Chapter 6
Agro-edaphic suitability classification (continued)

Wve

Histic H horizon

Hollic A horizon

Unbric A horizon

Ochric A horizon

Argillic B horizon

Hatric B horizon

Cubic B horizon

Spodic B horizon

Oxic B horizon

Calcic horizon

Gypsic horizon

Sulferic horizon

Albic E horizon (at.)

Calcareous material

CaCO3 > 5% (topsoil)

CEC high/very high

CEC low (< 24 meq)

CEC very low (<1.5 meq)

Base sat. > 50%. (pH> 5.5)

Base sat. Ml, (P/K > 5.5)

Cracking clay

Depth > 25 cm

Depth 25–50 cm

Drainage very poor/poor

Drainage imp./moderate

Drainage excessive

Plinthite

Salinity high

Salinity moderate

Alkalinity high

Alkalinity tolerate

Indurated subsoil

Cemented hardpan

Iron concretions

Texture coarse

Texture heavy

Abrubt textural change

Tonguing

Reddish colour

Yellowish colour

Nat. fertility high

Nat. fertility moderate

Nat. fertility low

Wve

TABLE 6.9
Soil unit ratings for high, intermediate and low levels of inputs

CROP

LEVEL OF INPUTS

SOIL UNITS

Aic

Aif

Aio

Ath

Bnc

Btc

Bte

Fnh

Fnr

Barley

High

Int.

Lot

Oat

High

Int.

Low

Cowpea

High

hi.

lot

Green gran

High

Int.

Lot

Pigeonpes

High

Int.

low

CROP

LEVEL OF INPUT

SOIL UNITS

Hnl

Hol

Hrl

Hth

Htl

Hvl

Lic

Barley

High

Int.

Lot

Oat

High

Int.

Lot

Cowpea

High

Int.

Lot

Green gran

High

Int.

Lot

Pigeonpea

High

Int.

Lot

CROP

LEVEL OF INPUTS

SOIL UNITS

Lif

Lio

Lnc

Lnf

Hvo

Hth

Hve

Hvn

Rtc

Barley

High

Int.

Lot

Oat

High

Int.

Lot

Cowpea

High

Int.

Low

green gran

High

Int.

Low

Pigeonpea

High

Int.

Low

CROP

LEVEL OF INPUTS

SOIL UNITS

Slo

Wve

Barley

High

Int.

Lot

Oat

High

Int.

Lot

Cowpea

High

int.

Lot

Green gran

High

Int.

Lot

Pigeonpea

High

Int.

Lot

TABLE 6.10
Stoniness ratings for high. Intermediate and low levels of inputs

CROP	LEVEL OF INPUTS	STONINESS TYPE
CROP	LEVEL OF INPUTS	GRAVELLY	VERY GRAVELLY	STONY	BOULDERY	STONY/ BOULDERY	BOULDERY/STONY
Barley	High	S3	N	N	N	N	N
	Int.	S2	S4	S3	S3	S4	S4
	Low	S2	S4	S3	S3	S4	S4
Out	High	S3	N	N	N	N	N
	Int.	S2	S4	S3	S3	S4	S4
	Low	S2	S4	S3	S3	S4	S4
Cowpea	High	S3	N	N	N	N	N
	Int.	S2	S4	S3	S3	S4	S4
	Low	S2	S4	S3	S3	S4	S4
Greengram	High	S3	N	N	N	N	N
	Int.	S2	S4	S3	S3	S4	S4
	Low	S2	S4	S3	S3	S4	S4
Pigeon pea	High	S3	N	N	N	N	N
	Int.	S2	S4	S3	S3	S4	S4
	Low	S2	S4	S3	S3	S4	S4

6.3.5 Slope limitations and soil erosion

Limitations imposed by slope are taken into account in three steps. Step one defines the slopes which are permissible for cultivation, and these are defined for the five crops as slopes less than 30 percent for the three input situations with soil conservation measures. Permissible slopes are less than 16 percent under high inputs without soil conservation measures.

Step two involves the computation of potential topsoil loss which is estimated, by input level, through a modified Universal Soil Loss Equation (Wischmeier and Smith 1978; Mitchell 1986).

Step three relates the estimated topsoil losses to yield losses through a set of equations given in Table 6.12, taking into account soil susceptibility, and level of inputs. The reduced impact at the intermediate and high levels of inputs is due to the compensating effect of fertilizer application at their normal rate of use.

Steps two and three are described in detail in Technical Annex 1.2 (Soil Erosion and Productivity). Table 6.13 shows the soil units ranked in three classes according to their susceptibility to yield loss in relation to loss of topsoil, on the basis of organic matter content, soil depth and on the presence of other unfavourable subsoil conditions.

In estimating yield losses from equations in Table 6.12 regeneration capacity of topsoil is taken into account in calculating net loss of topsoil. Regeneration capacities of topsoil by thermal and moisture regimes are given in Table 6.14.

TABLE 6.11
Soil phase ratings for high, intermediate and low levels of inputs

CROP

LEVEL OF INPUTS

SOIL PHASES

Barley

High

Int.

Lou

Oat

High

Int.

Low

Cowpea

High

int.

Lou

Green gram

High

Int.

Lou

Pigeonpea

High

Int.

Lou

CROP

LEVEL OF INPUTS

SOIL PHASES

Barley

High

Int.

Low

Oat

High

Int.

Low

Cowpea

High

Int.

Low

Green gram

High

Int.

Low

Pigeonpea

High

Int.

Low

TABLE 6.12 Relationships between topsoil loss and yield loss

Soil susceptibility ranking	Levels of inputs	Equation
Least susceptible	Low	Y = 1.0 X
	Intermediate	Y = 0.6 X
	High	Y = 0.2 X
Intermediate susceptibility	Low	Y = 2.0 X
	Intermediate	Y = 1.2 X
	High	Y = 0.4 X
Most susceptible
	Low	Y = 7.0 X
	Intermediate	Y = 5.0 X
	High	Y = 3.0 X

Y = productivity loss in percent;

X = soil loss in cm

TABLE 6.13
Ranking of soils (Kenya Son Survey) according to their susceptibility to productivity loss per unit of topsoil loss

Most susceptible	Intermediate susceptible	Least susceptible
Acrisols, except Humic Acrisols	Arenosols	Chernozems
Ferralic Cambisols	Cambisols, except Ferralic Cambisols	Fluvisols
Ferralsols, except humic Ferralsols	Gleysols	Histosols
Ironstone soils	Greyzems	Humic Andosols
Lithosols	Humic Acrisols	Mollic Andosols
Planosols	Humic Ferralsols	Vertisols
Rendzinas	Kastanozems
Solonchaks	Luvisols
Solonetz	Nitisols
	Phaeozems
	Regosols
	Vitric Andosols
	Xerosols
	Yermosols

TABLE 6.14
Regeneration capacity of topsoil (mm/year)

LGP					Thermal Zone
(days)	T1	T2	T3	T4	T5	T6	T7	T8	T9
< 76	0.6	0.6	0.6	0.6	0.6	0.26	0.26	0.26	0.26
76–179	1.0	1.0	1.0	0.6	0.6	0.26	0.26	0.26	0.26
180–269	1.6	1.6	1.6	0.76	0.76	0.6	0.6	0.6	0.6
270	2.0	2.0	2.0	1.0	1.0	0.6	0.6	0.6	0.6

Derived from Hammer (1981)

Chapter 6 Agro-edaphic suitability classification (continued)

Chapter 6
Agro-edaphic suitability classification (continued)