Chapter 8
Soil resources

8.1 Exploratory Soil Map of Kenya

The Exploratory Soil Map of Kenya (Siderius and van der Pouw 1980; Sombroek, Braun and van der Pouw 1982) at 1:1 million scale was used to compile the soil resources inventory for this assessment.

This soil map published by Kenya Soil Survey in 1980 provides the latest country-wide soil data base and includes information on distribution and characteristics of soils, landform and geology/parent material.

8.2 Soil Mapping Units

The soil mapping units are soil associations or soil complexes composed of dominant soils, associated soils and inclusions. The soil mapping units have been registered on the map by a symbol reflecting the landform in which they occur. In Table 8.1 the occurrences of soil mapping units are presented by landform.

The Exploratory Soil Map consists of 390 different soil mapping units. Extents of the individual soil mapping units are presented in Appendix 8.1.

For each soil mapping unit the following semi-quantified information in terms of description, classes and extents has been transferred to the soil resources data base of this assessment:

- Landform
- Geology/Parent material
- Soil units (with implied characteristics)
- Slope- gradient classes
- Soil texture classes
- Soil phases.

8.3 Landform

Landform is the first entry in the legend of the Exploratory Soil Map. It provides information on physiography, altitudinal position and slope patterns. A generalized map of landforms in Kenya is presented in Figure 8.1, and a description of landforms is presented in Table 8.1. The extents of individual landforms are presented in Table 8.2.

TABLE 8.1
Occurrence of soil mapping units by landform

Landform symbol	Landform description	Soil mapping units
A	Floodplains	A1 - A18
A	Floodplains	A8 + A12
B	Bottomlands	B1 – B16
D	Dunes or dune land	D1 – D3
D	Dunes or dune land	D1 + P13
F	Footslopes	F1 – F19
FY	Footslopes and piedmont plains undifferentiated	FY1 – FY3
	Hills and minor scarps
H		H1 – H22
	Step-faulted scarps of the Rift Valley
Hs		Hs1
	Plateaus and high level structural plains
L		L1 – L31
	Lava flows
La		Lava
	Coastal plateaus
Lc		Lc1 – Lc3
	Step-faulted floor of the Rift Valley
Ls		Ls1 – Ls3
	Plateau/upper-level upland transitions
Lu		Lu1 – Lu2
	Mountains and mayor scarps
M		M1 – M12
	Higher-level coastal plain
Pch	Lower-level coastal plain	Pc1 – Pc3
Pcl	Reef coastal plan	Pc4 – Pc7
Pcr		Pc8 – Pc10
	Dissected erosional plains
Pd		Pd1 – Pd6
Pf1	Sedimentary plains of large alluvial plains (older fans)	Pf1 – Pf3
Pf2	Sedimentary plains of large alluvial plains (younger fans)	Pf4 – Pf5
	Lacustrine plains
Pl		Pl1–Pl13
Pn	Non-dissected erosional plains	Pn1 – Pn35
Psh	Higher-level sedimentary plains	Ps1 – Ps6
Psh	Higher-level sedimentary plains	Ps3 + Ps15
Psl	Lower-level sedimentary plain	Ps21 – Ps27
Psm	Middle sedimentary plains (‘enclosed’ plains and sealing loam plain)	Ps7 – Ps20
Psm		Ps11 + D1
Psx	Sedimentary plains of undifferentiated levels	Ps28 – Ps29
Psx	Sedimentary plains of undifferentiated levels	Ps28 + D1
Pt	Sedimentary plain of upper river terraces	Pt1 – Pt4
Pv	Volcanic plains	Pv1 – Pv12
R	Volcanic footridges	R1 – R14
S	Swamps	S1 – S3
T	Mangrove swamps	T
Uc	Coastal Uplands	Uc1 – Uc11
Uh	Upper middle-level uplands	Uh1 – Uh19
Uh	Upper middle-level uplands	Uh3 + Uh4
Ul	Lower-level uplands	Ul1 – Ul21
Urn	Lower middle-level uplands	Um1 – Um29
Up	Upland/high-level plain transitional lands	Up1 – Up8
Uu	Upper-level uplands	Uu1 – Uu3
Ux	Uplands, undifferentiated lands	Ux1 – Ux10
V	Minor valleys	V1 – V2
W	Badlands	W1 – W2
Y	Piedmont plains	Y1 – Y13
Z1	Older coastal beach ridges	Z1
Z2	Younger coastal beach ridges	Z2
Z3	Lake side beach ridges	Z3

