Promoter: Prof. Dr. Eric Van Ranst

Tutor(s): Mathijs Dumon

Master dissertation submitted in partial

fulfilment of the requirements for the

degree of Master of Science in Physical

Land Resources

By: Joseph Tamale

Academic Year: 2013-2014

INTERUNIVERSITY PROGRAMME IN

PHYSICAL LAND RESOURCES Ghent University

Vrije Universiteit Brussel Belgium

Soil characterization in the landslide-prone area

Bumwalukani, Eastern Uganda.

i

COPYRIGHT

This is an unpublished MSc dissertation and is not prepared for further distribution. The

author and the promoter give the permission to use this Master dissertation for

consultation and to copy parts of it for personal use. Every other use is subject to the

copyright laws more specifically the source must be extensively specified when using results

from this Master dissertation.

Gent,

Author:

Joseph Tamale

Promoter:

Prof. Dr. Eric VanRanst

ii

ACKNOWLEDGEMENT

I would like to express my sincere appreciation and thanks to my promoter, Prof. Dr. Eric

Van Ranst, for being an awesome supervisor and mentor. I would like to thank you for the

encouragement throughout this period and also allowing me an opportunity to grow as a

soil scientist. Your advice both on research and my career has been priceless and will

forever be appreciated.

I also want to thank members of the Jury for my thesis, Prof. Dr. Peter Finke and Prof. Dr.

Jozef (Seppe) Deckers for accepting to read my work.

I wish also to extend my gratitude to my tutor, Drs. Mathijs Dumon for the time spared to

review my text and guide me throughout the preparation of my thesis. Thank you deeply.

I would also like to thank the great team of Laboratory staff, Nicole Vindevogel and Veerle

Vandenhende for the exceptional guidance during the analysis of my soil and rock samples. I

extend special thanks to Mr. Jan Jurcica for his effort investment in the timely preparation

of thin sections and Dr. Florias Mees for the help extended in interpretation of slides.

Many thanks to my colleague, Amaury Defrére from the Catholic University of Leuven for

the great team spirit and wonderful collaboration exhibited that enabled us to gather

enough field data within a short time frame.

A special thanks to my family; words cannot express how grateful I am to my mother, Miss

Erioth Matovu for all the sacrifices you’ve made on my behalf in order to see me through

school. Your prayers and encouragement are what have kept me going till this end. I would

also like to thank all my friends for the enormous support and incredible encouragement

during the entire period of my research.

I wish to categorically state that none of the people mentioned above is accountable for any

of the inaccuracies that may arise in this piece of work apart from the author himself.

iii

DEDICATION

I dedicate this piece of work to my friends and all those who have passion for soil science as

a discipline.

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ABSTRACT

Landslides have recently become more frequent and recurrent worldwide almost occurring

every year with devastating socio-economic problems. Research studies undertaken in

different landslide-prone areas in the world indicate a positive correlation between soil

properties and slope failures. In Uganda, however, information on soil properties in general

is to date extremely limited owing to a few general soil characterization studies so far

carried out and thus makes it hard to draw similar correlations. It is against this back ground

that a soil characterization study was done in the landslide-prone area Bumwalukani,

Eastern Uganda in order to contribute to a better understanding of soils in this area that

have been generally described in previous studies.

Three profiles P1, P2, P3 at the upper slope, middle slope, and valley bottom respectively

were selected for the study. Profile 2 (P2) was in close proximity to a scar left behind after

the occurrence of the 2010 Bududa landslide. Both rock and soil samples were obtained

from P1 and P2, but only soil samples were taken from profile 3. These were then analysed

to determine their physico-chemical, mineralogical and micmorphological properties at the

Laboratory of Soil Science, Ghent University, Belgium.

Chemical and mineralogical analyses of the selected rocks show that they are basic in nature

due to their low silica content (<50%). The rocks also have an appreciable amount of

amphiboles, feldspars, kaolinite, quartz and mica. Owing to their weathered state, rock 1

can be called amphibolitic while rock 2 and 3 are phonolitic. In contrast, soils from the three

profiles are deep, yellowish brown (5YR-7YR) in colour, have a moderately acidic to neutral

pH (5.5-6.5), high water dispersible clay content( >10%), high amount of clay and slit (>40%)

for P1 and P2, and a high sand content (>50%) (only for P3). Selective dissolution analysis of

the soil samples shows relative accumulation of crystalline Fe oxides over non-crystalline

(short range order) Fe oxides. The silt and clay fractions of the soils are dominated by

kaolinite, mica and quartz.

