Chapter 4
Land suitability assessment and selection of crop options

Land suitability assessments of single crops (Part I, Figure 2.1) is made according to the FAO-AEZ method (FAO 1978–81), and involves:

1. Selection and definition of land utilization types (e.g. crop, cropping type, produce, production system, inputs level).

2. Matching the thermal zones of the climatic inventory with the temperature requirements of the crops, and where these requirements are met, computation of agronomically attainable crop yields by length of growing period zones.

3. Matching the soil requirements of crops with the soil type, texture classes, stoniness, phases and slope classes of the soil inventory, by rating their limitations.

4.1 Crops and Land Utilization Types

A total of 25 crop species are considered in the assessment. The full list of crops and crop types is presented in Table 4.1. Coffee, cotton, pineapple, pyrethrum, sisal and tea are considered in the model to take account of the reported production and land area occupied by these crops as quantified by the land use inventory (Technical Annex 3). The remaining 19 crops are differentiated into 58 crop types to account for differences in ecotype adaptation, crop phenology and growth cycles within each crop species.

Each of the 58 crop types are considered at three levels of inputs circumstances (low, intermediate and high). The attributes of the three input level circumstances are listed in Table 4.2, and form the basis of the definition of the land utilization types in the assessment.

The following conditions apply to the crops considered:

1. cereal and legume crops are grown for dry grain production;

2. only sorghum varieties with white and yellow grain types are considered;

3. only maize varieties with white or yellow endosperm types are considered;

4. barley, oat and wheat cultivars comprise the daylength-neutral types;

5. groundnut is grown for dry kernel production from either sequentially branched cultivars (the Spanish and Valencia types) or alternately branched cultivars (the Virginia and Castle Cary types);

6. sugarcane is grown for sugar production using the ‘noble’ cane cultivars;

7. banana is grown for fruit (pulp) production using cultivars from the genome group AAA and AAB;

8. oil palm is grown for oil production from the fruit mesocarp using the African oil palm stock. It is assumed that the rotation length is 30 years and time to reach first harvest is 6 years.

TABLE 4.1
List of crops Included in the assessment

¹ Includes Phaseolus vulgaris (Common bean), P. lunatus (Lima bean), P. coccineus (Runner bean) and P. acutifolius (Tepary bean).

TABLE 4.2
Attributes of land utilization types

Note. No production involving irrigation or other techniques using additional water. No flood control measures.

4.2 Climatic Inventory

The climatic resources inventory of Kenya (Technical Annex 1) quantifies both heat and moisture conditions.

The quantification of heat attributes has been achieved by defining reference thermal zones representing the prevailing temperature regimes¹. Temperature seasonality effects of latitude are minor due to the equatorial position of Kenya.

¹The following equation closely represents the relationship between average daily temperature in degrees Celcius (T) and altitude in metres (A): T = 30.2 - 6.496(A/1000).

To cater for differences in temperature adaptability between crops, nine thermal zones (based on 2.5°C intervals) are distinguished in the climatic inventory of Kenya (Table 4.3). The temperature threshold values used in these definitions accord with those differentiating the four temperature adaptability groups of crops, pasture and tree species described in Technical Annexes 3, 5 and 6.

TABLE 4.3
Thermal zones

Quantification of moisture conditions was achieved through the concept of reference length of growing period (LGP), being defined as the duration (in days) when moisture supply can permit crop growth. A moisture supply from rainfall of half, or more than half, potential evapotranspiration has been considered to permit crop growth. The following main concepts, definitions and methods form the basis of the quantification of moisture conditions in the climatic inventory.

The growing period is the time when moisture supply from rainfall exceeds half potential evapotranspiration. It includes the time required to evapotranspire up to 100 mm of stored moisture from the soil profile. A ‘normal’ growing period has a humid phase, i.e. a period when moisture supply is greater than full potential evapotranspiration. When there is no humid period, the growing period is defined as ‘intermediate’.

The quantification of moisture regime is based on the analysis of the length of growing period for each year separately and the computation of:

1. number of separate lengths of growing periods per year, summarized as a historical profile of pattern of number of length of growing periods per year (LGP-Pattern);

2. length of each growing period and its various moisture periods, summarized as mean total dominant length, first associated length and second associated length, and the mean individual dominant and associated lengths making up the total lengths;

3. the quality of moisture conditions during the growing period and its various moisture periods;

4. year-to-year variability (frequency distribution) of each length of growing period and the associated moisture condition.

