5. STAGES IN THE DEVELOPMENT OF A METHODOLOGY

5.3 Land use requirements

Following description of land utilization types, the next and critical stage in evaluation procedures is determination of their land use requirements, the environmental conditions under which the specified land use is expected to function efficiently, moderately well or to fail. The requirements are needed for individual agroforestry components, principally multipurpose trees and crops, and for agroforestry land utilization types as a whole.

It is a matter of debate among land evaluation specialists whether land use requirements should be assessed in terms of land qualities or land characteristics (cf. Dent and Young, 1981, pp. 162-165). Land qualities are broad attributes of land, each of which acts in a district manner in its influence on the suitability of land for a particular kind of use; examples are moisture availability, rooting conditions, nutrient supply, erosion hazard, and potential for mechanization. Land characteristics are attributes of land that can be measured or estimated, e.g. mean annual rainfall, slope angle, soil drainage class. If land qualities are taken as the basis for assessment, they must normally be assessed in terms of land characteristics, singly or in combination. There are relatively few land qualities, less than 30 that are widely employed, but many hundred land characteristics.

All of the FAO guidelines to date base their assessments on land qualities measured or described in terms of land characteristics, Land qualities serve as a primary check, to ensure that all relevant influences of environment on land use have been taken into account. Decisions are then made on which land characteristics are to be employed to describe each quality; e.g. whether moisture regime is to be described in terms of mean annual rainfall, mean growing period, rainfall/evapotranspiration ratio, or specified confidence limits for any of these.

Land use requirements are specific to the use, e.g. moisture requirements of sorghum, tea, rice. Land qualities and characteristics are specific to land units, e.g. the moisture availability of Land Units A, B, C. To facilitate matching between the two, requirements and land qualities are expressed in closely corresponding terminology and units, for example:

Land use requirement                                                           Land quality

Temperature requirements                                                  Temperature regime
Moisture requirements                                                          Moisture regime
Nutrient requirements                                                           Nutrient availability
Tolerance of salts                                                                 Presence of salts
Requirements for mechanization                                         Land conditions affecting mechanization
Susceptibility to erosion hazard                                           Erosion hazard

As a starting point in determining land qualities relevant to agroforestry, a comparison has been made between the qualities listed in the FAO guidelines on rainfed crop production and forestry, together with those identified at the first workshop on evaluation for extensive grazing. Those qualities the influence of which occurs rarely are omitted and some qualities combined.

The results are shown in Tables 3 and 4. Table 3 is a summary of the land qualities, grouped into those which affect plant growth, management and conservation; direct estimates of growth, e.g. forest inventory, crop yield data, form a further group. There is finally a set of land characteristics which may influence two or more land qualities. These are classifications, employed as a substitute for more detailed data. Three, slope angle, soil texture and soil reaction, are characteristics commonly used to describe requirements or tolerance, without it being made clear which influence on the plant or the land use is being assessed; for example is soil reaction being considered as an indicator of aluminium toxicity, nutrient availability or sodicity?

Table 4 gives the land qualities in more detail, subdivided in some cases, e.g. temperature regime/requirements divided into general temperatures for growth, (tolerance of) extremes of heat, and (tolerance of) extremes of cold. The major kinds of land use for which each quality has been considered relevant are indicated. The last column gives land characteristics most commonly employed, singly or in combination, to describe each quality.

Thus the environmental features relevant to each of the types of land use contributory to agroforestry can be combined and grouped into 24 land qualities with 65 subdivisions. It should be noted, however, that in any specific evaluation study, some qualities do not affect the land utilization types under consideration, whilst others have no significant effects on suitability within the area concerned. It is frequently found that no more than 12 land qualities are relevant to a specific evaluation.

The use of land qualities, measured or estimated by means of land characteristics, has been taken as the initial method of suitability assessment for agroforestry, and as the basis of the requirements file of the environmental data base.

Table 3. LAND QUALITIES RELEVANT TO AGROFORESTRY: SUMMARY.

Table 4. CHECK LIST OF LAND QUALITIES FOR AGROFORESTRY. Source: check lists in guidelines on land evaluation for rainfed agriculture (R), forestry (F), irrigated agriculture (I) and extensive grazing (G).