For the Exploratory Soil Map of Kenya six slope classes have been employed in 12 combinations. The slope classes are:

0 – 2 %
2 – 5 %
5 – 8 %
8 – 16 %
16 – 30 %
> 30 %.

A generalized map of slope-gradient classes in Kenya is presented in Figure 8.2. The combinations of slope classes employed and their extents are presented in Table 8.3.

To each of the 12 slope classes, inventoried in the Exploratory Soil Map, 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 and included in the land resources inventory. The inventoried slope classes and associated slope classes are presented in Table 8.4.For the same purposes assumed mean slopes of quartiles of the land area of each of the slope classes have been assigned. These values are presented in Table 8.5.

TABLE 8.2
Extents of landform units FIGURE 8.2 Generalized map of slope gradient classes

Landform symbol	Extent (ha)	Percentage Total area
A	3,178,500	5.52
B	973,400	1.69
D	89,900	0.16
F	2,110,900	3.66
FY	614,000	1.07
H	3,203,600	5.56
Hs	514,600	0.89
L	4,051,800	7.03
La	959,800	1.67
Lc	212,800	0.37
Ls	810,800	1.41
Lu	97,600	0.17
M	2.375.200	4.12
Pch	429,500	0.75
Pcl	599,200	1.04
Pcr	82,600	0.14
Pd	1,895,300	3.29
Pf1	311,500	0.54
Pf2	377.000	0.65
Pl	862,500	1.50
Pn	6,007,100	10.43
Psh	3,885,400	6.74
Psl	3,138,300	5.45
Psm	5,618,100	9.75
Psx	247,100	0.43
Pt	322.700	0.56
Pv	998,000	1.73
R	3,121,500	5.42
S	94,700	0.16
T	133,700	0.23
Uc	533,000	0.93
Uh	786,200	1.36
Ul	1,400,000	2.43
Um	2,089,000	3.63
Up	385,900	0.67
Uu	131,900	0.23
Ux	1,744,000	3.03
V	112,400	0.20
W	722.000	1.25
Y	2,133,700	3.70
Z1	79,300	0.14
Z2	38,500	0.07
Z3	5,100	0.01
Lakes	117,600	0.20
Towns	10,700	0.02

Total extent	57.607.200	100.00

FIGURE 8.2
Generalized map of slope gradient classes

TABLE 8.3
Extents of slope classes

Slope class symbol	Slope class (%)	Extent (ha)	Percentage of total area
A	0 – 2	19 868 850	34.49
AB	0 – 5	14 540 600	25.24
B	2 – 5	3 098 150	5.38
BC	2 – 8	7 351 900	12.76
C	5 – 8	759 200	1.32
BCD	2 – 16	1 707 300	2.96
CD	5 – 16	1 557 450	2.70
D	8 – 16	1 616 200	2.81
DE	8 – 30	885 850	1.54
E, EF, F	> 16	6 093 400	10.58
Lakes		117 600	0.20
Towns		10 700	0.02
Total extent		57 607 200	100.00

TABLE 8.4
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

TABLE 8.5
Quartiles of slope classes

Slope class symbol	%	Gentlest Q1	Lower Q2	Upper Q3	Steepest Q4
A	0 – 2	0	1	1	2
AB	0 – 5	0	2	4	5
B	2 – 5	2	3	4	5
BC	2 – 8	2	4	6	8
C	5 – 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

