soils of the proposed wamumu extension ...2.2. geology and general landscape features the area, like...

REPUBLIC OF KENYA

MINISTRY OF AGRICULTURE-NATIONAL AGRICULTURAL LABORATORIES

KENYA SOIL SURVEY

SOILS OF THE PROPOSED WAMUMUEXTENSION

MWEA IRRIGATION SETTLEMENT SCHEME

by

F. N. Mnchena and G. Ngari

DETAILED SOIL SURVEY REPORT No. D2, 1975

REPUBLIC OF KENYA

MINISTRY OF AGRICULTURE - NATIONAL AGRICULTURAL LABORATORIES

KENYA SOIL SURVEY

SOILS OF THE PROPOSED WAMUMU EXTENSION

MWEA IRRIGATION SETTLEMENT SCHEME

b y

F.N. Muchena and G. Ngari

DETAILED SOIL SURVEY REPORT NO. D29 1975

, os

TABLE OF CONTENTS

SUMMARY

Page

1. INTRODUCTION 1

2. THE ENVIRONMENT 1

2.1. Location and communication 1

2.2 Geology and general landscape features 2

2.3. Climate and Vegetation 2

3 . SURVEY METHODS A

4 . THE SOILS . 4

4.1. Previous work 4

4.2 Systematics and nomenclature 7

4.3. General properties of the soils ..„. 7

4.4. Description of soil mapping units 9

4.5. Land suitability for basin irrigation of rice 16

LITERATURE CITED

APPENDICES

1. Detailed descriptions of representative profiles and analytical data.

2. Detailed soil map of the proposed Wamumu extension (scale 1:10,000)

3. Location map of profile pits and augerings Wamumu extension (scale

1:10,000).

II

LIST OF TABLES AND FIGURES

Page

Table 1 Average monthly and annual rainfall (in mm) at Mwea

Tebere and Mwea Experiinental Farm (E.A.M.D., 1974).,... 3

Table 2 Qualitative appraisal of rice growth in pots (during

6 weeks) on four bulk samples collected from the survey

area . 9

Table 3 Qualitative land suitability classification for basin

irrigation of rice 13

Figure 1 Location of the previous soil investigations in relation

to the present survey 6b

I l l

SUMMARY

This report describes the soils of the proposed Wamumu extension of

the Mwea Irrigation Settlement Scheme. The surveyed area comprises 305

hectares, of which 11 soil units have been identified (see appendix 2).

Chapter 4 gives an account of the prevailing soil conditions and also

revievs the previous soil investigations that had been carried•out

within the Mwea Irrigation Scheme.

In chapter 4.5 a qualitative land suitability classification for

basin irrigation of rice is given. Of the total, acreage covered by the

survey, 42.1 ha were found to be highly suitable, 63.5 ha highly suitable

to moderately suitable, 38.8 ha moderately suitable, 17.9 ha marginally

suitable, and 142.7 ha unsuitable for basin irrigation of rice

(see also appendix 2).

The surveyed area borders both the Wamumu and Karaba area where

recently rice crop failures occured largely due to an accute P-deficiency.

Exploratory pot tests on 4 soils from the survey area gave in 3 of the

4 soils responses to P-addition. Vegetative growth of rice in pots,

after having received N and P, appeared to be satisfactory.

- 1 -

1. INTRODUCTION

The present detailed soil survey was carried out at the request of the

National Irrigation Board in Nairobi, in view of plans to include the

area in the existing Mwea rice irrigation scheue. Topographical conditions

appeared suitable for such an inclusion. However, because of unexpected

crop failures in other recent extensions of the scheme (notably the

Karaba area) the fear was expressed that soil conditions might induce

similar crop failures in the present area. Therefore a detailed soil

survey was deemed necessary. The field work was carried out between December

1974 and January, 1975.

Though the area covered by the survey is 8ma11 (305 ha) the report is

relatively elaborate. It forms a number in the recently established

series "Detailed Surveys" of the Kenya Soil Survey and because of this

a standardised lay-out of the map and the report was called for. Also it

was deemed useful to include a substantial review of all earlier soil studies

in the Mwea area. Most of these early data are not easily accessible,

but they are nevertheless important for the assessment of the suitability

of the present survey area in its proper perspective.

The authors wish to acknowledge the co-operation of the staff of

the National Irrigation Board and especially that of Mr. Singh, the

topographic surveyor.

2. THE ENVIRONMENT

2.1. Location and Communication

The surveyed area, which coveres 305 hectares, is the proposed

Wamumu extension of the Mwea Irrigation Settlement. The area is

bounded to the North by the Thifea section (Blocks HA and H2), to the

Southeast by the Wamumu section (Blocks Wl, W4 and W5) and to the

West/Southwest the area touches the Mwea Irrigation Scheme boundary

(see Appendix 2 ). Elevation of the area lies between 1128m (3700ft)

and 1143m (3750ft) above sea level.

The area is well served by two main roads;one a tarmac road

from Nairobi via Sagana to Embu and the other (partly tarmac and partly

murram) via Makutano and Sagana Power Station, through Mwea and

continuing northwards to Embu. Within the Project area there are

numerous tracks which are motorable, particularly during the dry

season.

- 2 -

2.2. Geology and general landscape features

The area, like most of the Mwea volcanic plain, is underlain by

the Thiba olivine basalts ( a fairly uniform series of usually non-

porphyritic, fine-grained grey basalts erupted from Mt. Kenya), which

are Pleistocene in age (Fairburn, 1966).

Over much of the area, the basalts rest directly on the Basement

System rocks. D'Costa and Makin (1967) and Leyder and D'Costa (1969) have

been of the opinion that the basalt is now covered (unconformably)

by fine, montmorillonitic material of alluvial origin, which has

accumulated to a varying depth on the basaltic floor forming heavy

clay soils,, under conditions of poor drainage. They assert further

that the clay is not in any part developed from the underlying basalt.

However, this may be querried. From profile observations in the

upper and lower parts of the Mwea plains it has been noted that the

depth of the Vertisols('Black cotton soils') is more or less consistently

the same (1.00 - 1.50m deep) while nowhere any sign of sedimentary

stratification was apparent (cf. Belliss 1958, original survey).

Moreover, one can observe very gradual changes from red soils

(Nitosols) to black soils, with the same underlying basaltic rock,

when moving from undulating parts i.e. edges towards the flat plain

parts ("Catena"). Therefore, it may be inferred that the soils have

been derived from the underlying basalts.

Most of the survey area may be viewed as'part of a volcanic plain

which extends all over the Mwea area. The central part of the area;

which extends in a Northwest - South west direction, is flat to very

gently undulating with slopes in general less than 2%. "-ordering this

area (in ail directions) are minor scarps which have slopes ranging

from 2 to 5%.

2.3. Climate and Vegetation

Mwea is situated almost exactly at the point of transition between

the well watered, "high potential" upland zone and the surrounding

belt of drier marginal lands, where some shortage of (rain) water is

experienced almost evey year.

Mwea's altitude of about 1160 m (3800ft) is associated with a

milder climate than is usual in tropical equatorial zones. The annual

average maximum temperature is 28 C with peaks in July, August,

January and February, and with an average annual minimum temperature

of 15 C. The average relative humidity varies from 80 per cent at

9.00 a.m. to 50 per cent at 3.00 p.m., and the annual evaporation2

averages 7mm per day. The mean radiation varies from 650 gm. cal/cm2

per day in January and February to 400 gm.- cal/cm per day in July

/3

- 3 -

and August. The average monthly windspeeds fluctuate from i.l to 4.8

in.p.h. (Chambers and îîoris, 1973). The average annual rainfall at

Mwea Tebere is 1006 mm (over 16 years) and at Mwea Experimental farm

it is 890 mm (over 12 years).

Table 1 shows the average annual rainfall distribution at Mwea

Tebere (over 16 years) and at Mwea Experimental Farm (over 12 years).

According to Chambers and Moris (1973) the rainfall supplies much

of the water required for plant growth

Table 1

Month

Average monthly and annual rainfall (in mm) at Mwea

Tebere and Mwea Experimental Farm (E.A.M.D., 1974).

January

February

March

April

May

June

July

August

September

October

November

December

STATION

Mwea Tebere

(over 16 years)

27

29

84

264

167

14

16

23

16

112

199

54

live a Experimental Farm

(over 12 years)

38

33

77

224

145

20

12

11

11

82

195

52

1006 890

on the existing irrigation schemes, leaving the irrigation system to

supply the balance, for initial flooding of fields and for the constant

renewing of water in the earlier stages of plant growth.

The combination of the above climatic variables (rainfall,

evaporation, altitude etc) provide a suitable environment for paddy

production, especially if cropping is scheduled for the period

August to December. The dry period following in January/February makes

harvesting conditions ideal.

The natural vegetation could be described generally as an open

grassland with scattered stunted specimens of Acacia drepanolobium.

