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 ‘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 7.1 and the diagnostic horizons and properties of these soil units are presented in the 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 7.2).
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 2, and are listed in Table 7.3.
| Texture Symbol | Texture class |
| Coarse: | |
| S | Sand |
| LCS | Loamy coarse sand |
| FS | Fine Sand |
| LFS | Loamy fine sand |
| LS | Loamy sand LS |
| Medium: | |
| FSL | Fine sandy loam |
| SL | Sandy loam |
| L | Loam |
| SCL | Sandy clay loam |
| SL | Silt loam |
| CL | Clay loam |
| SIL | Silty clay loam |
| SI | Silt |
| Fine: | |
| SC | Sandy clay |
| SIC | Silty clay |
| PC | Peaty clay |
| C | Clay |
| Symbol | Name | Symbol | Name | Symbol | Name |
| Singles: | Combination of two: | Combination of three: | |||
| R | Rocky | RIB | Rocky and bouldery | R/B/AO | Rocky and bouldery and saline-sodic |
| B | Bouldery | R/S | Rocky and stony | R/P/S | Rocky and lithic and stony |
| BM | Boulder-mantle | B/S | Bouldery and stony | B/S/A | Bouldery and stony and saline |
| S | Stony | BM/AO | Boulder-mantle and saline-sodic | BM/S/AO | Bouldery and stony and saline-sodic |
| SM | Stone-mantle | S/R | Stoney and rocky | P/R/B | Lithic and rocky and bouldery |
| G | Gravelly | S/B | Stony and bouldery | P/R/S | Lithic and rocky and stony |
| GM | Gravel-mantle | S/K | Stony and petrocalcic | P/B/S | Lithic and bouldery and stony |
| P | Lithic | S/AO | Stony and saline-sodic | P/B/A | Lithic and bouldery and saline |
| PP | Paralithic | SM/O | Stone mantle and sodic | P/BM/AO | Lithic and boulder-mantle and saline-sodic |
| K | Petrocalcic | SM/AO | Stone mantle and saline-sodic | P/S/R | Lithic and stony and rocky |
| KK | Petrocalcic | P/R | Lithic and rocky | P/S/A | Lithic and stony and saline |
| C | Pisocalcic | P/B | Lithic and bouldery | P/S/AO | Lithic and stony and saline-sodic |
| CC | Pisocalcic | P/BM | Lithic and boulder-mantle | P/SM/AO | Lithic and stone-mantle and saline-sodic |
| M | Petroferric | P/S | Lithic and stony | P/GM/S | Lithic and gravel-mantle and saline |
| MM | Pisoferric | P/O | Lithic and sodic | ||
| A | Saline | P/AO | Lithic and saline-sodic | ||
| O | Sodic | PP/R | Paralithic and rocky | ||
| AO | Saline-sodic | PP/S | Paralithic and stony | ||
| F | Fragipan | K/S | Petrocalcic and stony | ||
| K/A | Petrocalcic and saline-sodic | ||||
| KK/A | Petrocalcic and saline | ||||
| KK/O | Petrocalcic and sodic | ||||
| M/R | Pisoferric and rocky | ||||
| MM | Pisoferric and pisoferric | ||||
| A/F | Pisoferric and fragipan | ||||
| O/F | Sodic and fragipan | ||||
TABLE 7.4
Associated slope classes
| Slope class symbol | % | Associated slope classes | ||||
| A | 0 – 2 | 100% | A | |||
| AB | 0 – 5 | 100% | AB | |||
| B | 2 – 5 | 100% | B | |||
| BC | 2 – 8 | 90% | BC | 5% | A | 5% D |
| C | 5 – 8 | 90% | C | 5% | AB | 5%D |
| BCD | 2 – 16 | 90% | BCD | 5% | A | 5% E |
| CD | 5 – 16 | 90% | CD | 5% | AB | 5% E |
| D | 8 – 16 | 90% | D | 5% | BC | 5% E |
| DE | 8 – 30 | 90% | DE | 5% | BC | 5% F |
| E | 16 – 30 | 90% | E | 5% | BCD | 5% F |
| EF | 16 – 56 | 95% | EF | 5% | BCD | |
| F | 30 – 56 | 95% | F | 5% | DE | |
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: 2–5%; 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%.
TABLE 7.5
Quartiles of slope classes
| Slope class symbol % | Gentlest | Lower | Upper | Steepest | |
| Q1 | Q2 | Q3 | Q4 | ||
| A | 0 – 2 | 0 | 1 | 1 | 2 |
| AB | 0 – 5 | 0 | 2 | 4 | 5 |
| B | 2 – 6 | 2 | 3 | 4 | 5 |
| BC | 2 – 8 | 2 | 4 | 6 | 8 |
| C | 6 – 8 | 5 | 6 | 7 | 8 |
| BCD | 2 – 16 | 2 | 6 | 11 | 16 |
| CD | 5 – 16 | 5 | 9 | 12 | 16 |
| D | 8 – 16 | 8 | 11 | 13 | 16 |
| DE | 8 – 30 | 8 | 16 | 22 | 30 |
| E | 16 – 30 | 16 | 21 | 25 | 30 |
| EF | 16 – 56 | 16 | 30 | 42 | 56 |
| F | 30 – 56 | 30 | 39 | 47 | 56 |
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 explicity in the soil resources inventory The 12 inventoried combination slope classes and the associated slope classes are presented in Table 7.4. For the same purposes of evaluation, assumed mean sloped have been assigned to each of the quartiles of the land area of each of the 12 slope classes (Table 7. 5)
