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The conditions that restrict crop growth on the earth's surface are complex but limited in number. They include the species and cultivar of the crop being grown, the nature of the climate, the kind of land on which the crop is grown, and the use of inputs such as fertilizers and pesticides applied to the land. The purpose of the AEZ study was to model the interaction of these factors to provide an estimate of how successfully specific crops could be grown in the different areas of the developing world.
The first, and by far the most important, step (Step 1 in the diagram below) was to analyse the needs of the crop in terms of both climate and soil - a process which was done in great detail for many crops by panels of experts. The accuracy with which crop requirements can be determined is one of the keys to the whole study. Inventories of climate and soil are useful only to the extent that they can be matched with crop requirements.
Next, 11 of the crops most widely grown were selected for the study. The temperatures needed by these crops for growth were then used to help define 14 major climates and thermal regimes. These are specific temperature regimes during the periods when moisture is available for the growth of the crops. Because the temperature needs of the crops had already been defined, and matched with the major climates, it was then easy to specify which crops could be grown in which climate.
The study then analysed information on climate from meteorological stations with long-term records. This information was used to map the location of the thermal regimes (major climates) and of the length-of-growing-period zones. These zones were defined in terms of the number of days in a year in which there was adequate moisture for crop growth. Information on the locations of the thermal regimes and the length-of-growing-period zones were then combined on to a further map, the climatic inventory (Step 2a).
The next step (2b) was to include data on soils. The study would not have been possible without more than a decade of earlier work by FAO and Unesco, in collaboration with many national and international bodies, which resulted in the FAO-Unesco Soil Map of the World. This provided both the actual map on which the AEZ study is based and the information on soils that is essential to it.
The climatic inventory was then combined with the soil map to produce the land inventory (Step 3). Grid lines 10 km apart were drawn over the land inventory, and the number of 10-km squares in each agro-ecological cell (i.e. areas with identical soil, climate and growing-period zones) were then counted in each developing country. This provided estimates of the area of each agro-ecological cell in each country. Undertaken before the availability of modern automated geographic information systems, this was a laborious task.
Five steps to potential land productivity
Calculations were then made of the maximum potential yields of each crop in each length-of-growing-period zone. These yields were then modified to take account of agroclimatic constraints in each zone for each crop, including rainfall variability, pests, diseases and the difficulties of harvest.
These modified yield potentials were then compared with the maximum attainable. On the basis of this comparison, an assessment was made of the suitability of each length-of-growing-period zone in each thermal regime for each of the crops. They were defined as very suitable for a crop if this would produce more than 80 percent of maximum yield; as suitable if yield potential was 40 to 80 percent of maximum; as marginally suitable if 20 to 40 percent of maximum; and as unsuitable if less than 20 percent of maximum. This was the agroclimatic suitability rating (Step 4).
The soil needs of each crop were then matched with the soil conditions prevailing in each agro-ecological cell, and the agroclimatic ratings further downgraded if soil conditions were less than ideal. The result was the land suitability assessment (Step 5) which, in final form, gives the area in each length-of-growing-period zone in each climate variously suited to the production of each of the 11 crops in terms of very suitable, suitable, marginally suitable, or not suitable.
All these calculations were carried out for two different levels of farming inputs - the high and low input levels summarized below. How crops are farmed (the amount of fertilizer used, the degree of mechanization and efforts made to conserve the soil, for example) has an important bearing on both crop yield and the total area over which a crop can be grown. Obviously, more plant nutrients and better pest control will increase yield. In the AEZ study, it was estimated that crop yields with low inputs would be only 25 percent of those attainable with a high level of inputs.
The ways in which farming techniques affect the area of potentially cultivable land are more complex. Obviously, some very heavy soils cannot be worked by hand while tractors can do the job easily. Yet some sloping or very stony land that can be worked with hand tools may be impossible to work with tractors. All these subtleties are taken account of in the AEZ study. Later studies, particularly those at the national level, make even more sophisticated use of the input variables.