Twenty two LGP-Patterns are recognized, and these with their composition are presented in Table 4.4. The LGP-Pattern code represents the number of growing periods per year in order of frequency of occurrence, e.g. in the pattern coded 2-1-3, the numeral 2 represents the number of lengths of growing periods per year (i.e. two) that occur in the majority of the years (i.e. 55 percent) - the dominant length number; the numeral 1 represents number of lengths of growing periods per year (i.e. one) that has the next most commonly occurring frequency (i.e. 25 percent) - the first associated length number; and the numeral 3 represents number of lengths of growing periods per year (i.e. three) that - has the smallest occurrence (i.e. 20 percent) - the second associated length number.

For each LGP-Pattern type, the mean total length of the dominant number is correlated with the mean total length of the associated numbers. Also, when the mean total length is a summation of more than one mean length, the latter lengths are again correlated with the former total length. These relationships are presented in Tables 4.5 and 4.6.

In the climatic inventory of Kenya, only the mean total dominant length has been inventoried on the map as 15 LGP zones. The boundary or isoline values used are 0, 30, 60, 90, 120,150, 180,210, 240, 270, 300, 330, 365^-and 365⁺ days respectively delineating the mean total dominant length of growing period zones of 0, 1–29, 30–59, 60–89, 90–119, 120–49, 150–179, 180–209, 210–239, 240–269, 270–299, 300–329, 330–364, 365^-and 365⁺days¹.

¹    365^- - year-round growing period but not humid year-round; 365⁺ - year-round humid growing period.

Additionally, the LGP-Pattern zones have been inventoried. Consequently, the relationships in Tables 4.5 and 4.6, together with the map of dominant LGP zones, and the LGP-Pattern zones provide the historical profile of any mean total dominat length of growing period in any of the 22 LGP-Pattern zones.

TABLE 4.4
Patterns of growing periods (LGP-patterns) - Historical profiles of occurrence of number of length of growing periods per year

H = 365⁺ days (i.e. year-round humid)
D = zero days (i.e. year-round dry)

TABLE 4.5
Relationships between mean total dominant and mean total associated lengths of growing period

L1 = Total length of one growing period per year
L2 = Total length of two growing periods per year
L3 = Total length of three growing periods per year
L4 = Total length of four growing periods per year

Reference tables relating the mean total dominant LGPs (mapped) and the corresponding mean total associated LGPs (unmapped) are presented in Technical Annex 1, together with generalized coefficients of variation of mean LGPs, and frequency of occurrence of intermediate LGPs.

4.3 Thermal Zone Suitability

The association between crop growth cycles and thermal zones in Kenya for the 64 crop types is presented in Table A4.1 in the Appendix. In general, growth cycle length (number of days to maturity) of wheat, barley, oat, phaseolus bean and white potato increases by some 5 to 6 days for each 100 m increase in altitude above 1500 m, or for each 0.5 °C decrease in mean temperature from 20.0°C. For maize and sorghum, there is generally about 20 days extention in maturation for each 100 m increase in altitude above 1500 m or for each 0.5 °C decrease in mean temperature from 20°C. The 20 days extension in maturity is made up of some 5 to 6 days delay in flowering (silking/anthesis) and some 14 to 15 days extention in the grain filling phase or time taken to reach black layer physiological maturity. For example, 110 and 130 days to maturity correspond respectively to 63 and 69 days to tasseling or heading, 73 and 79 to silking or anthesis, 110 and 130 days to physiological maturity.

TABLE 4.6
Relationship between individual component mean length and mean total length of growing period

L2₁ = First length of two growing periods per year
L3₁ = First length of three growing periods per year
L3₂ = Second length of three growing periods per year
L4₁ = First length of four growing periods per year
L4₂ = Second length of four growing periods per year
L4₃ = Third length of four growing periods per year

For wheat, barley, oat, phaseolus bean and white potato, mean temperatures of 10 to 12.5°C or below have been taken to correspond to a risk of frost damage too great for successful cultivation of these crops. Mean temperature range of 10 to 12.5°C corresponds to 2700 – 3100 m altitude range (Table 4.3).

For maize and sorghum, mean temperatures below 15 °C have been considered too low for normal production because of the very severe problems with seed set and maturation. Mean temperatures below 15 °C are reached at altitudes of 2350 m and above (Table 4.3).

The crop thermal zone suitability ratings for each crop type are presented in Table A4.2 in the Appendix. Five suitability classes are employed (i.e. S1, S2, S3, S4 and N), and the ratings apply to all three levels of inputs: where requirements are fully met, the zone is adjudged S1; where requirements are sub-optimal the zone is adjudged S2, S3 or S4; where requirements are not met, the zone is adjudged as N (not suitable).

