Contents - Previous - Next

Chapter 3: Land suitability classifications

3.1 General
3.2 Structure of the suitability classification
3.3 The range of classifications
3.4 The results of land suitability evaluation

3.1 General

Land suitability is the fitness of a given type of land for a defined use. The land may be considered in its present condition or after improvements. The process of land suitability classification is the appraisal and grouping of specific areas of land in terms of their suitability for defined uses.

In this chapter, the structure of the suitability classification is first described. This is followed by an account of the range of interpretative classifications recognized: qualitative, quantitative and of current or potential suitability. In accordance with the principles given in Chapter 1, separate classifications are made with respect to each kind of land use that appears to be relevant for the area. Thus, for example, in a region where arable use, animal production and forestry were all believed to be possible on certain areas, a separate suitability classification is made for each of these three kinds of use.

There may be certain parts of the area considered, for which particular kinds of use are not relevant, e.g. irrigated agriculture beyond a limit of water availability. In these circumstances, suitability need not be assessed. Such parts are shown on maps or tables by the symbol NR: Not Relevant.

3.2 Structure of the suitability classification

The framework has the same structure, i.e. recognizes the same categories, in all of the kinds of interpretative classification (see below). Each category retains its basic meaning within the context of the different classifications and as applied to different kinds of land use. Pour categories of decreasing generalization are recognized:

i. Land Suitability Orders:

reflecting kinds of suitability.

ii. Land Suitability Classes:

reflecting degrees of suitability within Orders.

iii. Land Suitability Subclasses:

reflecting kinds of limitation, or main kinds of improvement measures required, within Classes.

iv. Land Suitability Units:

reflecting minor differences in required management within Subclasses.

3.2.1 Land Suitability Orders

Land suitability Orders indicate whether land is assessed as suitable or not suitable for the use under consideration. There are two orders represented in maps, tables, etc. by the symbols S and N respectively.

Order S Suitable:

Land on which sustained use of the kind under consideration is expected to yield benefits which justify the inputs, without unacceptable risk of damage to land resources.

Order N Not Suitable:

Land which has qualities that appear to preclude sustained use of the kind under consideration.

Land may be classed as Not Suitable for a given use for a number of reasons. It may be that the proposed use is technically impracticable, such as the irrigation of rocky steep land, or that it would cause severe environmental degradation, such as the cultivation of steep slopes. Frequently, however, the reason is economic: that the value of the expected benefits does not justify the expected costs of the inputs that would be required.

3.2.2 Land Suitability Classes

Land suitability Classes reflect degrees of suitability. The classes are numbered consecutively, by arabic numbers, in sequence of decreasing degrees of suitability within the Order. Within the Order Suitable the number of classes is not specified. There might, for example, be only two, S1 and S2. The number of classes recognized should be kept to the minimum necessary to meet interpretative aims; five should probably be the most ever used.

If three Classes are recognized within the Order Suitable, as can often be recommended, the following names and definitions may be appropriate in a qualitative classification:

Class S1 Highly Suitable:

Land having no significant limitations to sustained application of a given use, or only minor limitations that will not significantly reduce productivity or benefits and will not raise inputs above an acceptable level.

Class S2 Moderately Suitable:

Land having limitations which in aggregate are moderately severe for sustained application of a given use; the limitations will reduce productivity or benefits and increase required inputs to the extent that the overall advantage to be gained from the use, although still attractive, will be appreciably inferior to that expected on Class S1 land.

Class S3 Marginally Suitable:

Land having limitations which in aggregate are severe for sustained application of a given use and will so reduce productivity or benefits, or increase required inputs, that this expenditure will be only marginally justified.

In a quantitative classification, both inputs and benefits must be expressed in common measurable terms, normally economic. In different circumstances different variables may express most clearly the degree of suitability, e.g. the range of expected net income per unit area or per standard management unit, or the net return per unit of irrigation water applied to different types of land for a given use.

Where additional refinement is necessary it is recommended that this should be achieved by adding classes, e.g. S4, and not by subdividing classes, since the latter procedure would contradict the principle that degrees of suitability are represented by only one level of the classification structure, that of the suitability class. This necessarily change e the meanings of class numbers, e.g. if four classes were employed for classifying land with respect to arable use and only three with respect to forestry, Marginally Suitable could refer to S4 in the former case but S3 in the latter.

An alternative practice has been adopted in some countries. In order to give a constant numbering to the lowest Suitable class, classes have been subdivided as, e.g. S2.1, S2.2. This practice is permitted within the Framework, although for the reason given in the preceding paragraph it is not recommended.

Suitability Class S1, Highly Suitable, may sometimes not appear on a map of a limited area, but could still be included in the classification if such land is known or believed to occur in other areas relevant to the study.

Differences in degrees of suitability are determined mainly by the relationship between benefits and inputs. The benefits may consist of goods, e.g. crops, livestock products or timber, or services, e.g. recreational facilities. The inputs needed to obtain such benefits comprise such things as capital investment, labour, fertilizers and power. Thus an area of land might be classed as Highly Suitable for rainfed agriculture, because the value of crops produced substantially exceeds the costs of farming, but only Marginally Suitable for forestry, on grounds that the value of timber only slightly exceeds the costs of obtaining it.

