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5. Assessing suitability

'Suitability is a measure of how well the qualities of a land unit match the requirements of a particular form of land use.'

Suitability is a measure of how well the qualities of a land unit match the requirements of a particular form of land use. Suitability is assessed for each relevant use and each land unit identified in the study.

Matching land use to land is the essence of land evaluation, and often influences all the other steps in the study. For example, early investigations may show that parts of an area are too dry to grow a desired crop; irrigation may therefore be needed. An additional land utilization type involving irrigation would then have to be recognized, and the increased relevance of slope and drainage might then lead to changes in the way land units were mapped. Needless to say, matching may suggest more subtle changes than these. The important point is that the systematic process of matching can really begin only after the most informative definitions of land uses and land units have been established.

The suitability classification aims to show the suitability of each land unit for each land use. In FAO's Framework for Land Evaluation, land is first classed as suitable (S) or not suitable (N). These suitability classes can then be further sub-divided, as required. In practice, three classes (S1, S2 and S3) are often used to distinguish land that is highly suitable, moderately suitable and marginally suitable for a particular use. Two classes of 'not suitable' can usefully distinguish land that is unsuitable for a particular use at present but which might be useable in future (N1), from land that offers no prospect of being so used (N2).


The process of suitability classification is often easier than it might appear. Although theoretically there are an almost unlimited number of land-use requirements and land qualities, in practice only a few have a major influence on suitability. These factors change from place to place and depend, of course, on the nature of the land use - and this means that a sharp eye must be kept open for unusual local factors or requirements.

Each land use has requirements and limitations that relate separately to its objectives, its management needs and to environmental issues. Different crops clearly have specific physiological requirements but land-use objectives such as recreation or watershed protection also have requirements related to soils and climate. Management requirements include factors such as workability of the land, access to markets and size of local land holdings. The environmental requirements of the land relate mainly to erosion and other forms of soil degradation but can also include pollution, increased salinity and waterlogging.

The requirements and limitations of each land use can usually be specified in terms of a limited number of land qualities, such as a growing period requirement, a moisture availability requirement or a water quality limitation. The FAO Guidelines for land evaluation for irrigated agriculture lists 32 such 'class-determining factors'. If suitability classes have been defined in terms of a range of values for each factor, the assessment is then relatively straightforward.

Of course, it is sometimes possible to change land qualities - swampy land can be drained, for example, or steep land terraced. If the suitability of the land for a particular use is assessed both with and without such improvements, then the evaluation will provide a measure of the value of the improvement itself.

The first step in classifying suitability is to decide which factors should be used to define each suitability class. Upper and lower limits for each relevant land characteristic or quality are then set for each class.

Local agronomic experiments are the best guide in this process. If these experiments cannot be carried out, class-determining limits must be based on the experience of local farms or on farms elsewhere. If no other information is available, estimates from first principles may have to be used to set the limits.

That done, the properties of each land unit are reviewed and compared to the class limits. Each land unit can then be assigned a suitability classification for each type of land use.

This stage is simplified if all the known information is first set out as a table that shows, for each land utilization type, the class rating of each class-determining factor together with any qualifying information or comment. The final assessment of suitability is usually based on one of three principles.

The limiting condition principle: the most unfavourable quality determines the suitability classification.

The subjective assessment principle: suitability classes are raised or lowered on the basis of judgements of the importance of different factors. Because subjective assessments are risky, the relevant factors are weighted, and account is taken of special limitations that may occur if two or more negative factors occur together.

The principle of arithmetic modelling: the fact that each land quality has an independent influence on suitability means that land qualities can be assigned values and manipulated arithmetically to provide a numerical assessment of overall suitability.

In practice a combination of these approaches is often employed.

Maps above show the agro-climatic suitability for the rainfed production of two crops - sorghum and maize - in South America. This assessment was made on a very small scale, for half a continent, and shows only four suitability classes: very suitable, suitable, marginally suitable and not suitable. High altitude areas were first eliminated from the study area.