The identified grass species (in the field) are Chloris sp.,

Pennisetum mezianum, Euelia diyinorum.keniensis and Typha domingensis.

- 4 - •

3 . SURVEY METHODS

Routine augerings were made following a rigid grid system which was

based on the existing 1000 ft grid system on the topographical map of the

area. Making use of the existing concrete bench marks as a reference,

auger holes to a depth of 120 cm were made at distances of approximately

100m(333 ft) apart (see observation map, appendix 3 ) . For easy

identification of the observation sites in the field by the soil surveyor»

a topographic survey team, made available by the National Irrigation Board,

had previously put wooden stakes at the above distances.

The soils were examined for depth, colour, texture, mottling, stoniness,

alkalinity, salinity etc. All the relevant surface and soil information of

each augering site was recorded on standard Kenya Soil Survey auger hole-

description sheets. For all the augerings on the plain lands the salinity

and alkalinity status was assessed in the field by determining, with portable

electrical equipment, the pE 1:5 and EC 1:5 of the topsoil and the subsurface

layers. Only the pH 1:5 was determined for the augerings on the scarps.

At benchmark sites, samples were taken, at various depths, for confirmative

analysis on salinity and alkalinity in the MAL Laboratory.

From representative soils, sites were selected for profile pits to be

dug. The pits were described in detail following Kenya Soil Survey

standards which are based on the Soil Survey Manual (U.S.D.A., 1951) and thei

F.A.O. Guidelines for s o i l description. Each horizon was sampled for

chemical and mechanical analysis in the laboratory. Additional mixed bulk

samples (30 kg) were taken from four s i t e s on the plain lands, at a depth

of 0-30 cm, for pot-testing by the KSS s o i l chemists on the so-called "Karaba

crop-failure effect»(see 4 .3 ) .

All the observation s i t e s were located on a base map at scale 1:5,000

surveyed in 1962. A total of 8 profi le p i t s were fu l ly described and

sampled. An additional 300 augerings were made/described (see appendix 3 ) .

After completing the "final f i e ld map" boundaries and symbols were transferred

to a translucent 1:5,000 topographical map, which was handed over to the

draughtsman for reduction to 1:10,000.

4. THE SOILS

A.I. Previous work

Prior to the present survey other investigations on soils and

hydrological conditions of the Mwea Irrigation Scheme had been carried

out. Since the results are available only as internal maps and/or

reports, they will be reviewed relatively amplyé One of the earliest

was a soil survey by the Ministry of Agriculture (Bellis, 1958),

draughted by the Water Development Department (W.D.D.). The area

surveyed covers approximately 24,300 hectares and lies between longitudes

37° 15' and 37° 28' East and latitudes 0° 37' and 0° 451 South.

- 5 -

On the Eastern side the area i s bound by river Nyamindi and on the

VJestem éide i t touches the Thika-Sagana road, (see figure 1). The

entire survey area i s covered by s ix s o i l maps, each at scale 1:10,000.

In these s o i l maps a l l the observation s i t e s are indicated. A total

of 242 profile p i ts and 1150 augerholes were made in the entire area

of the survey coverage. The s o i l s of the area were grouped into five

mapping units as follows:

(1) Black so i l over 3 f t . deep

(2) Black so i l 2 - 3 f t . deep

(3) Red s o i l over 3 f t . deep

(4) Red s o i l 2 - 3 f t . deep

(5) Shallow soils less than 2 ft. deep.

The soil maps, without any report, are available at the Kenya Soil

Survey Library.

D'Costa and Makin (1967)* carried out an investigation of the

soils of the Mwea (Karaba) extension. The purpose of the survey

was (a) to assess the soil status on the then existing irrigation

schemes with a view of determining any deterioration and (b) to

describe and estimate the acreage of the several soil types on the then

proposed Karaba extension area (Blocks 5 and 6) in the light of

development to rice paddy. During this survey, of about 3,860 acres,

38 soil inspection pits were dug. They were sited on basis of apparent

variations in natural phenomena (e.g. topography) likely to make

a significant impression upon the direction of soil development.

These soil investigations were supplemented by ground inspection on

foot and by extensive augering. 21 soil pits were sampled for

laboratory analysis. Additionally, some topsoil and auger samples were

analysed. The observation sites were located on a National Irrigation

Board map on a scale of 1:25,000.

The following'soil units were identified:

(a) shallow clays with basaltic boulders

(b) sloping clays above the Thiba river

(c) Thiba levee soils

These reports are available at the Kenya Soil Survey Library.

...../6

- 6 -

(d) Alkaline clays close to thé North western Escarpement

(è) Alkaline clays of the Karaba Trough

(f) Mixed colluvial clays close to the Karaba Pidge

(g) Soils of thé clay basin

The soils of units (a), (b), (c), (d) and (e), comprising about 970

acres were found to be unsuitable for rice paddy cultivation either due

to shallowness to bedrock or alkalinity. The soils of -units (f) and

(g), comprising about 2,890 acres were found suitable for rice paddy

cultivation.

Nyandat (1968) carried out investigations of Wamumu cotton scheme,

Kirinyaga District. The survey covered 416 hectares (1000 acres). A

map (scale 1:6250 approximately) plus a report was produced. Six

soil types were identified. The dominant soils are dark reddish

brown clays over dark red friable clays occurring on ridges and upper

slopes. Generally the soils vary from reddish brown clays on ridges

and upper slopes through very dark greyish brown clays on lower slopes

to dark brown to very dark greyish brown compact clays in depressions.

Also found are stony and gravelly soils. The former soils were found

to be suitable for cotton but the latter were unsuitable. All the soils

were found to be deficient in calcium and phosphate. In the report

a comprehensive account of the major factors governing cotton yields

is given.

Leyder and D'Costa (1969)* carried out soil investigations in

the then existing Mwea rice scheme. In their report (without any soil

map) they have attempted to establish interrelationships between soil,

leaf and yield data. A number of significant correlations between

these data were established and on the basis of these results a list

of recommendations was given for a follow up/monitoring programme.

Michieka and Oswaggo (1970) carried out soil investigations of

the Mwea TebereCotton Research Station. A map (scale 1:2,500) plus

a report was produced. The survey covered 79 hectares and 9 soil units

were identified.

A map (scale 1:25,000 approximately) of soils and crop failure

in the Karaba extension area has been compiled by Sotnbroek (KSS, 1974)

making use of the crop failure data over 1972 (per plot) provided by

the Mwea Irrigation Scheme management. This map also collates the

soil information as given in the Surveys of WDD (Bellis »958), D'Costa

and Makin (1967) and Michieka (1971). The map also attempts to

correlate crop failure occurrT're with a certain "blotchy" appearance

of the ground on old aerial photographs, as a prelude to assess crop

failure hazard in the Mutithi area./7

Fig.1 LOCATION OF PREVIOUS SOIL INVESTIGATIONS IN RELATION TO THE PRESENT SURVEY

0 40 ' S -

- 0 45 S

LEGENDTl WDD survey (1958).map sheets 1,JJ 2 , 3, 4.5 and 6 (scale 1:10,000)

KEY

f-:.:.:.:.:J detailed survey (D'Costa andf —'••'••'• •'•* Makin.1967)

|r~ 1 detailed survey (Nyandat, 1968)

KVVyödl detailed survey (MichiekaandrVfWOol Oswaggo, 1970)

detailed survey (Michieka,1971)

site evaluation Muthithi extensionarea (Sombroek,1974);maponly(scale 1:25,000)

site evaluation Karaba extension area(Sombroek, 1974);map only (scale 1:25,000)

present detailed survey, (Muchena andNgari.1975)

main roadirrigation section boundaryregional boundary

village

Prepared and drawn by Kenya Soil Survey in October,1975Drawing No. 75041

- 7 -

On this projected Mutithi extension area a provisional soil map

was thereupon compiled on basis of aerial photo interpretation in

conduction with augerhole observations of the W.D.D. survey (1958).

Leyder and Hinga (1974) conducted pot tests to investigate the

soil problem in the Karaba extension area. The results of these

limited pot tests suggested:

(1) a strong phosphorus deficiency which was not anticipated

in view of the generally adequate phosphorus status of the

soils of the previously existing irrigation schemes.

(2) the presence of a possible soil factor which does not manifest

itself after re-planting of the pots with a crop of rice.

Further investigations both in the field and in the laboratory

were recommended.

4.2. Systematics and nomenclature

At the highest level on the soil map (appendix 2) the soil mapping

units are subdivided according to physiographic land types, based on

geomorphology: minor scarps and plains. At the lowest level the soil

mapping units are subdivided on important profile characteristics

such as drainage, depth, colour, pH, presence of calcium carbonate

concretions etc.

