Chapter 6
Agro-edaphic suitability classification

As a medium in which roots grow and as a reservoir for water and nutrients on which crops continuously draw during their life cycle, soils are natural resource and valuable economic asset requiring protection, conservation and improvement through good husbandry.

The adequate agricultural exploitation of the climatic potential and sustained maintenance of productivity largely depends on soil fertility and management of soil on an ecologically sound basis. Soil fertility is concerned with the ability of the soil to supply nutrients and water to enable crops to maximize the climatic resources of a given location. The fertility of a soil is determined by its both physical and chemical properties whose understanding is essential to the effective utilization of climate and crop resources for optimum production.

In order to assess suitability of soils for crop production, soil requirements of crops must be known. Further, these requirements must be understood within the context of limitations imposed by landform and other features which do not form a part of soil but may have a significant influence on the use that can be made of the soil.

The basic soil requirements of crop plants may be summarized under the following headings, related to internal and external soil properties:

1. Internal requirements:

   -   the soil temperature regime, as a function of the heat balance of soils as related to annual or seasonal and/or daily temperature fluctuations;

   -   the soil moisture regime, as a function of the water balance of soils as related to the soil's capacity to store, retain, transport and release moisture for crop growth, and/or to the soil's permeability and drainage characteristics;

   -   the soil aeration regime, as a function of the soil air balance as related to its capacity to supply and transport oxygen to the root zone and to remove carbon dioxide;

   -   the natural soil fertility regime, as related to the soil's capacity to store, retain and release plant nutrients in such kinds and proportions as required by crops during growth;

   -   the effective soil depth available for root development and foothold of the crop;

TABLE 6.1 Crop edaphic adaptability inventory

Drainage classes - I = imperfectly drained;

MW = moderately well drained;

W = well drained;

SE = somewhat excessively drained;

E = excessively drained.

Flooding classes - F_o = no floods;

F₁ = occasional flooding.

Texture classes - MCs = montmorillonitic clay, structured;

C = clay (mixed unspecified);

KC = kaolinitic clay;

SC = sandy clay;

SiCL = silty clay loam;

CL = clay loam;

SCL = sandy clay loam;

L = loam;

SL = sandy loam;

LS = loamy sand.

-   soil texture at the surface and within the whole depth of soil required for normal crop development;

-   the absence of soil salinity and of specific toxic substance or ions deleterious to crop growth;

-   other specific properties, e.g. soil tilth as required for germination and early growth.

2. External requirements: in addition to the above internal soil requirements of crops, a number of external soil requirements are of importance, e.g.:

   -   soil slope, topography and characteristics determined by micro and macrorelief of the soil;

   -   occurrence of flooding as related to crop susceptibility to flooding during the growing period;

   -   soil accessibility and trafficability under certain management systems.

6.1 Crop Edaphic Adaptability

From the basic soil requirements of crops, a number of crop response related soil characteristics can be derived. One of these characteristics is, for instance, soil pH. For most crops and cultivars, optimal soil pH is known and can be quantified by a range within which it is not limiting to growth. Outside the optimal range, there is a critical range within which the crop can be grown successfully but with diminished yield. Beyond the critical range, the crop cannot be expected to yield satisfactorily unless special precautionary management measures are taken.

The same holds for other soil requirements of plants related to soil characteristics. Many soil characteristics can be defined in a range that is optimal for a given crop, a range that is critical or marginal, and a range that is unsuitable under present technology.

Table 6.1 presents for barley, oat, cowpea, green gram and pigeonpea, optimal and critical ranges of the following soil characteristics: soil slope, soil depth, soil drainage, flooding, texture and clay type, natural fertility (including cation exchange capacity, percent base saturation and organic matter), salinity, pH, free calcium carbonate content and gypsum content.

TABLE 6.2
Soil units

Many of the soil characteristics listed above are at least partly intrinsically related to the soil. This relationship has guided the definition of optimal and marginal ranges of the various soil characteristics and so simplified the subsequent matching of the different soil units with the inventoried soil requirements of crops.