6. Identifying environmental and socio-economic issues

Before a land use can be recommended in a development plan, its environmental and socioeconomic implications must be evaluated further. Conservation interests are taken into account in the matching process because suitability is assessed 011 a sustained use basis. But the environmental effects the land use may have on other land systems must also be assessed.

Irrigation schemes, for example, change the pattern of water distribution in an area and can also change water quality. These changes may be significant or minor but can rarely be ignored. While drainage may protect an irrigation scheme from waterlogging and salinization, the output from the drainage system can produce these effects further downstream. If groundwater is used excessively for irrigation, other wells, pumps and natural springs may dry up, and saline water may enter the aquifer with highly detrimental effects. But such difficulties can be foreseen.

Forestry activities, whether they involve clearance, production or plantation are also likely to affect the environment - particularly in relation to soil erosion, the rate of which may increase dramatically as forest cover is removed. The soil that is eroded may be deposited a long way downstream, where it may be even more damaging to agricultural production, at least in the short term.

Changes in microclimate, moisture retention and run-off associated with large-scale forest felling or plantation are more difficult to assess. Local changes in vegetation and wildlife which can result from forestry, and the prospects for regeneration after felling should be evaluated as part of the matching process. But these changes need to be assessed in a regional context: will enough land still be devoted to wildlife, for example?


Soil erosion and degradation of soil fertility, leading to loss of vegetation and further erosion, are the principal environmental hazards associated with rainfed crops. Land utilization types that involve the build-up of humus and ground cover on the plains, and the use of tree crops and agroforestry on the steeper slopes, should contain these problems on a local scale. But the effects of runoff, sedimentation and pollution from the use of excessive fertilizer accumulate downstream, and must be kept within acceptable limits.

A number of other awkward questions may well arise. Could future population growth render current precautions inadequate? Could over-stocking lead to migrating livestock trampling down land around villages and waterholes, leading to soil damage and erosion?

Not all such problems are amenable to complete solution but a first defence is to be aware of a threat before it develops beyond control.

Cattle trampling and eroding soil near a water-hole may be an undesirable outcome of new land-utilization.

A new or improved land use can succeed only if it can be adapted to fit local social and economic conditions. Socio-economic investigations are therefore a vital part of land evaluation, starting with the initial formulation of the study's objectives. Attention needs to be given to markets (local, national and perhaps even international), population levels and growth rates, problems of land tenure, the availability of skilled and unskilled labour, transport of products and inputs, and the ease with which subsistence needs for such things as firewood, building materials and local grazing can be met. Local religions and cultures may be important, particularly if new techniques, such as irrigation, are being considered. Political circumstances cannot be ignored, and any analysis should take account of the needs of all members of the population, including minority groups.

In quantitative land evaluation, more detailed and expert economic studies are necessary. These can take several forms and more than one form of presentation may be required. The balance of advantage - that is, profit or loss - can be assessed by financial analysis in relation to the individual farmer (or the enterprise of which he is part) and by economic analysis in a broader national or community context. Furthermore, profitability can be measured simply in terms of gross margins; or in almost limitless complexity, using techniques of discounted cash flow analysis that offset initial capital expenditure against the resulting gains in future years.

The choice of the most appropriate approach is, in itself, a matter for expert guidance.

It is important to distinguish the economics of land evaluation from project cost/benefit analysis. The former is concerned with land use on individual land units and develops assessments which, when combined with other information, can greatly increase the precision possible in project analysis. In quantitative land evaluation, the range of each suitability class is frequently expressed in monetary cost/benefit terms. Of special interest is the value below which a particular use is not to be recommended - the separation between the suitable (S) and not suitable (N) classes - because this can be determined accurately only by an analysis of costs and benefits. This limit will normally be set at some low level of positive return below which the risks and effort involved do not seem justified.

The precise numerical findings of economic analysis need to be viewed with caution. They reflect the time-dependent nature of land suitability assessment - a change in one economic parameter (the global price of oil, for example) can invalidate economic-based class limits almost overnight. Their significance depends greatly on the baseline against which they are measured - profit per hectare, per caput or per enterprise, for example. There is no single measure of profitability.