Each mapping unit is identified on the map by a mapping symbol;

of which S stands for minor scarps and P for plains. On the right hand

side of the soil map legend are presented the land suitability classes

for basin irrigation of rice, for each soil mapping unit. Next to the

suitability classes, the major limitations are presented (For the

details of the suitability classification see paragraph 4.5)

4.3. General properties of the soils

The soils of the proposed project area may broadly be subdivided

into two groups, (1) black, cracking clay soils ("Blad: cotton soils"

- pellic Vertisols) and (2) dark brown, friable, clay soils (Nitosols ?).

The black clay soils, which are dominant in the survey area, are

characterized by deep cracks and a mulchy granular structure at the

surface when dry. The solum is in general very hard when dry, very

firm when moist, sticky and very plastic when wet and with moderately

to strongly developed blocky structures with subsoil slickensides.

The texture is dominantly clay throughout the profile (clay %;60 - 80,

silt % s4 - 22, sand % : 10 - 26). The clay fraction is composed of

montmorillonite which causes the soil to be impermeable and have poor

internal drainage. External drainage is usually also poor because of

the flatness of the terrain. Variable amounts of free lime are

•..../o

- 8 -

precipitated in the profile, mostly in the form of calcium carbonate

concretions, which become more numerous with depth. Part of the soils

seem even very poorly drained, probably due to incoming seepage from

scarps and sloping upland through the subsoil and/or substratum. As

inferred from pH values* 7.5 - 8.5, some of the subsoils (greater than

20 cm depth onwards), show mild to moderate alkalinity. Alkaliness

proper (ESP > 15%) is however not present or rarely present (cf.

analytical data of representative profiles in appendix 1). From-the

conductivity measurements (ECe 1:1) these soils seem to be free from

salinity (ECe < 4 mmho's/cm)

The dark brown soils (cf soil mapping unit S3), which occur

mainly on the minor scarps, are deep, slightly hard when dry, friable

when moist and slightly sticky and slightly plastic when wet. The

texture is clay and the clay content increases with depth. These soils

do not suffer from any seepage but they seem to have excessive

percolation.

Generallys, the cation exchange capacity (CEC at pH 8.2) of all

the soils is high (over 30 m.e.%). The organic matter content is medium

to low and decreases with depth. Calcium and magnesium contents are

high but potassium is medium to low. Table 2 shows some qualitative

appraisal of rice grox th in pots (during 6 weeks in a greenhouse at

National Agricultural Laboratories, Nairobi) on four bulk topsoil

samples collected from the survey area» and fertilized with N and P.

The results indicate that all the soils show response to P (Leyder,

personal communication). Unfortunately, these bulk samples had to be

collected before the over-all pattern of soils, especially as regards

relatively high pH values, was known. For this reason the above

results may not be very conclusive as regards the so-called ''Karaba

crop failure" problem.

unless otherwise stated the pH refers to pH-IUO 1:1 suspension.

- 9 -

Table 2 Qualitative appraisal of rice growth in pots (during

6 weeks) on four bulk samples collected from the

survey area.

Sampling site

Field ref. No.

Soil mapping unit Qualitative appraisal

Auger 32/2

Benchmark23 With N only moderate

growth. Strong P response

Pit 2 S3 With N only poor growth.

Very strong P response

but even then the growth

is not optimal

Pit 625

With N only, good growth

slight P response.

Auger 5/4

Benchmark 27Good growth

Little P response

Data on dry matter yield is not available due to damage by birds.

For the location of the sampling sites see the observations map

(appendix 3).

4.4. Description of soil mapping units;

Soils are all developed on Pleistocene olivine basalts.

4.4.1.Soils of theMinor Scarps

Magping u.nit„s_lJE>

These are mostly well drained, shallow to very shallow» black

to very dark grey, very firm, stony, cracking clay soils.

Extent 95.6 ha

Macro relief? gently sloping; slopes from 2 to 5 %.

Range of characteristics:

The colour of both the topsoil and the subsurface soil ranges

from black ( 10YR 2.5/l+) to very dark grey (10YE. 3/1). The soil

depth ranges from 10 to 50 cm. The pH of the topsoil (0 - 20 cm)

ranges from 5.3 to 6.2 and that of the subsoil (20 - 50 cm) ranges

from 6.2 to 8.0. Surface stoniness varies from stony to very

stony.

+ Unless otherwise stated, the colour given is in the'tróist state.

/10

- 10 -

Di fferght i at in g cri te ri a ;The soils of this mapping unit differ from those of mapping unit S2 bybeing better drained, shallower and non-calcareous.Land use; Mostly used for grazing. In places the soils are cultivatedunder maize. •Additional remarksThese soi ls contain basaltic boulders and/or gravel on the surface.Rocks are normally encountered at depths less than 50 cm. On the upper

parts of the minor scarps slopes a r e l e s s than 2% but on the lower steepersides, the slopes range from 3 to 5%. Surface irregularities occurdue to presence of gilgai micro-relief (usually 0-20 cm in height).

Suitability for basin irrigation of riceClass IV; Unsuitable

Mapping unit S2

These are imperfectly drained, deep, black to very dark grey, veryfirm, non stony or slightly stony, cracking clay soi ls often with highpH (> 8.2) from 25 - 50 cm depth onwards and usually calcareous from50 - 80 cm depth onwards.

Extent; 13.9 haMacro relief; gently sloping; slopes from 2 to 3%.

Range of characteristics

The colour of the topsoil (0-20 cm) varies from black (10YR 2.5/1) to

very dark grey (10YR 3/1) but that of the subsoil (20 cm depth onwards)

is dondnantly black (10YR 2.5/1). The pH of the topsoil (0 - 20 cm)

ranges from 5.5 to 7.2 whereas that of the subsoil (10 cm depth onwards)

is usually greater than 8.0.

Differentiating criteria

The soils of this mapping unit differ from these of mapping unit S3 by

their colour and by being calcareous.

Land use; Grazing. In places the soils are under maize cultivation.

Additional remarks; The high pH in the subsoil (greater than 50 cm depth

onwards) is due to presence of CaCO_ concretions. There are surface

irregularities due to gilgai micro-relief.

Suitability for basin irrigation of rice;

Class IV; Unsuitable.

Mapping unit S3

These are imperfectly drained, mostly deep, mottled, dark brown to very

dark greyish brown, firm, non stony or slightly stony, clay soils.

- Il -

Extent; 33.2 ha

Ife er o-re 1 i_e f ; gently sloping slopes from 2 to' 5%.

Range of characteristics •' \,

The colour of the topsoi l (0-20 cm) varies from dark brown (10YR 3/3)

to very dark grey (10YR 3/1) and that of the subsoil varies from dark

brown (7.5YR 4/4) to very dark greyish brown (10YR 3 /2 ) . The pH

varies from 5.2 to 6.6 in the topsoil (0-20 cm) and 5.4 to 7.0 in the

subsurface layers. The s o i l depth ranges from 50 cm to greater than

120 cm.

Differentiating criteria

The soils of this mapping unit differ from those of the other mapping

units in colour and profile development. Soils of this mapping unit

do not exhibit vertic characteristics.

Land use; Cultivated under maize and beans. In places under grass

cover and usually used for grazing.

Additional Remarks; These soils show high percolation


Class IVs Unsuitable.

4.4.2.SoiIs of the Plain lands

Mapping unit P1

These are poorly draineds deep, black3 very firm, cracking clay soils.

Extent; 5.0 ha.

Macro-relief; level; slopes less than 2%.

Note; These soils occur on the upper-level plains. They are part

of the soils of the already existing irrigation scheme (Thiba section,

block H4). Consequently these soils are not treated in detail

in this report.

Suitability for basin irrigation ojE rice;

Class I (?); Highly suitable.

Mapping unit P21P

These are poorly drained, shallow, black, very firm, cracking clay

soils which are usually calcareous from 25 cm. depth onwards.

Extent ; 17.9 ha

Macro relief; level; slopes less than 2%.

Range of characteristics;

The colour of the topsoil (0-20 cm) varies from black (10YR 2.5/1)

to very dark grey (10YR 3/1) but that of the subsoil (20 cm depth

onwards) is dominantly black (10YR 2.5/1).

/12

- 12 -

The pH ranges from 5.9 to 6.9 in the topsoil and 6.0 to 8.1 in the

subsoil. There are inclusions of some deep soils.

Differentiating criteria;

The soils of this mapping unit differ from those- of mapping unit P22P

mainly in depth and drainage.

Land use; Mostly grazing

Additional remarks ;

The surface is very irregular due to gilgai micro-relief and cowfoetoes*.

The differences in level of the surface is about 20 - 30 cms. in

height.


Class III; Marginally suitable.

Mapping unit P22P

These are very poorly drained, moderately deep, black, very firm,

cracking clay soils which are usually calcareous from 25 - 50 cm

depth onwards.

Extent? 8.8 ha

Macro-relief; nearly level to very gently sloping; slopes from 1-2?.