It should be expected that boundaries between suitability classes will need review and revision with time in the light of technical developments and economic and social changes.

Within the Order Not Suitable, there are normally two Classes:

Class N1 Currently Not Suitable:

Land having limitations which may be surmountable in time but which cannot be corrected with existing knowledge at currently acceptable cost; the limitations are so severe as to preclude successful sustained use of the land in the given manner.

Class N2 Permanently Not Suitable:

Land having limitations which appear so severe as to preclude any possibilities Of successful sustained use of the land in the given manner.

Quantitative definition of these classes is normally unnecessary, since by definition both are uneconomic for the given use. The upper limit of Class N1 is already defined by the lower limit of the roast suitable class in Order S.

The boundary of Class N2, Permanently Not Suitable, is normally physical and permanent. In contrast, the boundary between the two orders, Suitable and Not Suitable is likely to be variable over time through changes in the economic and social context.

3.2.3 Land Suitability Subclasses

Land Suitability Subclasses reflect kinds of limitations, e.g. moisture deficiency, erosion hazard. Subclasses are indicated by lower-case letters with mnemonic significance, e.g. S2m, S2e, S3me. Examples are given in Table 5. There are no subclasses in Class S1.

The number of Subclasses recognized and the limitations chosen to distinguish them will differ in classifications for different purposes. There are two guidelines:

- The number of subclasses should be kept to a minimum that will satisfactorily distinguish lands within a class likely to differ significantly in their management requirements or potential for improvement due to differing limitations.

- As few limitations as possible should be used in the symbol for any subclass. One, rarely two, letters should normally suffice. The dominant symbol (i.e. that which determines the class) should be used alone if possible. If two limitations are equally severe, both may be given.

Land within the Order Not Suitable may be divided into suitability subclasses according to kinds of limitation, e.g. N1m, N1me, N1m although this is not essential. As this land will not be placed under management for the use concerned it should not be subdivided into suitability units.

3.2.4 Land Suitability Units

Land suitability units are subdivisions of a subclass. All the units within a subclass have the same degree of suitability at the class level and similar kinds of limitations at the subclass level. The units differ from each other in their production characteristics or in minor aspects of their management requirement e (often definable as differences in detail of their limitations). Their recognition permits detailed interpretation at the farm planning level. Suitability units are distinguished by arabic numbers following a hyphen, e.g. S2e-1, S2e-2. There is no limit to the number of units recognized within a subclass.

3.2.5 Conditional Suitability

The designation Conditionally Suitable may be added in certain instances to condense and simplify presentation. This is necessary to cater for circumstances where small areas of land, within the survey area, may be unsuitable or poorly suitable for a particular use under the management specified for that use, but suitable given that certain conditions are fulfilled.

The possible nature of the conditions is varied and might relate to modifications to the management practices or the input e of the defined land use (occasioned, for example, by localized phenomena of poor soil drainage, soil salinity); or to restrictions in the choice of crops (limited, for example, to crops with an especially high market value, or resistant to frost). In such instances, the indication "conditional" can avoid the need for additional classifications to account for local modifications of land use or local major improvements.

Conditionally Suitable is a phase of the Order Suitable. It is indicated by a lower case letter c between the order symbol and the class number, e.g. Sc2. The conditionally suitable phase, subdivided into classes if necessary, is always placed at the bottom of the listing of S classes. The phase indicates suitability after the condition(e) have been met.

Employment of the Conditionally Suitable phase should be avoided wherever possible. It may only be employed if all of the following stipulations are met:

i. Without the condition(s) satisfied, the land is either not suitable or belongs to the lowest suitable class.
ii. Suitability with the condition(s) satisfied is significantly higher (usually at least two classes).
iii. The extent of the conditionally suitable land is very small with respect to the total study area.

If the first or second stipulation is not met, it may still be useful to mention the possible improvement or modification in an appropriate section of the text. If the third stipulation is not met, then the area over which the condition is relevant is sufficiently extensive to warrant either a new land utilization type or a potential suitability classification, as appropriate.

As the area of land classed as Conditionally Suitable is necessarily small, it will not normally be necessary to subdivide it at the unit level.

It is important to note that the indication "conditional" is not intended to be applied to land for which the interpretation is uncertain, either in the sense that its suitability is marginal or because factors relevant to suitability are not understood. Use of "conditional" may seem convenient to the evaluator, but its excessive use would greatly complicate understanding by users and must be avoided.

3.2.6 Summary

The structure of the suitability classification, together with the symbols used, is summarized in Table 2. Depending on the purpose, scale and intensity of the study, either the full range of suitability orders, classes, subclasses and units may be distinguished, or the classification may be restricted to the higher two or three categories.


3.3 The range of classifications

The Framework recognizes four main kinds of suitability classification, according to whether it is qualitative or quantitative, and refers to current or potential suitability.