People are the ultimate beneficiaries of land evaluation; population levels and growth rates, labour supplies, markets and subsistence requirements are vital inputs to successful evaluation

Most important, perhaps, numerical findings are based unavoidably on major assumptions about the future of such factors as population growth, market growth, project lifetime and inflation. These assumptions may or may not prove to be correct.

7. Identifying the most suitable land use

'The best land use has to be selected in the light of economic, social, and sometimes political factors.'

Once evaluated, land units are rated according to their suitability for a range of uses, including the production of individual crops. The requirements of types of land use or crop are compared with the characteristics and qualities of each land unit. This matching process provides a measure of how successful any land use would be on a specific land unit.

Since land use normally involves the production of crops or products for consumption or sale, the matching process concentrates on soil and climatic factors, measuring the level of output that could be produced from a particular land unit using a given production system or level of inputs. In fact, the resulting suitability rating often represents average expected yield levels.

But the suitability-matching process does not take into account whether a potential market exists, or whether it would be necessary to transport products to that market. It is, as far as possible, a technical process which does not include off-farm or non-production factors such as the availability of credit, and does not make value judgements on the potential uses of the land. That is a separate and subsequent stage, at which environmental and social and economic issues are taken into account.


Furthermore, the evaluation process is carried out separately for each individual crop and land use, and results in an individual suitability rating. It does not, however, judge witch would be the 'better' land use between, say, oranges, which might be rated S1 and represent 80-100 percent of potential yields, or 30 tonnes/ha, and potatoes which might have a suitability rating of S2, or 60-80 percent of potential yield, or 20 tonnes/ha. Clearly, decisions about best or optimum land use must take into account the objectives of land users and the community, economic factors, input/ output relationships in terms of money, labour or produce, demand and other factors.

Typical land suitability assessment for subsistence farming

After: Soils and Land Suitability for Arable Farming of South-East District, Botswana, 1989. Government of Botswana/FAO project BOT/85/011, Field Document 3.

'Cultivation of land classified as S4 will not be economically profitable for a long time.

However, part of this land is used, and will be used for the specified system of production (improved traditional dryland farming) because:

• economic considerations are not significant, or only play a very minor part, as even very low yields are essential to bring total production up to subsistence level; and

• considerations such as employment justify crop production even at very low yields.

Land rated as N cannot provide an output or yield that would cover input costs.'

Therefore, once the land unit has been rated in terms of its suitability and potential for different crops or uses, the best use has to be selected in the light of economic, social, and sometimes political factors.

In general, this will entail:

• a review of objectives in order to select land uses which have the potential to produce the required outputs;

• an economic evaluation to compare input levels with expected output levels, establish which of the land uses would be economically viable, and whether the required inputs and infrastructure are available, or could be made so;

• a socio-political evaluation to establish whether potential land uses are socially, legally, and politically viable, and whether they could be made so; and

• an environmental impact assessment to establish the long-term effect of potential land uses and production systems on the land units for which they are proposed.

The result of the above procedure will often be a proposed modification of existing land uses and production systems, and may also indicate the need to modify the economic or legal environment, provide training, institutional or financial support and infrastructure as part of a proposed development plan.

These activities form the borderline between land evaluation and land use planing.

Land evaluation sequence

Farmers in Upper Volta using zebu cattle to work their land.

8. Planning land use

Tanzania: Land-use planning in practice

'Whilst land evaluation is focussed on the potential of individual land units for various uses, land planning must examine the relationships between uses.'

Land planning takes over from land evaluation after the latter has produced recommendations as to the most suitable land uses for the land units in the area. It is the job of planners to evaluate and discuss these recommendations with the beneficiaries and the authorities. Planners also decide how and where the plans can best be put into action to meet the social and economic needs of the community and ensure the environmental stability of the area as a whole. This kind of planning, to be successful, must be developed within the broader context of both the region and the nation itself. It must also include measures to encourage adoption of the land-use patterns proposed.