The colour of both the topsoil and the subsoil is dominantly black

(10YR 2.5/1). The pH varies from 5.6 to 7.1 in the topsoil (0-20 cm)

and from 6.2 to 8.0 in the subsoil (greater than 20 cm depth).

Yellowish red mottles (5YR 4/6) are found at depths greater than 20 cm..

Surface stoniness varies from non stony to slightly stony.

Diffe rent iating criteria;

The soils of this mapping unit differ from those of mapping unit P2IP

mainly in depth.

Land use; Under maize cultivation; in places under grass which is

used for grazing.

Additional remarks;

These soils occur at the foot of a minor scarp. Surface irregularity

is often observed due to gilgai micro-relief. These soils are also

succeptible to seepage from the higher-lying areas.


Class II; Moderately suitable.

* Cowfoetoes refers to surface irregularities due to cattle trampling

/13

- 13 -

Mapping unit P23

These are poorly drained, deep9 black, very firm, cracking clay soils which

are usually calcareous from 50 - 80 cm depth onwards.

Extent; 7.3 ha

Macro relief; level; slopes less than 2% '


The colour of the topsoil (0-20 cm) varies from black (10YR 2.5/1) to very

dark grèy (10YR 3/1) but that of the subsoil is dominantly black. The topsoil

pH ranges from 6.0 to 6.5 and that of the subsoil ranges from 6.5 to 8.0.

Calcium carbonate concretions occur at depths ranging from 20 to 50 cm

onwards.


The soils of this mapping unit differ from those of mapping unit P24 by

having lower pH values (pH < 8.0) and by being better drained.

Land use; Under grass cover.

Additional remarks ; These soils are susceptible to less seepage influence

than those of mapping unit P24. During the survey period it was observed

that the grass cover on these soils was drier than that on the soils.of

mapping unit P24. Surface irregularities occur due to gilgai micro-relief

(15 - 30 cm in height).

Suitability for basin irrigation of rices

Class I: Highly suitable.

Mapping uni t P24 ;

These are very poorly drained, deep, black, very firm, cracking clay soils,

often with high pH (> 8.0) from 20 - 50 cm depth onwards and usually

calcareous from 50 - 80 cm depth onwards.

Extent; 30.0 ha

Macro-relief; level; slopes less than 2%Range of characteristics;

The colour of both the topsoil and the subsoil varies from black (10YR 2.5/1)

to very dark grey (10YR 3/1). The pH of the topsoil (0 - 20 cm) ranges from

6.2 to 8.3 and that of the subsoil ranges from 7.1 to 9.0. Calcium

carbonate concretions occur at depths varying from 20 to 50 cm onwards.

Yellowish red mottles occur from 20 cm depth onwards.



having consistently higher pH values.

/14

- IA -

Land use; Mostly used for grazing

Additional remarks;

These soils are under the influence of seepage from the minor scarps and the

adjacent irrigation canals. The high pH of these soils indicate mild to

moderate alkaliness of the subsoil. There are some surface irregularities

due to gilgai micro-relief:


Class II: Moderately suitable.

Mapping unit P25

These are poorly drained, deep, black, very firm, cracking clay soils, often

with high pH (7.5 - 8.0 ) from 20 - 50 cm depth onwards and usually

calcareous from 50 - 80 cm depth onwards.

Extent: 32.5 ha

Macro-relief; level;,slopes less than 2%

twinge of characteristics:

The colour of both the topsoil and the subsoil variés from black (10YR 2.5/1)

to very dark grey (10YR 3/1). The pH of the topsoil (0 - 20 cm) ranges

from 6.3 to 7.8 and that of the subsoil ranges from 7.0 to 9.0. Calcium

carbonate concretions occur at depths ranging from 20 to 50 cm downwards.

Differentiating criteriat

The soils of this mapping unit differ from those of mapping unit P26

by having often rather high pH values and by being better drained.

Land use: Mostly used for grazing

Additional remarks;

These soils are susceptible to seepage influence from the higher-lying parts

(minor scarps). Along an old stream channel green vegetation cover (mainly

grass) was observed whereas in the adjacent areas the grass was drier.

Hence the flourishing of the vegetation cover may be attributed to higher

soil moisture due to seepage. These soils may also be mildly to moderately

alkali in the subsoil

Suitability for basin irrigation of rice:

Class I - Us Highly suitable to moderately suitable.

Mappingunit P26

These are very poorly drained, deep, black9 very firm, cracking clay soils

which are usually calcareous from 50 - 80 cm depth onwards.

/15

- 15 -

Extent; 31.0 ha

Macro-relief; leve l ; slopes l ess than 2%

Range of characteris t i c s ;

The colour of both the topsoil and the subsoil ranges from black (10YR 2.5/1)

to very dark grey (10YR 3 /1 ) . The pH of the topsoil (0 - 20 cm) ranges

from 6.0 to 7.3 and that of the subsoil (greater than 20 cm depth) ranges

from 6.2 to 7.9.

Differentiating criteria:


being calcareous and more poorly drained.

Land use ; Mostly used for grazing

Additional remarks;

In this unit there are slight surface irregularities due to gilgai micro-

relief.


Class I - Us Highly suitable to moderately suitable.

Mapping unit P27s

These are poorly drained, deep and non-calcareous, black, very firm,

cracking clay s o i l s .

Extent; 29.8 ha

Macro-relief: l eve l | slopes less than 2%

Range of characterist ics;

The colour of the topsoil (0-20cm) varies from black (10YR 2.5/1) to very

dark grey (10YR 3/1) whereas that of the subsoil (greater than 20 cm depth

onwards) i s dominantly black (10YR 2 .5 /1 ) . The topsoil pH ranges from

5.8 to 6.7. The subsoil pH ranges from 6.3 to 8.0. In general the pH

increases with depth.

Differentiatin& er i ter ia;

The s o i l s of this mapping unit differ from those of mapping unit P26 by

being non calcareous.

Land use: Mostly used for grazing

Additional remarks;

There are s l ight surface irregularit ies due to gi lgai micro-relief (usually

10 - 20 cm high).


Class I: Highly suitable. #

- 16 -

4.5. Land suitability for bas in irrigationof.rice

Land is classified to show its relative value for some purpose.

Different classifications are necessary for different purposes. During

former surveys in the area, no systematic land suitability determination

was carried out. In this report a qualitative land suitability classi-

fication for basin irrigation of rice is given with standardised

methodology (FAO, 1973, 1974). Because of the small size of the area,

only those limitations that are actually of importance in situ are

considered;, in a qualitative way.

In this land classification procedure, four classes of land

suitability have been used:

Class I; Highly suitable

Land+ having no significant limitations that would restrict its use

for basin irrigation of rice.

Class II; Moderately suitable

Land having slight to moderate limitations that restrict its use for

basin irrigation of rice.

Class III.. Marginally suitable

«and having moderate to severe limitations that restrict its use

for basin irrigation of rice.

Class IV; Unsuitable

Land having severe limitations that restrict its use for basin

irrigation of rice.

The land suitability classification data is presented in Table

3. The qualitative current suitability classification of each "mapping

unit" has been carried out separately, taking into account the following

limitations;

(a) Topography

Non irregular or slightly irregular flat land with slopes of less

than 1% is considered to present no limitation for the construction

of irrigation basins. Little or no levelling will be required. Land

with slopes over 1% will need some levelling. With increasing slope

gradients more levelling will be required, causing higher investments

and narrower irrigation basins. The considered slope classes are

1-2% » 2-3% and 3-5%, corresponding to degrees of limitation (t),

t and T respectively. Land with slopes greater than 5% is considered

to have an excessive topographic limitation, overruling any other

quality of the land, when mechanized basin irrigation of rice is

foreseen.

+ The term "land" also incorporates the soil.

- 17 -

The presence of a strong micro-relief (gilgai) degrades the degree

of limitation by one class. Gentle micro-relief differences (20 cm

or less) are not considered a limitation in view of the fact that this

would largely disappear at initial ploughing and that some initial

levelling will always be necessary for the construction of basin

edges, field canals etc (see however under e).

(b) Depth _qf soil

The depth of soil that can be effectively penetrated and

exploited by plant roots is obviously very iinportant. Soils with

an effective rooting depth of 80 cm or more are considered to have

no limitation for basin irrigation of rice. Depth of 50 - 80 cm

are considered to pose a slight limitations 25-50 cm a moderate

limitation and less than 25 cm a severe limitation: symbols

(d), d and D respectively.

(c) Stoniness/rockiness

Presence of stones and gravel in the soil and at the surface

hinders cultivation and also results in high development costs for

basin preparation. Non stony to slightly stony (< 0.01%) land is

considered to present a slightlimitation, fairly stony to stony

(0.01 - 3%) land presents a moderate limitation and very stony to

exceedingly stony (3% and more) land presents a severe limitation:

symbols (r)s r and R respectively.