FIGURE 8.3
Generalized map of geology/parent material

TABLE 8.6
Geology/parent material

Geology symbol	Geology description
A	(Alluvial) Sediments from various sources¹
B	Basic and ultra-basic igneous rocks (basalts etc.)
B+	As in B, but with volcanic ash admixture
BP	As in B, but volcanic ash predominant
D	Mudstones, claystones
E	Aeolian sediments (cover sands)
F	Gneisses rich in ferromagnesian minerals, hornblende gneises
G	Granites, granodiorites
G+	As in G, but with volcanic ash admixture
GF	Biotite-hornblende granites
GF+	As in GF, but with volcanic ash admixture
GP	As in G, but volcanic ash predominant
GR	Complex of G and R
I	Intermediate igneous rocks (syenites etc.)
I+	As in I, but with volcanic ash admixture
J	Lagoonal deposits
K	Siltstones
KT	Complex K and T
L	Limestones, calcitic mudstones
N	Biotite gneisses
N+	As in N, but with volcanic ash admixture
O	Plio-pleistocene bay sediments
P	Pyroclastic rocks
Q	Quartzites
R	Quartz-feldspar gneisses
S	Feldspar, grits, arkoses
T	Shales
U	Undifferentiated basement system rocks (predominantly gneisses)
U+	As in U, but with volcanic ash admixture
UP	As in U, but volcanic ash predminant
V	Undifferentiated or various igneous (volcanic) rocks
W	Marls
X	Undifferentiated or various rocks
X+	As in X, but with volcanic ash admixture
Y	Acid igneous rocks (rhyolite, aplite)
Y+	As in Y, but with volcanic ash admixture
-	Not defined

¹ If the source of alluvial sediments and bottomland infills is known (e.g. basalts), then the code for this rock is used, otherwise the code A applies.

8.4 Geology

Geology is the second entry in the legend of the Exploratory Soil Map. The geological subdivisions reflect mainly resistance to weathering and richness of parent material in order to provide linkage with soil formation. The first level subdivision comprises three types of rocks:

- Igneous rocks
- Metamorphic rocks
- Sedimentary rocks.

TABLE 8.7
Extents of geological units

Geology symbol	Extent (ha)	Percentage total area
A	2 241 600	3.89
B	6 786 800	11.78
B+	233 300	0.39
BP	52 200	0.09
D	102 900	0.18
E	235 400	0.41
F	1 680 600	2.92
G	477 900	0.83
G+	14 400	0.02
GF	38 500	0.07
GF+	70 700	0.12
GP	19 300	0.03
GR	50 700	0.09
I	499 900	0.87
I+	90 900	0.16
J	1 154 600	2.00
K	1 466 600	2.55
KT	94 100	0.16
L	2 056 400	3.57
N	400 600	0.70
N+	160 300	0.28
O	9 606 600	16.68
P	1 897 400	3.29
Q	405 900	0.70
R	58 900	0.10
S	647 400	1.12
T	164 000	0.28
U	14 007 000	24.31
U+	1 172 400	2.04
UP	36 700	0.06
V	8 536 300	14.82
W	168 500	0.29
X	1 350 200	2.34
X+	11 700	0.02
Y	155 800	0.27
Y+	79 000	0.14
-	1 263 400	2.19
Lakes	117 600	0.20
Towns	10 700	0.02

Total extent	57 607 200	100.00

Igneous rocks and metamorphic rocks are futher subdivided from basic to acid. The sedimentary rocks are futher subdivided from fine to coarse textured.

Each soil mapping unit is accordingly characterized for its geological setting/parent material.

A generalized map of geology/parent material is presented in Figure 8.3, and the descriptions and extents of geological units are presented in Table 8.6 and Table 8.7 respectively.

8.5 Soil Units

The individual soil units of the soil associations or soil complexes (soil mapping units of the Exploratory Soil Map) have been defined in accordance with the FAO-Unesco system. The soil units adopted were selected on the basis of present knowledge on the formation, characteristics and distribution of the soils, their importance as resources for agricultural production and their significance as a factor of the environment.