Whilst land evaluation is focussed on the potential of individual land units for various uses, land planning must examine the relationships between uses. For example, labour-intensive work may be suited to some areas, but it is not suitable to include in land planning if it draws labour away from other areas where it is needed. Similarly, it may matter little how well suited an area is to the growth of one particular product if the market for that product is limited.

The extent to which local demand should be met from local production needs careful consideration. A measure of self-sufficiency is always desirable, and in developing countries fuelwood, some building materials, essential livestock products and basic foods are nearly always produced locally in rural areas. But account must also be taken of outside markets, ease of communication and the social and nutritional implications of the choice between trade and self-sufficiency.


Food security is an important consideration in planning land use in developing countries. Food surpluses, and means of storing them, must be planned to carry the community through unfavourable circumstances. This requires some assessment of how well the planned land uses will meet the needs of future populations - in other words, an assessment of the future 'human carrying capacity' of the land.

To maximize food security the choice of land utilization types should aim to provide a variety of crops in order to guard against the ravages of selective pests and diseases, provide a balanced diet and, in the case of cash crops, cushion the effects of market changes. Similarly, thought must be given to the implications of exceptional meteorological conditions, as the potential of different types of land will change dramatically during particularly wet or dry seasons, and individual farmers should have access to alternative plots if their land is likely to be seasonally affected.

Although environmental hazards are examined before the detailed planning stage, the planner must ensure that all necessary precautions are included within the plan. The land-use pattern as a whole must be in keeping with the long-term conservation needs of the area, and local plans should include provision for safeguards such as wind breaks, contour terracing, adequate drainage, sensibly routed roads and control of pollution.

To be effective, a land-use planning authority needs to have a clearly established position in the organization of local or national government, with advisory access to the senior levels of the executive. The authority's planning proposals must be clear and attractively presented. The people who will be directly affected need to be convinced that the changes will be to their advantage. This may not be easy since in many development plans expenditure is required immediately while the benefits and rewards take time to materialize. Incentive schemes may well be needed during the period before benefits begin to be felt.

Tanzania: Land-use planning in practice

In 1976, the World Bank planned a Rural Integrated Development Project for the Tabora region of Tanzania. The importance of land use was recognized from the start. The project included a land-use component to survey land resources, develop a methodology of land-use planning at village level, and train local staff in using the new methodology.

Tabora, the largest of 20 regions in Tanzania, occupies 73,500 km2 in the west of the country. Three-quarters of the region is wooded, mostly infested with tsetse fly, and only about one-eighth is ever cultivated. Thus, despite generally infertile soils and low, rather unreliable rainfall, there is ample land to feed foreseeable populations. However, the present population of about 1 million is increasing and is not evenly distributed. More than half of the population occupies less than one-fifth of the area in two north-eastern districts. There is local over-population and serious land shortage - a situation demanding land-use planning at both regional and village levels.

Data collection, using air photo interpretation and extensive ground checking, was completed in two years. Meanwhile, agro-economic studies were made at village level to establish a basis for land planning. The survey mapped and analyzed the nature and distribution of land units, soils and vegetation throughout the region and was sufficiently detailed to identify regional development priorities. Furthermore, the survey provided insights into the factors that determine land suitability, the potential of land for agricultural use and the need for conservation. These insights allowed simple procedures to be drawn up so that these factors could be easily dealt with during village-level planning.

One of the project's achievements was to develop a method of calculating human carrying capacity of different areas, based on the proportion of the land available for cultivation, the length of fallow required (which varies with soil fertility and farming practice) and the area of land that an average family must cultivate to meet its needs. The family's land requirements for livestock and fuelwood were calculated separately.

The carrying capacity assessments were used first to examine the viability of existing villages, and then to establish planning priorities with district authorities. Finally, the assessments were used in a systematic, detailed survey and planning of individual villages. A land-use planning handbook was prepared and planning teams were trained.

Map (Tanzania)

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