(d) Drainage/flooding

In the case of basin irrigation of rice a poor internal drainage

is no limitation, but an asset (cf. table 3 and appendix 2). Uncont-

rolled flooding would be a limitation but does not apply in the survey

area. The only actual limitation as regards internal and external

drainage is detrimental water movement presented by the occurrence

of seepage into the area from the upland/scarps at both sides.

Continous subsurface inward seepage, as surmised by a homogeneously

(dark) grey subsoil as in some mapping units, rather than a reddish

mottled one should be detrimental at both the time of field

preparation and ripening of the rice crop. It is however considered

to constitute a slight limitation only; symbol (w).

of surface covered.

«..../18

- 17 -

'The presence of a strong micro-relief (gilgai) degrades the degree

of limitation by one class. Gentle micro-relief differences (20 cm

or less) are not considered a limitation in view of the fact that this

would largely disappear at initial ploughing and that some initial

levelling will always be necessary for the construction of basin

edges, field canals etc (see however under e).

Depth of soil

The depth of soil that can be effectively penetrated and

exploited by plant roots is obviously very important. Soils with

an effective rooting'depth of 80 cm or more are considered to have

nö limitation for basin irrigation of rice. Depth of 50 - 80 cm

are considered to pose a slight limitations 25-50 cm a moderate

limitation and leés than 25 cm a severe limitation: symbols

(d), d and D respectively.

(c) Stoniness/rockiness

Presence of stones and gravel in the soil and at the Surface

hinders cultivation and also results in high development costs for

basin preparation. Non stony to slightly stony (< 0.01%) land is

considered to present a slightlimitation, fairly stony to stony

(0.01 - 3%) land presents a moderate limitation and very stony to

exceedingly stony (3% and more) land presents a severe limitation:

symbols (r)s r and R respectively.

Drainage/flooding

In the case of basin irrigation of rice a poor internal drainage

is no limitation, but an asset (cf. table 3 and appendix 2). Uncont-

rolled flooding would be a limitation but does not apply in the survey

area. The only actual limitation as regards internal and external

drainage is detrimental water movement presented by the occurrence

of seepage into the area from the upland/scarps at both sides.

Continous subsurface inward seepage, as surmised by a homogeneously

(dark) grey subsoil as in some mapping units, rather than a reddish

mottled one should be detrimental at both the time of field

preparation and ripening of the rice crop. It is however considered

to constitute a slight limitation only: symbol (w).

% of surface covered.

••••«/18

Table Qualitative land suitability classification tor Dasm irïlgaciuii Ui LILET1

18

SOILMAPPINGUNIT

SI P

S2

S3

PI

P21P

P22p

P23

P24

P2T~

P26

P27

topo-graphy

3-5%

3-5%

3-5%

1-2%

IM

•

symbol

T

r

T

(t)

• •

- . •

-

- . •

-

depth ofsoil

25-50 -c.il.-7.

50-80 d»

-

-

-

-

L I M

symbol

••3)

d

(d)

stoni-ness

stony

non toslightlystony

non toslightlystony

-

-

-

-

-

I T A T '

symbol

r

(r)

(r)

---

- .

-

E O N S

drainage

-

-

it

—

very poor

-

very poor

—

very poor

'-

symbol

-

—

. (w)

-

(w)

-

(w)-

nutrientavailabi-lity

-

moderate

-

• • •

-

-

moderate

slight

-

-

symbol

-

f

-

••

-

-

f

(f)

-

-

LAND SUITABI-LITY CLASS

IV

IV

IV

I

III

II

I

II

I - II

I - II

-

ACEEAGE

in ha

95.6

13.9

33.2

5.0

17. $

8.8

7.3

30.0

32.5

31.0

29.8

Notes s

1) - means no limitations

2) * mean3 soils have excessive percolation

3) drainage limitation refers to the occurrence of seepage.

4) nutrient availability refers to "Karaba crop failure problem".

Total 305 ha

- • ?• . - •

(e) Nutrientfovaiïability

Limitations, in nutrient availability are normally related to lowamounts ànd/or low storage capacity for the main nutrients. (N,P, K) inthe soi l , as may be found in aA>ase saturation (lowpH) so i l and/or sandysoil or a s o u with a kaolinitic character. This i s hot the case inthe present area which has dominantly montmorillonitic clays ofrelatively high pH and i s rich in nutrients* Therefore no immédiateshortage of the main nutrients i s foreseen although nutrient removalunder cotttinous cropping will in the long run require applicationof fert i l izers , particularly N and P as experience at Mere a has shown;

In the present case there i s however the possibility pf nutrientunavailability of phosphorus and micro-nutrients induced by high pHat shallow depth. This relates to the so-called "Karaba crop failureeffect". I t has been demonstrated (cf. chapter 4.3) that at leastpartially this effect consists of fixation óf phosphorus in the in i t ia lstages of rice growth due to high pH (> 8.0) in the fcopsoil. In thesurvey area practically nowhere are the pH Values exceptionally highin the presentday topsoil, but some mapping unite have high pH valuesfrom some depth onwards (not due to excess Na)î Since by ploughingarid levelling (also in view of the slight Mcfo-relief) these highpH values may come near the cultivated surface, a pfesentday pH> 7.5 as from 20 cm depth onwards i s considered to constitute a slightlimitation and one of pH > 8.0 as from 20 cm depth onwards as amoderate limitation; symbols (f) and f respectively.

Rote: This evaluation may have to be revised after greenhouse pot-test experiments on the "Karaba crop failure effect" includingsamples for the mapping units concerned are satisfactorilycompleted«

Other limitations in connection with basin irrigation of ricecan be a significant percolation of irrigation water through the soil( > 5ran/day); At an excessive rate this precludes any basin irrigationand at a moderate rate this requires substantial puddling and plough-solecompaction» In tke present survey area a substantial percolation i sexpected to occur only in the reddish soi l of unit S3 which has anappreciable percentage of non-swelling clay minerals. Since fortopography reasons this unit i s already non-suitable, die limitationi s treated subordinately (* with drainage limitation).

Salinity (growth limiting amounts of free salts) and alkaliness(excess of Na on the adsorption complex) of the so i l , resulting in highdegree of dispersion and the presence of a shallow hard pan, are oftenimportant limitations but they do not apply to the present surveyarea. The same holds for shrubby and woody vegetation, that wouldhave to be cleared during land development. ,on

• « . . . / ^ w

- 20 -

Bellis, 1958s

Chambers, R.

and Moris J., 1973:

D'Costa, V. and Makin,

M.J., 1967:

E.A.M.D. 1974:

L I T E R A T U R E C I T E D

Original survey of the Mwea area by the Water

Development Department, 6 map shee t s , scale

1:10,000. Min .of Agr icul ture , Nairobi .

'*îwea. An I r r iga ted Rice Settlement in Kenya",

Weltforum Verlag, München. 539p.

"The so i l s of the Mwea extension, Karaba". Soil

Survey Unit , N.A.L., Nairobi

Fairburn, W.A., 1966:

F.A.O., 1968:

F.A.O. s ta f f , 1973:

F.A.O., 1974:

Kantor, W and

Schwertmann, Ü., 1974:

Leyder, R.A. and

DfCosta, V., 1969:

Leyder, R.A. and

Hinga G., 1974:

Michieka, D.0.9 1971:

Michieka, D.O., and

Oswaggo, 0., 1970:

"Sunmary of Rainfall in Kenya for the year 1972".

East African Meteorological Department (EAMD),

Nairobi.

"Geology of the Fort Ilall area (vri.th coloured map,

scale 1:125.000)", Report No. 73; Geological

Survey of Kenya, Nairobi.

"Guidelines for soil description", FAO, Rome.

"Soil Interpretation Handbook for Thailand" Land

classification Division, Bangkok, Thailand.

"Soil Survey in Irrigation Investigations",

Soils Bulletin, (Draft edition), FAO, Rome.

Mineralogy and Genesis of clays in Red-Black

Soil Toposequences on Basic Igneous rocks in

Kenya. J. Soil Sei . Vol 25 no. 1, P 67-78.

"Report on soil investigations in the Mwea Fi.ce

Scheme", Internal report, Ministry of Agriculture,

NAL., Nairobi.

"Programme of Investigations to solve the soil

problem in the Karaba extension of the Mwea

Irrigation Settlement Scheme". Internal report,

Kenya Soil Survey, NAL, Nairobi.

"Soils of the Wamumu extension-Mwea", Soil Survey

Unit, Ministry of Agriculture,NAL, Nairobi.

Soils of Mwea-Tebere Cotton Research Station ",

Soil Survey Unit, Ministry of Agriculture, NAL,

Nairobi

Munsell colour Co., 1959: Munsell colour charts

/21

- 21 -

Nyandat, N.N;* 1968 : ''The s o i l s of the Wainumu Cotton Scheme". Soil

Survey Unit, Ministry of Agriculture, NAL. Nairobi.

