3 THE DEVELOPMENT OF LAND EVALUATION METHODOLOGY

3.2 Rainfed crop production

Although entitled Guidelines for land evaluation for rainfed agriculture (FAO, 1983), the first of these manuals in fact refers not to agriculture in its wider sense but to crop production (annuals and perennials). It is the most detailed of the guidelines, setting out specific steps to be followed, and giving proformas for use at each stage.

Rainfed crop production can be regarded in some respects as the 'normal' or standard form of land use. Thinking in terms of suitability for maize, sorghum, rubber, tea, etc., within a given socio-economic context (smallholder farming, estate agriculture) formed the major basis on which most of the principles were formulated: in addition, the Framework has been more widely applied to assessment for crop production than for any other kind of land use.

The major systematic contributions arising from this study were to identify the land qualities which can affect suitability for crops, and to divide these into three groups: qualities affecting growth, management and conservation. The main qualities affecting plant growth (and survival) are radiation, temperature, moisture, nutrients, drainage, soil rooting conditions, and absence of various adverse conditions, e.g. flood hazard, soil toxicities, pests and diseases. Some growth requirements are common to all or most plants but many are specific to individual crops. Land qualities affecting management include soil workability, terrain conditions affecting ease of mechanized operations, accessibility and location. These conditions are often specific to kinds of management (e.g. mechanized cultivation) but to some degree independent of the crop grown. In crop production the principal land quality affecting conservation is the degree of soil erosion hazard.

These three groups of land use requirements can be treated independently to some extent. This permits, for example, the identification of areas suited to the growth of maize, irrespective of method of management; those suited to mechanized agriculture, irrespective of crop; and areas which are suited for annual crops on grounds of conservation, such assessments being different for cultivation with and without soil conservation works. For a specified land utilization type these component suitabilities can be combined, to give areas suitable for, e.g., "large-scale mechanized maize cultivation with conservation bunds", or " smallholder non-mechanized maize cultivation without physical soil conservation works".

This technique of disaggration of the land utilization type,  and of its land use requirements, clearly has potential applications to agroforestry. At the simplest level, growth requirements of the component elements in an agroforestry system---crops, trees, pastures ---could first be assessed in isolation, providing necessary but  not sufficient conditions for their success in combination. It might also be possible to isolate management requirements of specific agroforestry techniques, e.g. alley cropping.