Additionally, there are significant differences between the mineralogical composition of the

silt and clay fractions which is attributed to both physical and chemical weathering.

Leaching, clay illuviation, goethite and hematite formation, and bioturbation are the main

soil-forming processes indicated in thin sections. Furthermore, it is evident that soils are

v

highly leached and weathered, with a high spatial variability in soil properties (i.e. Cambisol

at the upperslope, Nitisol at midslope, and Fluvisol at valley bottom) due to differences in

parent materials and topographic positions. Profile 2 (close to the scar left behind by the

occurrence of 2010 Bududa landslide) has a small amount of swelling minerals in its Bt

horizon owing to low intensity peaks from XRD analysis. However, this soil profile (Nitisol)

also has a good internal drainage and therefore infiltration of rainwater to the depth with

swelling minerals can consequently lead to liquefaction, creating a sliding plane at that

depth and thus occurrence of a landslide. Therefore presence of some swelling minerals in

a soil coupled with good internal drainage increases sensitivity to landslides.

Key words: Landslide-prone area, mineralogy, micromorphology, phonolitic rocks,

amphibolitic rocks and weathering.

vi

TABLE OF CONTENTS

ACKNOWLEDGEMENT ................................................................................................................ ii

DEDICATION .............................................................................................................................. iii

ABSTRACT .................................................................................................................................. iv

LIST OF FIGURES ...................................................................................................................... viii

LIST OF TABLES .......................................................................................................................... ix

ABBREVIATIONS AND ACRONYMS ............................................................................................. x

CHAPTER ONE: INTRODUCTION ................................................................................................. 1

1.1. Background of the study ............................................................................................. 1

1.2. Justification ................................................................................................................. 3

1.3. Objectives .................................................................................................................... 3

CHAPTER TWO: LITERATURE REVIEW ........................................................................................ 4

2.1. Soil-forming processes in Mount Elgon Region, Eastern Uganda ................................... 4

2.1.1. Ferralitization ....................................................................................................... 4

2.1.2. Biotic activity ........................................................................................................ 5

2.1.3. Erosion ................................................................................................................. 7

2.1.4. Eluviation, leaching and illuviation ...................................................................... 8

2.2. Common soil types in the landslide-prone area ......................................................... 9

2.3. Soil properties ........................................................................................................... 10

2.3.2. Morphological properties .................................................................................. 10

2.3.3. Physico-chemical properties .............................................................................. 11

CHAPTER THREE: METHODOLOGY ........................................................................................... 14

3.1. Choice of study area .................................................................................................. 14

3.2. General description of the study area ...................................................................... 15

3.2.1. Location .............................................................................................................. 15

3.2.2. Geology .............................................................................................................. 16

3.2.3. Climate ............................................................................................................... 16

3.2.4. Hydrology and drainage ..................................................................................... 16

3.2.5. Vegetation .......................................................................................................... 17

3.2.6. Population .......................................................................................................... 17

3.2.7. Land use ............................................................................................................. 17

vii

3.3. Field sampling ............................................................................................................ 18

3.3.1. General site information .................................................................................... 18

3.3.2. Sampling design ................................................................................................. 18

3.4. Methods .................................................................................................................... 19

3.4.1. Physico-chemical analyses ................................................................................. 19

3.4.2. Mineralogical analysis ........................................................................................ 23

3.4.3. Micromorphological analysis ............................................................................. 23

CHAPTER FOUR: RESULTS ........................................................................................................ 24

4.1. Rocks ......................................................................................................................... 24

4.2. Soils ........................................................................................................................... 28

4.2.1. Soil morphological properties ............................................................................ 28

4.2.2. Soil physico-chemical properties ....................................................................... 28

4.2.3. Selective dissolution composition ..................................................................... 34

4.2.4. Soil mineralogical properties ............................................................................. 36

4.2.5. Soil micromorphological features ...................................................................... 36

CHAPTER FIVE: DISCUSSION ..................................................................................................... 43

5.1. Rocks ......................................................................................................................... 43

5.2. Soils ........................................................................................................................... 44

5.2.1. Soil-forming processes ....................................................................................... 44

5.2.2. Evolution of soil morphological and physico-chemical properties in relation to

weathering ......................................................................................................... 47