A rating of S1 indicates that the temperature conditions for growth and yield physiology and phenological development are optimal and that it is possible to achieve the maximum attainable agronomic yield potential if there are no additional climatic and/or edaphic (including landform) limitations. Ratings of S2, S3 and S4 indicate that temperature conditions for growth and development are sub-optimal and that there would be a suppression of yield potential in the order of 25, 50, and 75 percent respectively. A rating of N indicates that temperatures are not suitable for production of the crop.

4.4 Length of Growing Period (LGP) Zone Suitability

4.4.1 Individual component LGP suitability

Potential yields with constraints for individual LGPs were derived according to the method developed by the FAO-AEZ project (FAO 1978) for all crops except coffee, cotton, pineapple, pyrethrum, sisal and tea. The background details for maize, pearl millet, wetland rice, sorghum, phaseolus bean, soybean, cassava, sweet potato and white potato are given in (FAO 1978); the details for groundnut, dryland rice, sugarcane, banana and oil palm are given in (FAO 1980); the details for barley, oat, cowpea, green gram and pigeonpea are given in Technical Annex 3.

Agronomically attainable yield potentials from the agro-climatic viewpoint (i.e. on suitable soils and terrain) for suitable thermal zones (i.e. thermal zones with S1 rating) are presented in the Appendix in Table A4.3 for high level of inputs, in Table A4.4 for intermediate level of inputs and in Table A4.S for low level of inputs.

Yields in Tables A4.3, A4.4, and A4.S refer to single crops which act as building blocks in the formulation of annual cropping patterns and crop rotations, taking into account LGP-Patterns and soil-landform constraints. Single crop yields attainable with low inputs are set at 25 percent of those attainable with high inputs. Single crop yields at the intermediate level of inputs are set half-way between the high and low inputs yields.

Yields in Table A4.3, A4.4 and A4.5 apply to normal lengths of growing periods, i.e.growing period with a humid period during which precipitation is greater than full potential evapotranspiration. For intermediate growing periods, i.e. growing period with no humid period, full crop water requirements cannot be met and yield reductions are assumed to be of the order of 50% on all soils except Fluvisols and Gleysols. The percentage of occurrence of intermediate lengths of growing periods in all LGP-Pattern zones is 100% in LGP zone 1–29 days, 65% in LGP zone 30–59 days, 25% in LGP zone 60–89 days, 10% in LGP zone 90–119 days and 5% in LGP zone 120–149 days (see Technical Annex 1).

4.4.2 LGP-Pattern suitability

All annual crops are matched to individual component length of growing periods, i.e. L1, L21, L22, L32, L32, L33, L41, L42, L43 and L44. The LGP-Pattern evaluation for each annual crop is achieved by taking into account all the constituent component lengths in each LGP-Pattern, thus providing a profile of variability in potential yields over time (e.g. average yield, maximum yield, minimum yield). From such information, it is then possible to set the desired level of yield stability (e.g. in terms of percentage difference between maximum yield and minimum yield or in terms of percentage difference between average yield and minimum yield) in the selection of optimum crops and crop rotations.

Perennial crops (cassava, banana, oil palm, sugarcane) are matched to total lengths of H, L1, L2, L3 and L4, with yield potential downgraded as shown in Table 4.7 for LGP-Patterns comprising L2, L3 and L4. Reduction ratings S1, S2 and S3 in Table 4.7 correspond to zero, 25% and 50% yield reduction respectively due to moisture stress. Rating of N represents unsuitable moisture conditions for crop production.

4.4.3 Cash crops LGP and LGP-Pattern allocation ratings

For coffee, cotton, pineapple, pyrethrum, sisal and tea allocation ratings by LGP and LGP-Patterns were formulated to enable these crops to be allocated to their suitable climatic zones on suitable soils. The LGP allocation rules at high, intermediate and low levels of inputs are given in the Appendix in Tables A4.6 – A4.23 for coffee (Tables A4.6, A4.7, A4.8), cotton (Tables A4.9, A4.10, A4.11), pineapple (Tables A4.12, A4.13, A4.14), pyrethrum(Tables A4.15, A4.16, A4.17), sisal (Tables A4.18, A4.19 and A4.20) and tea (TablesA4.21, A4.22, A4.23).