6.2 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, phases, stoniness 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 soil units. The soil units inventoried in the Kenya soil resources inventory (Exploratory Soil Map of Kenya) are listed in Table 6.2.

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 6.3) 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 up to three. They are described in Technical Annex 1, and are listed in Table 6.4.

TABLE 6.3
Texture Texture

The presence of coarse material (stoniness) in the soil profile has been inventoried seperately 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).

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: 2–5%; 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: 2–16%; CD: 5–16%; D: 8–16%; DE: 8–30%; 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 upto 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 6.5. 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 6.6).

TABLE 6.4
Soil phases

6.3 Agro-edaphic Suitability

From the basic soil requirements of crops, a number of responses related soil characteristics can be derived. A correlation between soil requirements listed above and soil characteristics that can be used as soil factors to rate crop performance is given in Table 6.7.

As explained earlier, the soil units (Table 6.2) have been defined in terms of measurable and observable properties of the soil itself, and specific clusters of such properties are combined into ‘diagnostic properties’, which are used in the definition of the soil units.

TABLE 6.5
Associated slope classes

The diagnostic horizons have been used as defined in the FAO/Unesco Soil Map of the World Legend. Diagnostic properties however have been narrowed down in case of ferric properties and widened in case of vertic properties. In Table 6.8 for each soil unit diagnostic horizons and properties of the soil units are summarized. For the ease of interpretation of Table 6.8 these characteristics are summarized below.

TABLE 6.6
Quartiles of slope classes

TABLE 6.7
Relation between basic soil requirements for crops and soil characteristics

¹ Moisture availability is influenced by climatic factors

² Oxygen availability is influenced by inundation and flooding characteristics

³ Chemical properties of soil parent material may also be involved in some cases.

The agro-edaphic suitability classification is input-specific and based on:

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

2. modification of the soil unit evaluation by limitation imposed by, texture, stoniness, phase and slope conditions.

6.3.1 Soil unit evaluation

The soil unit evaluation is expressed in terms of ratings based on how far the soil conditions of a soil unit meet crop requirements under three specified levels 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 for crop production are optimal and that suppression of potential yields (if any) are assumed to be slight or nil. The rating of S2 indicates that there are slight to moderate limitation which would suppress potential yields by some 25 percent. The rating of S3 indicates sub-optimal soil conditions with moderate to severe limitations which would suppress potential yields by some SO percent. The rating of S4 indicates sub-optimal soil conditions with severe limitations which would suppress potential yields by some 75 percent. The rating of N indicates that crop production is not possible. The ratings are presented in Table 6.9 for high, intermediate and low level inputs circumstances.

6.3.2 Texture evaluation

All ratings of soils with coarse texture (sand, loamy coarse sand, fine sand, loamy fine sand and loamy sand) for the five crops are classified one class lower for 50% and two classes lower for the remaining 50% of its extents, except for Arenosols (Q), Albic Arenosols (Qa), Cambic Arenosols (Qc), Ferralic Arenosols (Qf), Calcaro-cambic Arenosols (Qkc), Luvic Arenosols (Ql) and Vitric Andosols (Tv), which should remain unchanged since coarse texture limitations have already been applied in the soil unit ratings. All ratings of soils with medium and fine textures remain unchanged since limitations imposed by these textures have been included in the soil unit ratings.

6.3.3 Stoniness evaluation

The limitations imposed by presence of coarse material (stoniness) in the soil profile have been rated (using the five basic classes described in 6.3.1) by crop and inputs level. The ratings are presented in Table 6.10.

6.3.4 Soil phase evaluation

The limitations imposed by presence of soil phases which occur individually or in combinations of up to three phases have been rated (using the five basic classes described in 6.3.1) by crop and inputs level. The ratings are presented in Table 6.11.

TABLE 6.8
Diagnostic horizons and properties of soil units

(indicates autually exclusive horizons)

(indicates autually exclusive horizons)

(indicates mutually exclusive horizons)