In the legend of the Exploratory Soil Map some adaptations of the FAO-Unesco legend have been introduced. At the first level (great group) the terminology for Lithosols and Nitosols has been modified. At the second level (unit level) new subgroups have been introduced (cambic and orthic Rendzinas) and others modified (vertic Gleysols, mollic Nitisols, chromic Acrisols, chromic Luvisols and chromic Cambisols). In order to reflect thegreater amount of detail of the Exploratory Soil Map of Kenya, a third level of terminology (subunit level) has been introduced for subdivision of soil units into subunits. The prefixes used to distinguish subunits are ando, calcaro, chromo, ferralo, luvo, nito, ortho and verto.

In the Exploratory Soil Map 123 different soil units and five miscellaneous units occur. Table 8.8 presents the soil units and miscellaneous units and their extents.

The 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’.

The diagnostic horizons have been used as defined in the FAO-Unesco legend. Diagnostic properties however have been narrowed down in case of ferric properties and widened in case of vertic properties. In Appendix 8.2 for each soil unit diagnostic horizons and properties of the soil units are summarized. For the ease of interpretation of Appendix 8.2, these characteristics are summarized below.

The complete definitions of the soil units are given in Volume 1 (Legend) of FAO-Unesco Soil Map of the World (FAO 1974) and Exploratory Soil Survey Report (KSS 1982).

Histic H horizon:	Surface layer of organic material more than 20 cm thick.
Mollic A horizon:	Surface horizon with dark colour, medium to high humus content, high base saturation.
Umbric A horizon:	Surface horizon with dark colour, medium to high humus content, low base saturation.
Ochric A horizon:	Surface horizon with light colour, low humus content
Argillic B horizon:	Subsoil horizon with accumulation of illuvial clay.
Natric B horizon:	Subsoil horizon with accumulation of illuvial clay and high exchangeable sodium.
Cambic B horizon:	Subsoil horizon with a structure and/or colour different from overlying and underlying horizons.
Spodic B horizon:	Subsoil horizon with accumulation of iron and/or humus.
Oxic B horizon:	Subsoil with residual accumulation of sesquioxides and low CEC.
Calcic horizon:	Horizon of accumulation of calcium carbonate.
Gypsic horizon:	Horizon of accumulation of calcium sulphate.
Sulphuric horizon:	Horizon with strong acidity and prominent mottling.

