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Assessing agroclimatic suitability
The next stage is to assess the potential productivity of the crops themselves. This, of course, varies with the growing conditions which prevail and hence is different in each length-of-growing-period zone.
The initial step is to calculate, from known theory about photosynthetic rates, the maximum yield as limited by climate which could be expected from each crop in each length-of-growing-period zone where the crop could grow. These theoretical yields are then reduced to take account of a number of agroclimatic constraints (divided into four groups) that would limit production. These include losses in yield owing to water stress, pests, diseases, weeds and harvesting difficulties (such as waterlogging at harvest time).
These constraints vary according to the level of farm inputs used; for example, in very humid areas maize might be grown successfully with high inputs if those inputs include means of drying the crop, but maize for grain could not be harvested satisfactorily on a large scale under these conditions using only low inputs.
These reduced yields are then expressed as a percentage of the maximum attainable yield in the most productive length-of-growing-period zone. These percentages form the basis of the suitability assessment. For each crop, and for each input level, zones which would produce less than 20 percent of the maximum are classified as not suitable; those that produce 20 to less than 40 percent as marginally suitable; those that produce 40 to less than 80 percent as suitable; and those that produce 80 percent or more as very suitable.
Maps of agroclimatic suitability can then be produced for each crop. Note that these maps do not yet include any assessment of soil suitability.
How the assessment is made: maize in Africa (low inputs)
Range of maximum yield for each crop in each length-of-growing- period zone results from gross and nett biomass calculations involving estimated radiation intensity and moisture availability.
Four groups of constraints are applied to reduce yield:
A: moisture variability or stress;
B: effect of pests, diseases and weeds on crop growth;
C: factors affecting yield and quality;
D: wetness effecting workability and handling of produce.
Constraints are rated at three levels:
2 reduces yield by 50 percent;
1 reduces yield by 25 percent
0 factor has no effect on yield
Reduced yield, after applying agroclimatic constraints In the order A to D, is calculated for each length of growing period. Reduced yields are expressed as a percentage of the maximum attainable in the most productive growing period zone (18 tonnes/ha in this example).
Agroclimatic suitability assessments are presented in map form (right, for maize in Africa). At this stage effects of soil conditions have jet to be included.
Suitability classes for each growing-period zone are assessed as: very suitable 110 percent or more of maximum yield; suitable, 40 to less than 110 percent, marginally suitable, 20 to less than 40 percent unsuitable, less than 20 percent.
Agroclimatic suitability for maize in Africa (low inputs)
