specific factor ratings follow the early approach of Stone (1933) and more recently that of the Alberta Soils Advisory Committee (1987).
Changes from the CLI are reflected in some of the following guidelines and assumptions:
As a basis for developing specific ratings for various factors, the following relationship was established:
|LSRS Manual Table 2.1 -- Relationship of suitability class to index points.|
|Suitability Class||Index Points||Limitations for specified crop*|
|1||80 -100||none to slight|
|5||20 29||very severe|
|6||10 19||extremely severe|
|*Limitations are for production of the specified crops. This does not imply that the land could not be developed for other crops or for other uses.|
The system has two categories: "Classes" based on the degree of limitation of land for production of the specified crop or crops (LSRS Manual Table 2.1), and "Subclasses" based on the kind of limitation. This information is useful for land use planning and for determining conservation and management requirements. The first three classes are considered suitable for sustained production of the crop in question, Class 4 is considered marginal, and Classes 5 to 7 are not considered capable of supporting sustained production of the crop using presently recommended practices. Subclasses reflect the kind of climate, soil and landscape limitations.
Provision is made for a third category "Units" which are groupings of soil-landscapes based on management considerations. For example, all areas having similar requirements for conservation practices, or drainage, or fertility amendments might be grouped. This category has not been developed here but attempts have been made where intensive land management is practiced (cf Luttmerding, 1984).
It must be emphasized that land areas assigned to the same suitability class are similar only with respect to the degree, and not the kind, of limitation for production of a crop. Each class can include different soil and landscape characteristics which may require different management practices.
|Class 1||Land in this class has no significant limitations for production of the specified crops (80 - 100 index points).|
|Class 2||Land in this class has slight limitations that may restrict the growth of the specified crops or require modified management practices (60 - 79 index points).|
|Class 3||Land in this class has moderate limitations that restrict the growth of the specified crops or require special management practices (45 - 59 index points).|
|Class 4||Land in this class has severe limitations that restrict the growth of the specified crops or require special management practices or both. This class is marginal for sustained production of the specified crops (30 - 44 index points).|
|Class 5||Land in this class has very severe limitations for sustained production of the specified crops. Annual cultivation using common cropping practices is not recommended (20 - 29 index points).|
|Class 6||Land in this class has extremely severe limitations for sustained production of the specified crops. Annual cultivation is not recommended even on an occasional basis (10 - 19 index points).|
|Class 7||Land in this class is not suitable for the production of the specified crops (0 - 9 index points).|
CLIMATE (C): A general climatic restriction.
|Temperature (H)||This subclass indicates inadequate heat units for the optimal growth of the specified crops.|
|Moisture (A)||This subclass indicates inadequate moisture for the optimal growth of the specified crops.|
SOIL (S): A general soil restriction.
|Water holding capacity/texture (M)||This subclass indicates land areas where the specified crops are adversely affected by lack of water due to inherent soil characteristics.|
|Soil structure (D)||This subclass indicates land areas where the specified crops are adversely affected either by soil structure that limits the depth of rooting, or by surface crusting that limits the emergence of shoots. Root restriction by bedrock and by a high water table are considered separately (see Rock and Drainage).|
|Organic matter (F)||This subclass indicates mineral soil with a low organic matter content in the Ap or Ah horizon (often considered a fertility factor).|
|Depth of topsoil (E)||This subclass indicates mineral soil with a thin Ap or Ah horizon (often resulting from erosion).|
|Soil reaction (V)||This subclass indicates soils with a pH value either too high or too low for optimum growth of the specified crops.|
|Salinity (N)||This subclass indicates soils with amounts of soluble salts sufficient to have an adverse effect on the growth of the specified crops.|
|Sodicity (Y)||This subclass indicates soils having amounts of exchangeable sodium sufficient to have an adverse effect on soil structure or on the growth of the specified crops. Its use is restricted to reconstructed soils.|
|Organic surface (0)||This subclass indicates mineral soils having a peaty surface layer up to 40 cm thick.|
|Drainage (W)||This subclass indicates soils in which excess water (not due to inundation) limits the production of specified crops. Excess water may result from a high water table or inadequate soil drainage.|
|Organic soil temperature (Z)||This subclass recognizes the additional temperature limitation associated with organic soils - particularly where the regional climate has less than 1600 Effective Growing Degree Days (EGDD).|
|Rock (R)||This subclass indicates soils having bedrock sufficiently close to the surface to have an adverse effect on the production of the specified crops.|
|Degree of Decomposition or Fibre Content (B)||This subclass identifies organic soils in which the degree of decomposition of the organic material is not optimum for the production of the specified crops.|
|Depth and Substrate (G)||This subclass indicates shallow organic soils with underlying material that is not optimum for the production of the specified crops.|
LANDSCAPE (L): a general landscape restriction.
|Slope (T)||This subclass indicates landscapes with slopes steep enough to incur a risk of water erosion or to limit cultivation.|
|Landscape Pattern (K)||This subclass indicates land areas with strongly contrasting soils and/or nonarable obstacles that limit production of the specified crops or substantially impact on management practices.|
|Stoniness and Coarse Fragments (P)||This subclass indicates land that is sufficiently stony (fragments coarser than 7.5 cm) or gravelly (fragments smaller than 7.5 cm diameter) so as to hinder tillage or limit the production of specified crops.|
|Wood content (J)||This subclass indicates organic soils with a content of wood or of Eriophorum species sufficient to limit the production of the specified crops.|
|Inundation (I)||This subclass indicates land areas subject to inundation or flooding that limits the production of the specified crops.|
Units should be considered as soil management groups. That is, groupings of soils or map units with similar relevance and response to a particular management objective. It is suggested that criteria and groupings could change with objective of decision and scale of data or application. These types of groupings are more appropriate at levels of detailed land management such as individual farms or site plans and the need for national guidelines has not been established. However, some general statements are included for orientation.
The original concept (Klingebiel and Montgomery, 1961) was that the capability unit should group soils that were nearly alike in their suitability for plant growth and responses to management. Thus, soils in the same unit should be sufficiently uniform to (a) produce similar kinds of cultivated crops and pasture plants with similar management practices, (b) require similar conservation treatment and management under the same kind and condition of plant cover, and (c) have comparable potential productivity. The principal control-ling parameters are texture, drainage, slope and climate although others such as fertility requirements, salinity or stoniness can be locally important.
Use of the rating system requires information for each factor within the climate, soil and landscape components. The information may be for a specific site or estimated from maps, reports and local information. In some cases, data for factors can be estimated from data for other parameters. For example, the AWC can be estimated from information on texture and structure, or the water table of undrained mineral soils might be estimated from the classification of the soil. The level and purpose of the suitability rating have a bearing on the degree of specificity of the data required. Regional assessments of the suitability of land can generally be made using published data. Assessments of specific tracts of land for the production of specified crops, however, usually require specific data for the sites involved, including on-site inspection, unless the avail-able data are unusually comprehensive.