TABLE 8.8
Extents of soil units

Soil unit symbol	Soil unit name	Extent (ha)	Percentage total area
A	Acrisols	45 080	0.08
Ac	Chromic Acrisols	82 830	0.14
Ag	Gleyic Acrisols	49 950	0.09
Ah	Humic Acrisols	151 345	0.26
Aic	Ferralo-chromic Acrisols	772 685	1.34
Aif	Ferralo-ferric Acrisols	127 760	0.22
Aio	Ferralo-orthic Acrisols	325 475	0.56
Ao	Orthic Acrisols	167 920	0.29
Ap	Plinthic Acrisols	14 880	0.03
Ath	Ando-humic Acrisols	14 600	0.03
B	Cambisols	8 800	0.02
Bc	Chromic Cambisols	1 000 700	1.74
Bd	Dystric Cambisols	70 350	0.12
Be	Eutric Cambisols	630 140	1.09
Bf	Ferralic Cambisols	53 840	0.09
Bg	Gleyic Cambisols	31 150	0.05
Bh	Humic Cambisols	513 185	0.89
Bk	Calcic Cambisols	1 890 630	3.28
Bnc	Nito-chromic Cambisols	296 195	0.51
Btc	Ando-chromic Cambisols	436 600	0.76
Bte	Ando-eutric Cambisols	181 750	0.32
Bv	Vertic Cambisols	20 825	0.04
Ch	Haplic Chernozems	39 800	0.07
Ck	Calcic Chernozems	56 800	0.10
Ec	Cambic Rendzinas	14 700	0.03
Eo	Orthic Rendzinas	184 790	0.32
F	Ferralsols	128 080	0.22
Fa	Acric Ferralsols	59 400	0.10
Fh	Humic Acrisols	6 100	0.01
Fnh	Nito-humic Ferralsols	13 600	0.02
Fnr	Nito-rodic Ferralsols	225 900	0.39
Fo	Orthic Ferralsols	855 490	1.49
Fr	Rodic Ferralsols	2 695 015	4.68
Fx	Xanthic Ferralsols	73 860	0.13
G/Ge	Gleysols/Eutric Gleysols	9 270	0.02
Gc	Calcaric Gleysols	95 100	0.17
Gd	Dystric Gleysols	22 610	0.04
Gh	Humic Gleysols	52 830	0.09
Gm	Mollic Gleysols	124 840	0.22
Gv	Vertic Gleysols	820 070	1.42
Hg	Gleyic Phaeozems	147 180	0.26
Hh	Haplic Phaeozems	96 355	0.17
Hnl	Nito-luvic Phaeozems	21 900	0.04
Hol	Orthic-luvic Phaeozems	319 120	0.55
Hrl	Chromo-luvic Phaeozems	523 270	0.91
Hth	Ando-haplic Phaeozems	36 190	0.06
Htl	Ando-luvic Phaeozems	263 980	0.46
Hvl	Verto-luvic Phaeozems	642 225	1.11
I	Lithosols	2 344 045	4.07
Ir	Ironstone soils	216 285	0.38
J	Fluvisols	34 800	0.06
Jc	Calcaric Fluvisols	1 434 600	2.49
Je	Eutric Fluvisols	299 400	0.52
Jt	Thionic Fluvisols	79 650	0.14
Kh	Haplic Kastanozems	57 640	0.10
L	Luvisols	194 750	0.34
La	Albic Luvisols	124 625	0.22
Lc	Chromic Luvisols	2 321 880	4.03
Lf	Ferric Luvisols	582 150	1.01
Lg	Gleyic Luvisols	167 395	0.29
Lic	Ferralo-chromic Luvisols	784 840	1.37
Lif	Ferralo-ferric Luvisols	44 955	0.08
Lio	Ferralo-orthic Luvisols	50 325	0.09
Lk	Calcic Luvisols	981 995	1.70
Lnc	Nito-chromic Luvisols	34 120	0.06
Lnf	Nito-ferric Luvisols	36 260	1.81
Lo	Orthic Luvisols	1 039 855	0.29
Lv	Vertic Luvisols	169 710	0.12
Mo	Orthic Greyzems	70 700	0.07
Mvo	Verto-orthic Greyzems	38 100	0.05
Nd	Dystric Nitisols	31 195	0.05
Ne	Eutric Nitisols	600 610	1.04
Nh	Humic Nitisols	635 860	1.10
Nm	Mollic Nitisols	190 610	0.33
Nth	Ando-humic Nitisols	212 530	0.37
Nve	Verto-eutric Nitisols	33 365	0.06
Nvm	Verto-mollic Nitisols	3 240	0.01
Od	Distric Histosols	79 800	0.14
Q	Arenosols	15 750	0.03
Qa	Albic Arenosols	63 880	0.11
Qc	Cambic Arenosols	702 790	1.22
Qf	Ferralic Arenosols	2 200 890	3.82
Qkc	Calcaro-cambic Arenosols	446 780	0.78
Ql	Luvic Arenosols	187 860	0.33
R	Regosols	13 250	0.02
Rc	Calcaric Regosols	1 256 110	2.18
Rd	Dystric Regosols	138 640	0.24
Re	Eutric Regosols	696 390	1.21
Rtc	Ando-calcaric Regosols	301 440	0.52
S	Solonetz	634 580	1.10
Sg	Gleyic Solonetz	434 030	0.75
Slo	Luvo-orthic Solonetz	4 664 100	8.10
Sm	Mollic Solonetz	43 820	0.08
So	Orthic Solonetz	2 536 730	4.40
Th	Humic Andosols	455 520	0.79
Tm	Mollic Andosols	528 950	0.92
Tv	Vitric Andosols	81 600	0.14
U	Rankers	154 670	0.27
V	Vertisols	149 580	0.26
Vc	Chromic Vertisols	763 920	1.33
Vp	Pellic Vertisols	1 514 390	2.63
W	Planosols	4 380	0.01
Wd	Dystric Planosols	114 205	0.20
Wh	Eutric Planosols	323 710	0.56
We	Humic Planosols	195 800	0.34
Ws	Solodic Planosols	2 472 405	4.29
Wve	Verto-eutric Planosols	251 205	0.44
X	Xerosols/Yermosols	102 920	0.18
Xh	Haplic Xerosols/Yermosols	650 620	1.13
Xk	Calcic Xerosols/Yermosols	3 620 670	6.29
Xy	Gypsic Xerosols/Yermosols	168 500	0.29
Z	Solonchaks	577 750	1.00
zg	Gleyic Solonchaks	187 000	0.32
Zo	Orthic Solonchaks	1 601 410	2.78
Zt	Takyric Solonchaks	147 800	0.26
Lava		1 015 960	1.76
Lava flows		128 990	0.22
Lava fields		56 460	0.10
Ice cap		557 885	0.97
		5 335	0.01
Lakes		117 600	0.20
Towns		10 700	0.02
Total extent		57 607 200	100.00