Sombroek", W.G., 1974 : - "Provisional s o i l map of the Projected Muthithi

extension area,Mwea Irrigation Scheme". Internal

document, Kenya Soi l Survey*

- "Hap of s o i l s and crop fa i lure , Karaba extension

area, Mweà irrigation Scheme". Internal document

Kenya Soil Survey, NAL, Nairobi.

U.S. Bureau of

Reclamation (USBR),

1953 :

U.S. Soil Survey Staff

1951 :

Reclamation Manual, Vol. V "Irrigated Land use",

part 2, Land classification Bureau of Reclamation

U.S. Dept. of the Interior, Denver, Colorado.

"Soil Survey Manual", Agriculture Handbook No. 18,

United States Dept. of Agriculture, Washington, DC.

,/22

- 22 -

Appendix I to "Soils of the Proposed Wamuniu Extension".

Detailed description of representative profiles and anlytical data.

,/23

5 0 i l A T 0 n Y D A T A 3 H E H -

R-iuti 10 i

O I L P R O F I L E

S;: c i u l ?.rv.ly3ü£

135/2-7 (PIT 5)

A;"'.. : WAllUMl! EXTENS

i L.'ir.crntory n o . / "

!jHcrizor»

iDepth i n cm,

MDistura ^ a i r dry so i l

)j Gravel °fr

i Toxture-Hydrcmater:

Sand % 2.0 - 0.05 mm

S i l t <& 50 - 2 u

Clay. $ 2 - 0 u

Texture class

Dispersed clay f:

F locculat ian index

S o i l - l i q u i d 1:1 susp:

pH-H?O

pK-KCl

EC i n mmho's/cm

Saturat ion extract :

pH

EC

Saturation °j?

CaC03 *

CaS0a.2H20 7i

C *

N *

C/N

CEC (NaOAc.pH 8.2) me«i

CEC (isH^OAc.pH 7.0) ••

Exch. Ca "

Mg

11 K ••

H N a . .

Sura of cations "

Basa sot. $ at pH 8.2

•I « M 11 7 Q

E.S.P. at pH 8.2

Exch. ac id i t y me%

Miscellaneous

207 208 ; ; i

A : c ; • ;

0-20 ; 20-50 !

j

14

18

68

C

6.1

4 .6

0.30

1.2!

0.11

56.4

35.6

12.0

0.40

0.35

48.35

86

<5

; 1 4

• 2 2

:64

;c

;7.1

5 .4

;o.4O

1.12

52.6

— -.28.8

12.8

1.25

Trace

42.85

81

<5!

• :

! ;

: :

; :

1 j

' ;

I

t • i

1

i

;

;

1

1

|

j

:

;

1

;

.. — ;

Df.:th i n cm.

Toxturc U.S.O.A.:

Scii-.d °/r 2.0 - 1.0 mm

1.0 - 0.5 "

0.5 - 0.25"

0.25- 0.1 "

0.1 - 0.05"

total sand "je

Si l t ?: 50 - 20 u

2 0 - 2 u

total s i l t ^

Clay i 2 - 0 u

Texture class

Bulk dunsity

Particlo density

Porosity

Muisturc i: w/u at: ;

pF 2.0

pF 2.3

pF 2.7

pF 3.7

pF 4.2

Clay mineralogy:

S i0 2 /A l 2 0 3

SiO2/R2O3

Fe203%

X-ray repor t :

Soil f e r t i l i t y aspects

Available Ca me=

" Mg "

•• K "

" Na "

" Mn "

" P ppm

Total Ca mcfli

Mg »

n K ,<

" P ppm

, - - - - •

•

ti „

:

: i

: ;

; i

': ; •

• \ - - -

; !

! ;

;

<

j

•

I ,

-,

;

___ .._ — -

- 2 4 .

Mapping unit SljP

Geological formation:

Local petrography:

Physiography:

Relief, macro:11 me so, mi er o:

Vegetation/land use:

Erosion:

Rock outcrops:

Surface stoniness:

Slope gradient:

Salinity/alkalinity:

Surface sealing:

Effective soil depth:

Internal drainage class:

Observation s i te 135/2-7 (pit 5)

Thiba basalts

01ivine basalts

Minor scarp

Gently sloping

Nil

Grassland/grazing

Nil

Nil

Stony

2%

Nil

2-4 mm, moderate

Shallow, 50 cm

Well drained

Description óf profile:

A 0 - 20 cm Black (10YR 3/1 dry, 10YR 2.5/1 moist),

clay; moderate, medium, angular blocky

structure; extremely hard when dry, •

very firm when moist, very sticky and

plastic when wet; few, fine pores; few

medium roots; pH 6.1; clear and wavy

transition to:

20 - 50 cm+ Black (10YR 2.5/1 moist) , clay; moderate,

medium to coarse prismatic structure;

extremely hard when dry, firm when moist,

very sticky and p las t i c when wet;

common, moderate to strong sl ickensides;

few, very fine to fine pores; few,.fine

roots; some calcium carbonate

concretions (2%, 2-3 mm); pH 7 .1 ,

./25

L A .5 0 a A T 0 R Y D A T A 3 H t C I - 5

301L

O I L P H G F I L [ !.

Z:: s.:i - l a-TîîyEuS

135/2 - 4 (PIT 2)

."»ro-.: WAMUMU KXJKNSION

: L J tsarn t o r y ne . / 7 5

i Horizon

Doptb in cm.

Moisture *! &ir dry soil

• Gravai $

i Textura-Hydrometer:

Sand % 2.0 - 0.05 mm

Silt % 50 - 2,u

d a / # 2 - 0 u

Texture class

Cispersed clay *j'

Flocculation index

Soil-liquid 1:1 susp:

PH-H2O

pH-KCl

EC i n mmho's/cm

Saturation extract:

• P H

EC

Saturation %

CaC03 £

CaSO^.SH 0 «i

C *

N<& .0

C/N

CEC (NaOAc.pH 8.2) DIE£

CEC (^OAc.pH 7.0) »

Exch. Ca ••

" Mg

" K "

" Na "

Sum of cations "

: Base sa t . $ at pH 8.2

II II II II 7 Q

E.S.P. at pH 8.2

Exch. acidity mö)ó

Miscellaneous

. . .

199 200 : 201 :

A

0-20

AB ; B2 :

20-50 : 50-110 :

16

14

70

C

6 . 0

4.6

0.17

1.56

0.14

59.6

. _

32.0

17.9

0.70

Trace

50.60

85

<5

- • i

; ' 2

10

: 7 8

: C

• j - -

i 6.3

; 4.6

: 0.95

0.76

65.5

32.0

17.8

0.80

0.30

50.90

78

<5

• • • !

! - •

: 1 0

:10 I•80

Ie

16.6

|0.95

fl.SO

55.0

33.0

'7.9

).70

P.30

51.90

80

<5

:.. .. . .

r- —

Di.vLri in cm.

Taxtxire U.S.O.A.:

Sand ^ 2.0 - 1.0 mm

1.0 - 0.5 "

C.5 - 0.25"

0.25- 0.1 "

0.1 - 0.05"

total sand °/c

Silt c/. 50 - 20 u

2 0 - 2 u

total silt $

Clay % 2 - 0 u

Textures class

Bulk uunsity

Particle density

Porosity

Moisture £ v;/y at: •

pF 2.0

pr 2.3

pF 2.7

pF 3.7

pF 4.2

Clay mineralogy:

SiO2/Ai2o3

S i 0 2 / R 9 0 3

Fe2O3%

X—ray r e p o r t :

Soil fer t i l i ty aspects

Available Ca me%

•• M g • '

" K ••

.. Na ..

11 Mn ••

" P ppm

T o t a l Ca ras^

" Mg "

.. K n

P ppm

j . . . . . . .

• • • • -

| -

•

.

- 26 -

Mapping unit S3 Observation site 135/2-4 (pit 2)

Geological formation: Thiba basalts

Local petrography: Olivine basalts

Physiography: Minor scarp

Relief, macro : Gently sloping

" meso,micro:: Nil

Vegetation/land use: Grassland/maize cultivation and grazing

Erosion: Nil

Rock outcrops: Nil

Surface stoniness: Nil

Slope gradient: 1%

Salinity/alkalinity: Nil

Surface sealing: 5 mm, moderate

Effective soi l depth: Deep, 110 cm.

Internal drainage class:Imperfectly drained.