Each classification is an appraisal and grouping of land units in terms of their suitability for a defined use.

3.3.1 Qualitative and Quantitative Classifications

A qualitative classification is one in which relative suitability is expressed in qualitative terms only, without precise calculation of costs and returns.

Qualitative classifications are based mainly on the physical productive potential of the land, with economics only present as a background. They are commonly employed in reconnaissance studies, aimed at a general appraisal of large areas.

A quantitative classification is one in which the distinctions between classes are defined in common numerical terms, which permits objective comparison between classes relating to different kinds of land use.

Quantitative classifications normally involve considerable use of economic criteria, i.e. costs and prices, applied both to inputs and production. Specific development projects, including pre-investment studies for these, usually require quantitative evaluation.

Qualitative evaluations allow the intuitive integration of many aspects of benefits, social and environmental as well as economic. This facility is to some extent lost in quantitative evaluations. The latter, however, provide the data on which to base calculations of net benefits, or other economic parameters, from different areas and different kinds of use. Quantitative classifications may become out of date more rapidly than qualitative ones as a result of changes in relative costs and prices.

3.3.2 Classifications of Current and Potential Suitability

A classification of current suitability refers to the suitability for a defined use of land in its present condition, without major improvements. A current suitability classification may refer to the present use of the land, either with existing or improved management practices, or to a different use.

A classification of potential suitability refers to the suitability, for a defined use, of land units in their condition at some future date, after specified major improvements have been completed where necessary.

Common examples of potential suitability classifications are found in studies for proposed irrigation schemes. For a classification to be one of potential suitability it is not necessary that improvements shall be made to all parts of the land; the need for major improvements may vary from one land unit to another and on some land units none may be necessary.

In classifications of potential suitability it is important for the user to know whether the costs of amortization of the capital costs of improvements have been included. Where these are included, the assumptions should state the extent to which input e have been costed and the rates of interest and period of repayment that have been assumed.

Classification with amortization is only possible if the repayment of capital costs can be apportioned to identifiable areas of land. If the benefits from major expenditure are not confined to the agricultural sector (as in multipurpose irrigation and power schemes), responsibility for capital repayments is difficult to assess. In these circumstances, amortization costs will usually be excluded from the evaluation.

The distinction between qualitative and quantitative classifications, and between current and potential suitability, do not fully describe the nature of a classification. Two further considerations of importance are treatment of the location factor and of amortization of capital costs, but these by no means exhaust the range of possibilities. They are not distinguished as further specific types of classification. A suitability classification needs to be read in conjunction with the statement of the data and assumptions on which it is based (Chapter 4).

3.4 The results of land suitability evaluation

The results of an evaluation will usually include the following types of information, the extent to which each is included varying with the scale and intensity of the study. Some examples are given in Chapter 5.

i. The context, physical, social and economic, on which the evaluation is based. This will include both data and assumptions.

ii. Description of land utilization types or of major kinds of land use which are relevant to the area. The more intensive the study, the greater will be the detail and precision with which these are described.

iii. Maps, tables and textual matter showing degrees of suitability of land mapping units for each of the kinds of land use considered, together with the diagnostic criteria. Evaluation is made separately for each kind of use. Examples of land suitability maps and tables are given in Fig. 2 and Table 3.

iv. Management and improvement specifications for each land utilization type with respect to each land mapping unit for which it is suitable. Again, as the survey becomes more intensive, so the precision with which such specifications are given increases; thus in a semi-detailed survey a need for drainage might be specified, whilst in a detailed survey the nature and costs of drainage works would be given.

v. Economic and social analysis of the consequences of the various kinds of land use considered.

vi. The basic data and maps from which the evaluation was obtained. The results, particularly the suitability classification itself, are based upon much information of value to individual users. Such information should be made available, either as an appendix to the main report or as background documentation.

vii. Information on the reliability of the suitability estimates. Such information is directly relevant to planning decisions. It will also aid any subsequent work directed towards improving the land suitability classifications, by indicating weaknesses in the data and aspects which might repay further investigation.

The meaning of other subclass letters are the same as in Table 5. Based on Young (1976, p.409). NR: Not Relevant. b: biological resources

Land Mapping Units

Potential Suitability for Irrigation


Rainfed Cultivation of Annual Crops

Improved Pastures


Forestry Plantations

Tourism and Conservation

It has sometimes been thought that a land classification map is the main output from land evaluation. At least in quantitative surveys, however, the information on land utilization types, their required inputs and management specifications may be equally important.

Suitability evaluation does not necessarily identify a single form of use as "best" on each land unit. Suitability class limits are defined separately for each use. It follows that suitability classes for different uses cannot be compared in a routine, automatic manner. Thus a particular land mapping unit might be classified as S1 for forestry and S3 for arable farming, but this does not necessarily mean that the former use will be selected. The physically and economically viable alternatives are presented, with information on the consequences of each, as a basis for planning decisions.

Contents - Previous - Next