Albic E horizon:	Eluvial horizon from which clay and free iron oxide have been removed, light colour.
Calcareous material:	Calcium carbonate present at least between 20 and SO cm from the surface.
CEC high or very high:	Exchange complex dominated by allophane or montmorillonite.
CEC low:	Exchange complex dominated by kaolinite (CEC less than 24 meq/100 g clay).
CEC very low:	Less than 1.5 meq/100 g clay.
Cracking clays:	Formation of deep and wide cracks upon drying.
Plinthite:	Mottled subsoil layer which irreversibly hardens upon exposure to repeated wetting and drying.
High salinity:	Electrical conductivity (EC) higher than 15 mmhos/cm.
Moderate salinity:	Electrical conductivity (EC) between 4 and 15 mmhos/cm.
High alkalinity:	Saturation with exchangeable sodium of more than 15 percent.
Moderate alkalinity:	Saturation with exchangeable sodium of to 15 percent.
Indurated subsoil:	Subsoil layer with firm or hard consistence, but can still be penetrated by spade or auger.
Cemented hardpan:	Extremely hard continuous subsoil layer which cannot be penetrated by spade or auger.
Coarse texture:	Less than 18 percent clay and more than 65 percent sand.
Heavy texture:	More than 35 percent clay.
Abrupt textural change:	Considerable increase in clay content within a very short vertical distance.
Tonguing:	Deep and irregular penetration of an albic E horizon into an argillic B horizon.

8.6 Soil Textures

Soil textures may vary within the range of textures defined for a particular soil unit. In the legend of the Exploratory Soil Map textural classes for individual soil units by soil mapping unit are presented. The three major textural divisions are subdived into 17 classes. The three major textural divisions are shown in the texture diagram. Textural classes by major division are also listed below.

Percentage occurrence of textural classes is presented in Table 8.9.

8.7 Soil Stoniness

The presence of coarse material (stoniness) in the soil profile has been inventoried separately from soil textures. The presence of coarse material is subdivided into six types:

Gravely	(G)
Very gravely	(VG)
Stony	(S)
Bouldery	(B)
Stony/bouldery	(SB)
Bouldery/stony	(BS)



Coarse textures	- Sand	Fine textures	- Sandy clay
	- Loamy coarse sand		- Silty clay
	- Fine sand		- Peaty clay
	- Loamy fine sand
	- Loamy sand

Medium textures	- Fine sandy loam
	- Sandy loam
	- Loam
	- Sandy clay loam
	- Silt loam
	- Clay loam
	- Silty clay loam
	- Silt

Textural classes by major division

TABLE 8.9
Percentage occurrence of textural classes

Texture class	Texture symbol	Percentage¹
Sandy	S	0.2
Loamy coarse sand	LCS	0.5
Fine sand	FS	0.2
Loamy fine sand	LFS	0.8
Loamy sand	LS	3.8