Description of profile;

A 0 - 20 cm Dark brown (7.5 YR 3/2 dry and moist), clay; moderate,

fine to medium subangular blocky structure; hard when

dry, firm when moist,slightlysticky and plastic when

wet; few, fine pores; common, medium and fine roots;

pE 6.0; clear and smooth transition to:

AB 20 - 50 cm Dark reddish brown (7.5 YR 4/4 dry, 5YR 3/4 moist)

clay; few, fine, distinct, yellowish red mottles;

porous massive breaking to some we ale, fine and medium

subangular blocks; slightly hard when dry, friable when

moist, sticky and plastic when wet; common, fine pores;

few, very fine roots; some manganese concretions (5%,

2 mm); pH 6.3; diffuse and smooth transition to:

B2 5 0 - 1 1 0 cm+ Reddish brown (7.5YR 4/4 dry, 5YR 4/4 moist), clay;

common, fine and medium, prominent yellowish red

mottles; porous massive; slightly hard when dry,

friable when moist, sticky and plastic when wet; few,

fine, pores; some manganese concretions (10%, 2 mm)

pH 6.6.

,/27

L A B O R A T O R Y C » T AKeJV. SOIL yC.-jry

H E E T - S C I t . P n 0 F I L. u MO. 133/2-3 (PIT I)

Spécial s r n l y ; s ,\rj\: vfAMUML' EXTENS

. Lci'jorntory ne, '

I Horizon

Depth in en.

Moisture ^ a i r dry soi l

i Grav'el i<

i Texturs-Hydrometer:

Sand i: 2.0 - O.CS mm

S i l t % 50 - 2.u

Clay $ 2 - 0 u

Texture class

196

A

0-30

197

14

6

AC

198

C

30-80 ; 80-105;

14

6

10

Da.;-In in en." T

e U.G.n,A.:

°/: 2.0 - 1.0 mm

1.0 - O.S "

• Ü.5 - 0.25"

Û.25- 0.1 "

C l - 0.05"

total s;jnd %

80

C

80

C

10

80

C

Silt * 50 - 20 u

2 0 - 2 u

Dispersed clay i

Flucculation index


total s i l t

Clay % 2 - 0 o

Texture class

Bulk durœity6.7

5.3

0.55

7.6 8.2

pH-KCl

EC in mmho's/cm

Saturation extract:

pH

EC

6.2

0.40

6.6

0.85

Porticle density

Porosity

Moisture fs w/u at:

pF 2.0

pF 2.3

- - - - - -Saturation

i. pF 3.7

pF 4.2

Clay mineralogy:a 1.20 1.08 0.79

ni

C/N

0.10

CEC (NaOAc.pH 8.2} me$

CEC (^40Ac,pH 7.0) »

67.0 68.8 67.2

X-ray report:

Exch. Ca

Mg

" K

38.0 39.6 38.8

19.2

0.34

17.7

1.34

18.9

0.74

1.80" Na

Sum of cations

Base sat. r/i at pH 8.2

1.95

59.49

89

0.42

59.06

86

60.24

90

Soil fertility aspects

M M IT M 7 0

E.S.P. at pH 8.2

Exch. acidity me%

Miscellaneous

Available Ca me?

" Mg "

<5

Mn

P ppm

Ca tnçffaTotal

My "

K "

P ppm

- 28 -

Mapping unit P 24


Local petrography:

Physiography:

Relief, macro :

" meso,micro :


Erosion:

Rock outcrops:

Surface stoniness:

Slope gradient:

Salini ty/alkalini ty:

Surface sealing:



Description of profile

Aj 0 - 3 0 cm

Observation site 135/2-3 (pit 1)

Thiba basalts

01 ivine basalts

Plain

Level

Gilgai

Grassland/maize cultivation

Nil

Nil

Nil

1%

Nil

2-4 mm, strong.

Very deep, more than 120 cm

Very poorly drained.

AC 30 - 80 cm

80 - 105 cm+

Black (10YR 2.5/1 dry and moist), clay; moderate,

medium, angular blocky structure; extremely hard

when dry, very firm when moist, very sticky and

very plastic when wet; few, fine pores, common, very :

fine pores; few very fine roots; wide cracks; pH 6.7,

clear and wavy transition to:

Black (10YR 2.5/1 dry and moist), clay; strong

medium to coarse prismatic structure breaking to

moderate, medium, angular blocks; extremely hard


plastic when wet; abundant, moderate to strong

slickensides; few, fine pores; pH 7.6; diffuse

and wavy transition tos

Black (10YR 2.5/1 moist), clay; moderate, medium to

coarse prismatic structure breaking to some

moderate fine to medium angular blocks; extremely

hard when dry; firm when moist, sticky and plastic •

when wet; abundantj moderate to strong slickensides;

few, very fine pores; some calcium carbonate

concretions; pH 8.2.

/29

L A . i Ü R A T i l R Y D A T A 3 H £ E T - S O I L . P R O F I L E K 1 . , 3 5 / 2 - 5 ( p I T 3 )

R IL. Li 10 *m nyso^j

, Lnloratory no. ƒ.75

i Horizoc

Depth in cm.

Moisture $ sir dry soil

i Grax/el $

Textura-Hydrometer:

Sand % 2.0 - 0.05 mm

Silt $ 50 - 2.u

Clay, £ 2 - 0 u

Textura class

Dispersed clay $

Flocculatian index


pH^O

pH-KCl

EC in mmha's/cm

Saturation extract:

PH

EC

Saturation $

CaCO3 i

CBS04.2H20 *

c *

C/N

CEC (NaOAc.fjH 8.2) I»E#

CEC (^OAc.pH 7.0) "

Exch. Ca

" Mg "

Na , "

Sum of cations "

Base sat. % at pH 8.2

?» 11 « ** 7 0

E.S.P. at pH 8.2

Exch. acidity mef£

Miscellaneous

• • •

202

A l

0-25

16

16

68

C

7.2

5 . 8

0.40

1.00

0.09

51.6

32.8

7.9

1.50

Trace

45.20

88

<5

203.

AC

25-75

14

22

64

C

7.9

6.5

0.60

0.76

50.4

33.2

11.1

0.90

0.30

45.50

90

<5

2Q4

Cca

75-110

1

26 ;

14

60

C

8.3

6.8

0.75

0.79

49.2

33.2

12.0

0.25

1.55

46.82

95

<5

-

T . 1 1

• "

Special arvilysisS

Oopth in cm.

Texture U.S.D.A.:

Sand $ 2.0 - 1.0 mm

1.0 - 0.5 "

0.5 - 0.2S"

0.25- O.I "

0.1 - 0.05"

total sand °fc

Silt f 50 - 20 u

2 0 - 2 u

total s i l t $

Clay $ 2 - O u

Texture class

Bulk density.

Particle density

Porosity

Moisturo °f> w/u at! •

pF 2.0

pF 2.3

pF 2.7

pF 3.7

pF 4.2

Clay mineralogy:

•—£- —F82°3*

X-ray report:


Available Ca me t

Mg "

" K "

" Na "

" Mn "

P pom

Total Ca md%

" Mg ' •

it K "

" P ppra

• A r - - . : WAMIIMU KX:

•

1

1—

__ 1

—

Mapping unit P 25


Local petrography:

Phys i ography:

Relief, macro :11 meso,micro:


Erosion:

Rock outcrops:

Surface stoniness:

Slope gradient:

Salini ty/alkal ini ty:

Surface sealing:

Effective s o i l depth:


Description of prof i le:

A. 0 - 25 cm

•AC 25 - 75 cm

Cca 75 - 110 cm+


Thiba basalts

01ivine basalts

Plain

Level

Gilgai

Grassland/grazing

Nil

Nil

Nil

1%

Nil

2-4 mm, strong

Deep, 110 cm

Poorly drained.

Black (10YR 2.5/1 moist), clay; strong, medium, angular,

blocky structure; extremely hard when dry, very firm Ï

when moist, very sticky and plastic when wet; fex*

fine pores; common, fine roots; pH 7^2j clear and

wavy transition to:

Black (10YR 2.5/1 dry and moist), clay; moderate,

coarse prismatic structure breaking to some

moderate, medium angular blocke; extremely hard when

dry, very firm when moist, very st icky and p las t i c .

when wet; abundant, moderate to strong s l icken-

sides; few, fine pores; few, fine roots; some

calcium carbonate concretions (2%, 2 mm); pH 7.9;

diffuse and wavy transition to:

Black (10YR 2.5/1 moist), clay; few, f ine , prominent,

yellowish red mottles (5YP 4/6); moderate, medium

angular blocky structure; extremely hard when dry

firm when moist,, very sticky and plas t ic when

wet;.abundant, moderate to strong sl ickensides;

few, very fine to fine pores; some calcium.carbonate

concretions (5%, 2-3 mm) ; pH 8 .3 .

/31

L A B O R A T O R Y D A T A H E E T -KF.'JYA 301L

analyses

S O I L P R O F I L E -ia

Sp~ciil niiilysüs

135/2-8 (PIT 6)

Arr.-.:WAMUMü EXTENSI

.; Laboratory no. /75

Horizon

Depth i n cm.