TABLE 4.7
Reduction ratings for perennials matched to total length of growing periods (LGP, days) L2. L3 and L4

4.4.4 Fluvisols suitability

Cultivation of Fluvisols is governed by the depth, intensity and duration of flooding which occurs in the low-lying areas of these soils. These flooding attributes are generally controlled, not by the amount of ‘on site’ rainfall but by external factors such as river flood regime, hydrological features of catchment area and catchment-site relationship. Additionally, cultivation of these soils is normally confined to post-flood periods, the crops being grown on moisture remaining in the soil profile.

As a result of these factors Fluvisols were rated separately for all crops at high, intermediate and low levels of inputs, and the ratings are presented in the Appendix in Tables A4.24, A4.25 and A4.26 respectively.

4.5 Soil Inventory

The soil resources of Kenya (Technical Annex 1) have been inventoried in terms of associations of soil units, and the corresponding characterization of soil textures, stoniness, phases, and slopes.

Soil units have been defined in terms of measurable and observable properties of the soil itself, and specific clusters of such properties are combined into ‘diagnostic horizons’ and ‘diagnostic properties’, which are used in the definition of the soil units. The soil units inventoried in the Kenya soil resources inventory (Exploratory Soil Map of Kenya) are listed in Table 4.8, and the diagnostic horizons and properties of these soil units are presented in Technical Annex 1.

Soil texture may vary within the range of textures defined for a particular soil unit. In the legend of the Exploratory Soil Map, textural classes for the individual soil units by soil mapping unit are presented. The three major textural divisions (coarse, medium and fine) are subdivided into 17 classes (Table 4.9).

Soil phases indicate land characteristics which are not considered in the definition of the soil units but are significant to the use and management of land. Soil phases recognized on the Exploratory Soil Map of Kenya can be grouped into phases indicating a mechanical hindrance or limitation (rocky, bouldery, boulder-mantle, stony, stone-mantle, gravel-mantle), phases indicating an effective soil depth limitation (lithic, paralithic, petro-calcic, piso-calcic, petro-ferric, piso-ferric), and phases indicating a physico-chemical limitation (saline, sodic and saline-sodic). Soil phases occur either individually or in combinations of upto three. They are described in Technical Annex 1, and are listed in Table 4.10.

TABLE 4.8
Soil units

The presence of coarse material (stoniness) in the soil profile has been inventoried separately from soil textures. Six types of coarse material or stoniness have been inventoried: Gravelly (G), Very Gravelly (VG), Stony(S), Bouldery (B), Stony/Bouldery (SB)and Bouldery/Stony (BS).

TABLE 4.9
Texture classes

Six basic slope classes, in 12 combinations, have been employed in the Exploratory Soil Map of Kenya. The six basic slope classes are: A: 0–2%; B: 25%; C: 5–8%; D: 8–16%; E: 16–30% and F: > 30%. The 12 combination slope classes are: A: 0–2%; AB: 0–5%; B: 25%; BC: 2–8%; C: 5–8%; BCD: 216%; CD: 5–16%; D: 8–16%; DE: 830%; E: 16–30%; EF: 16–>30%; F: >30%.

To each of these 12 slope classes,associated slope classes have been assigned. These associated slope classes,covering up to 10% of the land area of the 12 slope classes, are used for evaluation purposes only. They are not included explicitly in the soil resources inventory. The 12 inventoried combination slope classes and the associated slope classes are presented in Table 4.11. For the same purposes of evaluation, assumed mean slopes have been assigned to each of the quartiles of the land area of each of the 12 slope classes (Table 4.12).

4.6 Edaphic Suitability

The edaphic suitability assessment is input-specific and based on:

1. Matching the soil requirements crops with the soil conditions of the soil units described in the soil inventory (soil unit evaluation).

2. Modification of the soil unit evaluation by limitation imposed by texture, stoniness and phase.

4.6.1 Soil unit evaluation

The soil unit evaluation is expressed in terms of suitability ratings based on how far the soil conditions of a soil unit meet the crop requirements under a specified level of inputs. The appraisal is effected in five basic classes for each crop and level of inputs, i.e. very suitable (S1), suitable (S2), moderately suitable (S3), marginally suitable (S4), and not suitable (N).

A rating of S1 indicates that the soil conditions are optimal, and that suppression of potential yields (if any) are assumed to be slight. Ratings of S2, S3 and S4 indicate that soil conditions are sub-optimal for crop production and that potential yields would be suppressed by 25%, 50% and 75% respectively. A rating of N indicates that soil conditions are so severe that the soil unit is not suitable for crop production. The soil unit ratings for all 25 crops are presented in the Appendix in Table A4.27.