Coarse textures		5.5

Fine sandy loam	FSL	1.3
Sandy loam	SL	3.1
Loam	L	6.4
Sandy clay loam	SCL	15.4
Silt loam	SL	0.6
Clay loam	CL	28.5
Silty clay loam	SICL	0.4
Silt	SI	< 0.1

Medium textures		55.8
Sandy clay	SC	10.2
Silty clay	SIC	0.3
Peaty clay	PC	< 0.1
Clay	C	28.2

Fine textures		38.7

¹ Total extent where texture is applicable is 55,714,270 ha (96.9% of total area).

8.8 Soil Phases

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. The 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). Some of these phases have been defined in the FAO-Unesco Legend, while others have been introduced in the Exploratory Soil Map. The soil phases are briefly outlined below.

Rocky phase (R)¹

The rocky phase marks areas where the presence of rock outcrops makes the use of mechanized agricultural equipment impracticable.

Bouldery phase (S)¹

The bouldery phase marks areas where the presence of boulders in the surface layers makes the use of mechanized agricultural equipment impracticable.

Boulder-mantle phase (BM)¹

The boulder-mantle phase marks areas where an excessive amount of boulders at the surface makes any arable use impossible and renders extensive range very difficult.

Stony phase(S)¹

The stony phase as for the bouldery phase, but with stones.

Stone-mantle phase (SM)¹

The stone-mantle phase as for the boulder-mantle phase, but with stones.

Gravely phase (G)

The gravely phase marks areas, where the presence of gravel in the surface layers makes the use of mechanized agricultural equipement impractable.

Gravel-mantle phase (GM)¹

The gravel-mantle phase as for the boulder-mantle phase, but with gravel.

Lithic and Paralithic phases (P) and (PP)¹

The lithic phase reflects continuous coherent and hard rock occurs within SO cm of the surface (P). The paralithic reflects occurrence of soft rock (PP).

Petrocalcic phase (K) and (KK)

The petrocalcic phase marks areas in which the upperpart of a petrocalcic horizon occurs within 100 cm of the surface. In the Exploratory Soil Map a differentiation has been made between occurrence of petrocalcic horizons within 100 cm (K), and within SO cm from the surface (KK).

Pisocalcic phase (C) and (CC)¹

The pisocalcic phase marks soils having a layer, at least 25 cm thick and starting within 100 cm of the surface, with 40 % or more, by volume, of loose fragments of secondary calcium carbonate, usually concretions. The symbols C and CC are applied as described for the petrocalcic phase.

Petroferric phase (M)

The petroferric phase marks soils in which the upper part of the petroferric horizon occurs within 100 cm of the surface. In areas where the petro ferric horizon usually starts within SO cm of the surface are termed ironstone soils (see soil units).

Pisoferric phase (N)¹

The pisoferric phase marks soils having a layer consisting of 40 % or more by volume of oxidic concretions or of hardened plinthite, or ironstone with a thickness of at least 25 cm, the upper part of which occurs within 100 cm of the surface.

Saline phase (A)

The saline phase marks soils that show electric conductivity values of the saturation extract higher than 4 mmhos/cm at 25 °C in some horizons within 100 cm of the surface.

Sodic phase (O)

The sodic phase marks soils having more than 6 % exchangeble sodium in some horizons within 100 cm of the surface.

Saline-sodic phase (AO)

The saline-sodic phase marks soils with a combination of the saline and sodic phases as defined above.

Fragipan phase (F)

The fragipan phase marks soils which have a loamy subsurface horizon which has a high bulk density relative to the horizons above it. The fragipan is occurring within 100 cm of the surface.

Percentage occurrence of soil phases are presented in Table 8.10.

¹ Defined in the Exploratory Soil Map of Kenya.

8.9 Soil Mapping Unit Composition

At the exploratory level a soil mapping unit only rarely comprises a single soil; usually it consist of one main soil with minor associates. When the various soils of a soil mapping unit occur in a recognizable geographical pattern in defined proportions, they constitute a soil association; if such a pattern is absent, they form a soil complex.