5.2.3. Mineralogical composition of silt and clay fraction ........................................... 49

5.2.4. Evolution of mineralogy of silt and clay fractions in relation to weathering .... 50

5.2.5. Classification of the selected soil profiles .......................................................... 54

5.2.6. Implications for landslide susceptibility ............................................................ 55

CHAPTER SIX: CONCLUSIONS AND SUGGESTIONS FOR FURTHER RESEARCH ......................... 57

6.1. General conclusions ...................................................................................................... 57

6.2. Recommendations for further research ....................................................................... 58

REFERENCES ............................................................................................................................. 59

APPENDICES ................................................................................................................................ I

viii

LIST OF FIGURES

Figure 1. A profile of a Ferralsol (Oxisol) with indication of different horizons ....................... 5

Figure 2. Schematic representation of the effects of bioturbation on pedogenesis (Wilkinson et al., 2009). ............................................................................................. 7

Figure 3. A fresh scar left behind by the occurrence of 2010 landslide in Bumwalukani (photo by Joseph Tamale, 2013). ............................................................................ 14

Figure 4. A map of Uganda with a window showing the study area ...................................... 15

Figure 5. (a) Cracks in weathered amphibolitic matrix (plane-polarised light); (b) cracks in weathered amphibolitic matrix (crossed-polarised light), (c) grey amphibole matrix with cracks filled with unknown material (crossed-polarised light), (d) laminated illuvial clay infilling in amphibolitic matrix (plane-polarised light), (e) advanced weathering stage of amphibolitic rock, with fragments of goethite, anatase and quartz (plane-polarised light), (f) advanced weathering stage of amphibolitic rock, with fragments of goethite, anatase and quartz (crossed-polarised light) for the bed rock in profile 1 ................................................................................................ 26

Figure 6. (a) Pseudomorph after nepheline replaced by clay (plane-polarised light); (b) pseudomorph after nepheline replaced by clay (crossed-polarised light), (c) plagioclase feldspar (plane-polarised light), (d) plagioclase feldspar (crossed-polarised light) for the bed rock in profile 2 ............................................................ 27

Figure 7. Evolution of O.C (a), pH (b), and CEC (c) with depth for the selected soil profiles. . 32

Figure 8. XRD patterns of silt powders for the respective horizons for profile 1 (all d-spacing of the peaks are in nm) ............................................................................................. 37

Figure 9. XRD patterns of silt powders for the respective horizons for profile 2 (all d-spacing of the peaks are in nm ............................................................................................. 38

Figure 10. XRD patterns of silt powders for the respective horizons for profile 3 (all d-spacing of the peaks are in nm) ............................................................................. 39

Figure 11. (a) Clay illuviation in the saprolite (plane-polarised light); (b) illuvial clay with incorporated coarse kaolinite aggregates (plane-polarised light), (c) illuvial clay along planar voids in soil matrix (plane-polarised light), (d) quartz (white)- amphibole (green) dominated rock fragment (plane-polarised light), (e) fibrous

ix

goethite coating (plane-polarised light), (f) goethite coating consisting of radial aggregates (plane-polarised light) from Profile 1 .................................................. 41

Figure 12. (a) Laminated clay infillings (plane-polarised light); (b) infilling with crescent structure related to termite activity (plane-polarised light), (c) angular feldspar dominated rock fragment (crossed-polarised light), (d) angular feldspar dominated rock fragment (plane-polarised light), (e) fragmented fibrous goethite coating (plane-polarised light), (f) fragmented fibrous goethite coating (plane-polarised light) from Profile 2 ................................................................... 42

Figure 13. Bowen reaction series (1956) ............................................................................... 52

Figure 14. XRD patterns of oriented clay samples of the B2t horizon of profile 2 after K+ and Ca2+ saturation. ...................................................................................................... 53

LIST OF TABLES

Table 1. General site information ............................................................................................ 18

Table 2. Total elemental composition of rock samples related to the selected soil profiles . 25

Table 3. Morphological and physical properties of selected profiles .................................... 30

Table 4. Chemical properties of the selected soil profiles ...................................................... 33

Table 5. Selective dissolution analysis of the selected soil profiles (expressed in %) ............. 35

Table 6. Qualitative mineralogical composition of the silt fraction based on XRD analysis .. 40

Table 7. Qualitative mineralogical composition of the clay fraction based on XRD analysis.. 40