TABLE 4.10
Soil phases

TABLE 4.11
Associated slope classes

4.6.2 Texture evaluation

Soil unit ratings apply as indicated in Table A4.27 if there are no additional limitations imposed by texture. Modifications are required where limitations are imposed by texture.

Soil unit ratings remain unchanged for Arenosols (Q), Albic Arenosols (Qa), Cambic Arenosols (Qc), Ferralic Arenosols (Qf), Calcaro-cambic Arenosols (Qkc), Luvic Arenosols (Ql) and Vitric Andosols (Tv), since coarse texture limitations have been already applied in the soil unit ratings.

TABLE 4.12
Quartiles of slope classes

Soil unit ratings remain unchanged where textures are medium: fine sandy loam (FSL), sandy loam (SL), loam (L), sandy clay loam (SCL), silt loam (SL), clay loam (CL), silty clay loam (SICL) and silt (SI); or fine: sandy clay (SC), silty clay (SIC), peaty clay (PC) and clay (C).

In all other cases, i.e. soil units with coarse textures: sand (S), loamy coarse sand (LCS), fine sand (FS), loamy fine sand (LFS), and loamy sand (LS), the soil unit rating is lower by 25% for all crops except for groundnut and white potato.

4.6.3 Stoniness and phase evaluation

Soil unit ratings apply as indicated in Table A4.27 if there are no additional limitations imposed by stoniness or phase.

Limitations imposed by stoniness and phase are rated using the five basic suitability classes described above. The stoniness modification ratings are presented in the Appendix in Table A4.28 and the phase modifications ratings in the Appendix in Table A4.29.

4.7 Soil Erosion and Yield Loss

Limitations imposed by slope are taken into account in three steps. Step one defines those slopes which are permissible for cultivation by crop/land use and inputs level (Table 4.13).

Step two involves the computation of potential topsoil, and step three relates topsoil loss to productivity loss according to the method described in Technical Annex 2.

Topsoil loss is estimated for each crop and inputs level using a modified Universal Soil Loss Equation (USLE: Wischmeier and Smith 1978).

TABLE 4.13
Slope-cultivation association screen

¹   For wetland crops, terracing is required.

TABLE 4.14
Relationships between topsoil loss and yield loss

Y = productivity loss in percent;
X = soil loss in cm.

The estimated topsoil losses are related to yield losses through a set of equations given in Table 4.14, taking into account the susceptibility of the individual soil units and level of inputs. The reduced impact under intermediate and high levels of inputs is due to the compensating effect of fertilizer applications at their normal rates of use.

Table 4. 15 shows the soil units ranked in three classes according to loss of topsoil, on the basis of organic matter content and on the presence of other unfavourable subsoil conditions.

In estimating yield losses from equations in Table 4.14 regeneration capacity of topsoil is taken into account in calculation of net topsoil loss. Regeneration capacities of topsoil by thermal and moisture regimes are given in Table 4.16.

TABLE 4.15
Ranking of soils (Kenya Soil Survey) according to their susceptibility to productivity loss per unit of tonsoil loss

TABLE 4.16
Regeneration capacity of topsoil (mm/year) by length of growing period (LGP) and thermal zone

Derived from Hammer (1981)

4.8 Crop Options

The land suitability assessment part of the productivity model (Part I, Figure 2.1) when applied to the land resources inventory (Technical Annex 1) allows the assessment, by agro-ecological cell, of potential crop performance and consequently crop options to be selected for further processing in Part II of the model. At the same time, land that is reserved for other uses, such as cash crops zones, irrigation schemes, forest zones, reservation and conservation areas, is taken into account as appropriate.

For a crop to qualify for selection of the list of crop options for a particular agro-ecological cell, its attainable yield in that cell must be more than the ‘threshold or critical’ minimum percentage of the maximum attainable yield potential, after taking into account limitations due to climate, soil and erosion hazards.

The threshold minimum yield percentage parameter is a model variable and is set at 20%. Thus, land whose yield potential for any crop is less than 20% of its maximum attainable yield, is regarded as not suitable for production of that crop. Instead, the land would be set aside for further consideration in Part V of the model for livestock and fuelwood production (Technical Annexes 5 and 6).

The introduction of a threshold minimum yield potential allows the identification of a ‘gross’ list of crop options from which a further selection of crops can be made based on additional selection criteria or constraints. In the model, this additional criteria (a model variable) can be set as required, depending on the objective function driving the productivity model.