The productivity potential of different soil units within a soil mapping unit consequently may vary widely. The suitability of soil association (soil complex) for specific use cannot be assessed without taking account of each individual soil unit within the association.

TABLE 8.10
Extents (%) of soil phases in Kenya

Soil phase	Symbol	Percentage¹
Rocky	R	6.7
Bouldery	B	1.6
Boulder-mantle	BM	1.1
Stony	S	4.6
Stone-mantle	SM	5.1
Gravely	G	0.2
Gravel-mantle	GM	0.9
Lithic	P	3.7
Paralithic	PP	0.7
Petrocalcic	K	0.2
Petrocalcic	KK	3.1
Pisocalcic	C	0
Pisocalcic	CC	0.4
Petroferric	M	1.2
Saline	N	0.1
Sodic	A	12.8
Saline-sodic	0	9.7
Fragipan	AO	10.6
	F	0.1
One soil phase		62.8
Rocky and bouldery	R and B	3.7
Rocky and stony	R and S	2.9
Bouldery and stony	B and s	3.6
Boulder-mantle and saline-sodic	BM and AO	1.1
Stony and rocky	S and R	0.3
Stony and bouldery	S and B	<0.1
Stony and petrocalcic	S and K	<0.1
Stony and saline-sodic	S and AO	1.0
Stone-mantle and sodic	Sm and O	1.0
Stone-mantle and saline-sodic	P and R	0.1
Lithic and rocky	P and B	1.8
Lithic and bouldery	P and BM	1.1
Lithic and boulder-mantle	P and S	1.3
Lithic and stony	P and O	2.6
Lithic and sodic	P and AO	<0.1
Lithic and saline-sodic	PP and R	0.1
Paralithic and rocky	PP and S	<0.1
Paralithic and stony	K and S	0.7
Petrocalcic and stony	K and AO	1.6
Petrocalcic and saline-sodic	KK and A	<0.1
Petrocalcic and saline	KK and O	0.1
Petrocalcic and sodic	N and R	0.1
Pisoferric and rocky	N and M	0.1
Pisoferric and pisoferric	A and F	0.2
Pisoferric and fragipan	O and F	0.1
Sodic and fragipan		0.1
Two soil phases		23.7
Rocky and bouldery and saline-sodic	R and B and AO	1.1
Rocky and lithic and stony	R and P and S	0.1
Bouldery and stony and saline	B and S and A	0.2
Bouldery and stony and saline-sodic	BM and S and AO	0.8
Lithic and rocky and bouldery	P and R and B	0.2
Lithic and rocky and stony	P and R and S	2.8
Lithic and bouldery and saline	P and B and S	0.2
Lithic and boulder-mantle and saline-sodic	P and B and A	1.0
Lithic and stony and rocky	P and BM and AO	1.1
Lithic and stony and saline	P and S and R	0.8
Lithic and stony and saline-sodic	P and S and A	0.6
Lithic and stone-mantle and saline-sodic	P and S and AO	0.1
Lithic and gravel-mantle and saline	P and SM and AO	3.2
	P and GM and S	1.3
Three soil phases		13.5

¹ Total extent where soil phases occur is 28 096 674 ha (48.9% of total area).

The legend of the Exploratory Soil Map does not provide explicit information in a quantified manner on the composition of the soil mapping units. This however has been provided to FAO by Stiboka in Wageningen in close cooperation with Kenya Soil Survey in Nairobi and the International Soil Reference and Information Centre in Wageningen (van der Pouw 1983).

The complete mapping unit composition table is presented in Appendix 8.3. This table consist of the percentage allocation of soil units by slope class, soil texture and soil phases for each soil mapping unit, and of information on landform and geology/parent material.

The composition of an example soil association (Ps3) is illustrated and explained in Figure 8.4.

FIGURE 8.4
Example of soil mapping unit composition of soil mapping unit Ps3