Table 8. Geological classification of rocks (Tilley, 2010). ........................................................ 43

x

ABBREVIATIONS AND ACRONYMS

a.s.l Above sea level

AEC Anion exchange capacity

BS Base saturation

CEC Cation exchange capacity

FAO Food and Agriculture Organisation

GoU Government of Uganda

LOI Loss on ignition

N Normal

NEMA National Environmental Management Authority

O.C Organic carbon

rpm Rounds per minute

WDC Water dispersible clay

WRB World Reference Base

1

CHAPTER ONE: INTRODUCTION

1.1. Background of the study

Landslides rank 10th among the most devastating natural disasters in the world occurring

almost across all terrains with steep slopes singled out as most susceptible to sliding

(Schuster and Highland, 2003; Highland and Bobrowsky, 2008; Leroy and Grachera, 2013).

They occur almost every year leading to significant property damages and deaths (Huabin et

al., 2005; Kirschbaum et al., 2010; Msilimba and Holmes, 2010). Within the period of 2004–

2011, more than 32,000 landslide-related fatalities were recorded worldwide with the 2010-

Bududa landslide in Eastern Uganda being among the major landslides documented there

(Pankow et al., 2014). This landslide event destroyed a lot of valuable property, claimed

over 300 lives and rendered about 5000 people homeless. It was the most devastating

catastrophe ever recorded in the history of Uganda (Gorokhovich et al., 2013). Mugagga et

al. (2012) noted that this landslide event was attributed to recurrent heavy rainfall and

slope failure due to overloading. The latter is a consequence of human development

expansion on unstable hilly slopes increasing their vulnerability to sliding (Smyth and Royle,

2000; Huabin et al., 2005).

However, Highland and Bobrowsky (2008) noted that most of the slope failures in landslide-

prone areas are by far linked to the nature of slope materials (rock and soil). Soil materials in

landslide-prone areas have considerable amounts of swelling minerals in the fine earth

fraction and therefore seasonally swell and shrink following wet and dry periods (Yalcin,

2007). Upon drying up, cracks develop which grow with subsequent periods of swelling and

shrinking. These cracks increase preferential entry of water in soils and consequently lead to

early saturation following a rainfall event causing liquefaction incase of a trigger like an

earthquake (Galeandro et al., 2011). If cracked fine-grained soils are overlying a more

permeable material, water flow through this type of soil system can be affected by closing

of cracks due to swelling of overlying fine-grained soils. Rainfall infiltration in such soil

systems can consequently lead to a dramatic decrease in suction of unsaturated soils

triggering various instability phenomena, such as the slip of steep surface soil layers.

2

Basharat et al. (2014) additionally, suggested earthquakes as a trigger for landslides

resulting into rock falls, debris falls, rockslides and rock avalanches.

Unlike developed countries, e.g. China and USA, where enormous efforts have been

invested in research about landslides (e.g. Hua-xi and Kun-long (2014) in China, Chung et al.

(2014); Tao and Barros (2014) in the USA) to better understand this phenomenon and foster

adoption of feasible mitigation measures in case of an occurrence, developing countries

(e.g. Uganda) still lag behind despite the socio-economic threats posed by the occurrence of

this catastrophe (landslide) on their populations.

In Uganda, a few studies have been carried out in the frame work of landslides with major

emphasis put on evaluation of causal factors and physico-chemical properties of the soils.

However, mineralogy of the soils was completely neglected despite its strong influence on

stability of slopes. Moreover, the two studies i.e. Kitutu et al. (2009) and Muggaga et al.

(2011) that focussed on physico-chemical properties were incomprehensive and don’t give a

complete picture about the nature of the soils. To highlight, Kitutu et al. (2009)

characterized and classified these soils as Cambisols, Ferralsols, Lixisols and Nitisols.

However, the classification was general and inconclusive since it was based on incomplete

analytical data. No soil mineralogical analysis was done at all yet it’s useful in understanding

the type of clay minerals (e.g. swelling minerals) present in the soils. Additionally, no

information on the presence or absence of weatherable minerals can be traced from their

study. In regard to this, one wonders how soils were classified as Ferralsols when analytical

data required in verifying presence or absence of a Ferralic diagnostic horizon was never

obtained. Mugagga et al. (2011) only focussed on particle-size distribution of the soils in the

region and used this to extrapolate and draw conclusions about their mineralogy. Therefore

there is still a big information vacuum on the properties of soils in Bumwalukani which

needs to be filled. In this study, a complete and comprehensive analysis of the physico-

chemical, mineralogical and micromorphological parameters of both bulk and undisturbed

soil samples from Bumwalukani, Eastern Uganda, was done.

