2. THE NATURE AND PURPOSE OF LAND EVALUATION

2.2 Procedures of land evaluation

The procedures of a land evaluation study are shown in simplified form in Fig. 1. They commence with formulation of the objectives of the study, frequently within the framework of a land development project. Two parallel sets of activities are then initiated: studies of promising kinds of land use, and surveys of the land. The studies of land use lead to the identification of a range of alternative land use systems which appear to meet the objectives and to be relevant both to the physical and to the social and economic context of the area. These systems are progressively amplified and refined, leading to their description as land utilization types.

Next, the land use requirements of each land utilization type are identified; requirements, for example, for temperature regime, moisture, nutrients, soil erosion hazard, or absence of pests and diseases. Each land utilization type has different requirements: for example, each crop requires different climatic and soil conditions, the soil nutrient requirements differ according to whether mechanized or manual harvesting is specified.

Parallel with the activities directed towards land use come surveys and specialized studies of the land. The first task is to identify and survey relatively homogeneous areas of land, called the land units. This term refers not to a particular scale or type of mapping unit, but to any area of land taken as the basis for land evaluation; for example, in broad reconnaissance surveys, land systems might form the mapping units, whilst in more detailed surveys they might consist of land facets, soil series or vegetation communities. Having identified and mapped each area, the relevant properties of the land units are surveyed and recorded; for example moisture availability, soil depth, soil nutrient availability, erosion hazard, presence of pests and diseases, or terrain conditions which affect mechanization.

These two sets of activities are brought together in the comparison of land use with land. The first stage of comparison is matching, in which the requirements of each kind of land use are compared with the properties of each area of land. Thus moisture requirements of a crop of land use system are compared with the moisture availability of each land unit, nutrient requirements with nutrient availability, the susceptibility of the land use to erosion with the erosion hazard, etc.. Matching leads to a first approximation of land suitability, in terms of physical land use requirements only.

These provisional suitabilities are then subjected to further stages of analysis, in terms of environmental impact, economic analysis, and social consequences. Some combinations of land use with land which appeared suitable in physical terms may be rejected on grounds of adverse environmental consequences, negative economic return or unacceptable social effects.

These further stages of analysis lead to land suitability classification, giving the suitability of each surveyed land unit for each defined land utilization type. In the land suitability classification normally employed there are two suitability orders, S Suitable and N Not Suitable, divided into five suitability classes as follows:

S1 Highly suitable                     N1 Currently not suitable (physically possible, but not economically viable)

S2 Moderately suitable

S3 Marginally suitable               N2 Permanently not suitable

Further subdivision is possible, into land suitability subclasses and units. Subclasses show the type of limitation, e.g. moisture deficiency, salinity, adverse conditions for mechanization. Land suitability units consist of further subdivision into areas with relatively uniform management requirements and a specified range of productive capacity.

Figure 1. Outline of Procedures in Land Evaluation.
Source: FAO (1984a, p. 27).