Moisture £ oir dry soil

i GraCel °fc

Texture-Hydrometer:

Sand % 2.0 - 0.05 mm

Sil t % 50 - 2.u

Clax H> 2 - 0 <J

Texture class

Dispersed clay %

Flocculation index


pH~H2O

pH-KCl

EC in mmho's/cm

Saturation extract:

pH

EC

Saturation %

CaCCL Ü

CaS0d.2H20 %

dHi

C/N

CEC (NaOAc.pH 8.2) me#

CEC (NH^OAc.pH 7.0) »

Exch. Ca

Mg

' " K "

" Na "

Sum of cat ions "

• Base s a t . $ a t pH 8.2

» •• •> •• 7 . 0

E.S.P. a t pH 8.2

Exch. ac id i ty me%

Miscellaneous

.

i

209 210 ; 211 : !

A : AC : C '

0-30 ;

20 :

lo :

70 ;

c [

6.0

4 .4

0.60

1.77

0.13

-

56.6

32.8

12.0

0.55

0.30

45.65

81

<5

.

30-60 60-105

10 :

10 ;

80 j

c

7.6

5.6

0.75

0.97

61.0

32,8

23.4

0.55

1.35

58.10

95

<5

T"

\ r-

26

14

60

C

8.0

6.2

0.85

0.47

56.0

32.0

23.4

0.55

Trace

: 55.95

; 100

! <5

1

L , .

~!

_.__

;

J

<

j

Oopth in cm.

Texture U.S.O.A.:

Sand i-. 2.0 - 1.0 mm

1.0 - 0.5 "

0.5 - 0.25"

0.25- 0.1 "

0.1 - 0.05"

total sand °fc

Sil t f. 50 - 20 u

2 0 - 2 u

total s i l t %

Clay % 2 - 0 u

. Tsxture class

Bulk density

Particle density

Porosity

Moisture $ w/v at: ;

pF 2.0

pF 2.3

pF 2.7

pF 3.7

pF 4.2

Clay mineralogy:

SiO2/Al2O3

SiO2/R2O3

F e2°3*

X-ray repor t :


Available Ca möjk

" Mg "

« K "

" Na "

" Mn "

" P ppm

Total Ca mi=$

" Mg "

» K H

" P ppm

:

j-

•

.i

-—_-„

,

- —

1

....1

1 ._

_

., .

T

•J

j —

J

-32 _

Mapping unit P 27


Local petrography:

Physiography:

Relief, macro :11 meso,micro :


Erosion:

Rock outcrops:

Surface stoniness:

Slope gradient:

Salinity/alkalinity:

Surface sealing:



Description of profile

A 0 - 30 cm

AC 30 - 60 cm

60 - 105 cm+


Thiba basalts

Olivine basalts

Plain

Level

Gilgai

Grassland/grazing

Nil

Nil

Nil

Nil

2 ran. moderate

Deep, 105 cm

Poorly drained

Black (10YR 2.5/1 moist), clay; common, fine,

prominent yellowish red mottles; strong, medium

angular blocky structure; extremely hard when dry,

very firm when moist, very sticky and plastic when

wet; few, fine pores; common, medium and fine

roots; pH 6.0; clear and wavy transition to:

Black (10YR 2.5/1 moist), clay; common, fine to

medium prominent yellowish red mottles; moderate,

medium to coarse prismatic structure; extremely hard


plastic when wet; abundant, moderate to strong

slickensides; few, fine pores; few, fine roots;

pH 7.6; gradual and wavy transition to:

Black (10YP. 2.5/1 moist), clay; common, fine to

medium prominent yellowish red mottles; moderate,

medium prismatic structure; very hard when dry,

very firm when moist, very sticky and plastic

when wet; common, moderate to strong slickensides;

few, fine roots; pH 8.O.

29

APPENDIX 2

» v4yOsT-

** * . , ,. E T H I O P I A

K E N Y A

v0 NAIROBI

A 1 '

K E Y

! J

L\A\\\\j

S I PB

(VIWEA IRRIGATION SETTLEMENT SCHEMELEGEND

MINOR SCARPS (gen

L

SÏ

S3

( g e n t l y s l o p i n g ; s l o p e s f r o m 2 S O , )

—J d a r k g r e y , v e r y f i r m , s t o n y , c r a c k i n g c l a y

1 ' ^ P e r f e c t l y d r a i n e d , d e e p , b l a c k t o v e r y d a r K g r e y , v e r y f i r m .n o n 0 ' s l i g h t l y s t o n y , c r a c k i n g d a y o f t e n v v i i h n i g h p H { >" >' ' ' ' ' - " " 2 5 - 5 0 c m d e p t i i o n w a r d s a n d u s u a l l y c a l c a r e o u sf r o m 5 0 - 8 0 c m d e p t i i o n . v a r d s

" 1 " d s e t f e c ! l y d t a i n e n . m o s t l y d e t t e m o t t l e d d a r k n i o w n t o

• J v ' . a y . j . i r K g r e y i s h b r c n ' n i f i r m , n o n o r s l i g h t l y s t o n y , c l a y

LAND SUITABILITY CLASSIFICATIONFOR BASIN IRRIGATION OF RICESUITABILITY CLASS LIMITATION

IV

r , ( r ) , f

S1 P

a

P L A I N S ( l e v e l , s l o p e , ( e s s . n a n 2 a , )Upper level plain vvi ih b l a c k , very f irm, c r a c k a v j c l a y s o i l s

pel Me V E I V f l S O L S

Pj j Pt iuny ' . j ramod. :,;eep. u a d i ü e i e n t i a t e c !

Lower - leve l plain •:, a li b l ack very f i i m.cr.ncKing c l a y soi is

pel l ie V E H r t S O L S

[ P2JP ] Peoe» d ,a ; n e , ; i s h a l l o w , e s u a i l y c a l c a r e o u s from 25 can

depi t i en .var..ss

[ P;) ; , - "" V W V poor iy d raa ien , i n o d e r a t e U deep, a s n a l l y c a l c a t e o a s

P21 I '" ' - 'y pood y .aviaied. deep .o f ten A I U I h i ' j h pH( , ; ö 0 ) f iom~ ' 20 - 50 can dep th o i uvmds and u s u a l l y c a l c a r e o u s I'oai

50 • àO cm d e p m ornva ids

[ P2S j p a ' v i y d i a i n e d deep, of ten ,vd li ra tne i h ign pH (7 5 - 8.0 )' | :-1'1' d0 • 50 cm r iept l i o n w a r d s and nsuaMy c a l c a i e o e sf i 0 " i 5 0 - 30 cm depth orovasdi.

I P26 I v c r y poor l y d r a i n e d , deep, u s j a i ! y e a i c a t e o u s ' l oa i 50 • BO1 - - 1 <-'" dep i ' i e iuva tds

P2I d i a m e d deep and n o n - c a l c a r e o u s

KEY TO LAND SUITABILITY CLASSIFICATION FOR BASIN IRRIGATION OF RICESUITABILITY CLASS LIMITATION excessive severe moderate sl ight

L O C A

topography _ I

depth of s o d D Js ton i ness _ r

d r a i n a g e -nn tnen t a v a i l a b i l i t y " • - - f

* drai i ' iaqe l i m i t a t i o n re fe rs to d ie o c c u r r e n c o of seepncis" n u t r i e n t a v a i l a b i l i t y re fe rs to the " K a r a b a c rop fa i lu re p r o b ' e i n "

S!)

(d)

(n

50-80 j [ I A I I

30-120 !

.. a s io a fad v t

Î EX I id RAO COASSES

percent sand

SCALE 1:10,000

0 1 9 KmPrepared and drawn by Kenya Soli Suivey in October, 1975

Drawing No, 75036

APPENDIX 3: LOCATION IVfAP OF PROFILE PITS AND AUGERÏNGS "WAlUfVIU EXTENSION11

MWEA IRRIGATION SETTLEfWENT SCHEME

±

.,,., » , ETHIOPIA

K E N Y A

*v NAIROBI

~T> MOM BASA

L o c a t i o n of the map

A1 'I • ' /•sis

A 15n O ^"'

\ ƒƒ/ / ••' \

Äw

/°\ .

• \ Av, - \ .

• \ .

\\\

A13 '

< <

O

•A3

O 7 Si. ,

'O

M

\

\

\

\

!/

\LOCATION OF SURVEY AREA WITHIN MffEA IRRIGATION SCHEME: \

\A12

f,\; \

KEY

1OSL,-<>

:nain roadformer dra inage wascheme bourvJai y

i t r i ga i i on canal

augerho'e observat ion

soi l piOfiiG p i l . \vi!h sainpi inq

buik sampling for greenhouse 'est

A 'OS .

. A2

l 0

SCALE 1:10,000

2 3 4

* 3 US 1.5 O

O'

• 4

A

\

W

7 3 ) Km Prepared and drawn by Kenya Soil Survey in October, 1975

Drawing No. 75036a

soils of the proposed wamumu extension ...2.2. geology and general landscape features the area, like...

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