All three assessments: the climatic suitability, the edaphic suitability and the soil erosion hazard, are required to determine the ecological land suitability for crop production of each climate-soil unit of the land resources inventory. In essence the land suitability assessment takes account of all the inventoried attributes of land and compares them with the requirements of the crops, to give, an easy understood picture of the suitability of land for crop production.
The results of the land suitability assessment are presented in five basic suitability classes, each linked to attainable yields for the three levels of inputs considered. For each level of inputs, the land suitability classes are: very suitable (VS) - 80% or more of the maximum attainable yield; suitable (S) - 60% to less than 80% of the maximum attainable yield; moderately suitable (MS) - 40% to less than 60% of the maximum attainable yields; marginally suitable (mS) - 20% to less than 40%; and not suitable (NS) - less than 20%.
Land suitability assessment is achieved by applying the programme illustrated in Figure 7.1. The assessment is carried out separately for each crop and level of inputs.
Firstly, the temperature requirements of the crops with regard to photosynthesis and phenology are compared with the prevailing temperature conditions of each thermal zone. If they do not match, all the growing period zones in that thermal zone are classified as not suitable. If the temperature conditions of a thermal zone partially or fully match the crop thermal requirements, all growing period zones in that thermal zone are considered for further suitability assessment according to the thermal zone rating.
This further asessment comprises application of length of growing period suitability to the computed areas of the various growing period zones by LGP-Pattern zone. Thus if the thermal zone rating of a particular growing period zone is S1, then potential yield biomass value for the growing period zone is not modified. If the thermal zone rating of the growing period zone is S3, then the potential yield biomass value for the computed extents of the period zone is decreased by 50%. The thermal and moisture suitability assessments are described in Section 6.
The length of growing period suitability is applied according to the LGP-Pattern make-up. All five crops are matched to the individual component length of growing period, i.e. L1, L21 L22, L31 L32, L33,L41, L42, L43 and L44. The LGP-Pattern evaluation for each crop is achieved by taking into account the constituent component lengths of each LGP-Pattern, thus providing a profile of variability in potential yields over time (e.g. average yield, maximum yield, minimum yield).
FIGURE 7.1
Schematic presentation of the land suitability assessment programme for crop production
The next step is an appraisal of the soil units present in each growing period zone. The rating of soil units, for the crops and level of inputs under consideration, is applied to the computed area of the growing period zone occupied-by each soil unit. The appraisal, undertaken on the basis of the soil ratings as described in Section 6, leads to appropriate modifications of the climatic suitability assessment and the attainable yield. Subsequently, the ratings for the different soil textures, phases and stoniness are applied consecutively.
Finally, limitations imposed by slope are taken into account to arrive at the final land suitability appraisal for the crops, for the level of inputs under consideration.
The five classes of land suitabilities are related to attainable yield as a percentage of the maximum attainable under the optimum climatic, edaphic and landform conditions. Consequently the results provide for each land unit an assessment of crop production potentials which can be aggregated for any given area in Kenya.
The generalized results of land suitability assessment at intermediate level of inputs for barley, oat, cowpea, green gram and pigeonpea are presented in Figures 7.2, 7.3, 7.4, 7.5 and 7.6 respectively. It should be noted that the generalized results presented in the map-form include a subdivision of the not suitable class zero to less than 20% of maximum attainable yield) into two classes (1) very marginally suitable (more than zero to less than 20 % of maximum attainable yield) and (2) not suitable (zero yield).