3

1.2. Justification

It’s been said that scientists know more about soils of Mars than those of Africa (Sanchez et

al., 2010). This to a great extent explains why Uganda to-date still uses the conventional less

detailed and out-dated soil map of 1967 made at a scale of 1:1,500,000. Isabirye et al.

(2004) mentioned scarcity of information about soils in Uganda as a major hindrance and

obstacle in sustainable management of this resource. The little information that is available

is also not digital. Therefore there is a need to do more soil characterization and

classification to contribute to the soil resources information data base of Uganda. This

study was oriented towards collection of a considerable amount of information on the

properties of soils in the landslide-prone area and grouping studied profiles into their

respective reference groups. Additionally, results from the study would be used as a basis

for drawing linkages between soil types and landslide risk in future research studies.

1.3. Objectives

The main objective of the study was soil characterization in the landslide-prone area,

Bumwalukani in Eastern Uganda.

The specific objectives of the study were:

(1) to identify soil-forming processes using observable micromorpholgical features

indicated in thin sections; and

(2) to determine physico-chemical and mineralogical properties of the soils in order to

understand better their sensitivity to land sliding.

4

CHAPTER TWO: LITERATURE REVIEW

2.1. Soil-forming processes in Mount Elgon Region, Eastern Uganda

Soil formation is a result of soil forming processes whose rate is influenced by the intensity

of soil forming factors like climate, living organisms, parent material, topography and time

(Brady and Weil, 2002). In regard to Eastern Uganda, the following soil-forming processes

have been reported and they include; ferralitization, biotic activity, erosion, eluviation,

leaching and illuviation.

2.1.1. Ferralitization

Ferralitization process is associated with strong weathering leading to formation of soils

with a ferralic B horizon (i.e. Oxisols (USDA) or Ferralsols (FAO)). Oxisols (USDA) or Ferralsols

(FAO) (Figure 1) are extremely weathered soils with diffuse horizon boundaries (van

Breemen and Burman, 2002). Additionally, these soils are dominated by gibbsite, Fe oxides

and kaolinite in the clay fraction and as a consequence have a low CEC (<16 cmol(+)/kg at

pH7) and low cation retention (<10 cmol(+)/kg at soil pH). Neufeldt et al. (1999) noted that

ferralitization is a common soil-forming process in humid and subhumid tropics because of

the extended periods of wet and hot climate. However, it can also occur in ustic and drier

climates provided there is a wet period in between a long dry period. Ferralitization

(residual accumulation of (hydr)oxides of Fe and Al) is preceded by desilication (net loss of Si

from primary silicates and quartz) and kaolinitization (formation of kaolinite) (Neufeldt et

al., 1999). Desilication (chemical weathering of silicates) typically involves hydrolysis as the

most important reaction. It involves splitting of water molecules into their H (often

substitutes a cation from the mineral lattice) and OH components. Van Breemen and

Buurman (2002) used hydrolysis of feldspars (highly abundant Al-silicate minerals) to explain

both desilication and kaolinitization reactions in soils. Feldspars are more stable in the

earth‘s interior than at the surface because of favourable conditions (high temperatures and

little water) there compared to the earth‘s surface (low temperature, abundant water)

(Hieronymus et al., 1990; Hai and Egashira, 2008). The hydrolysis of feldspar involves H+ ions

from water reacting with Oxygen of feldspars which leads to disintegration of the primary

mineral thus desilication. The K and part of the silica (Si) are removed from the soil resulting

into formation of kaolinite (secondary clay mineral) hence kaolinitization. However, both

5

processes are extremely slow (Preetz, 2008). Conversely, residual accumulation of

sesquioxides (Al/Fe oxides) is a result of strong depletion of basic cations owing to their high

solubility leaving Al/Fe oxides to precipitate and accumulate (Schwertmann, 1985). As a

result of strong weathering of primary minerals in Ferralsols, their silt content is relatively

low (i.e. silt/clay ratio in most Ferralsols is <0.15) (van Breemen and Buurman, 2002).

Figure 1. A profile of a Ferralsol (Oxisol) with indication of different horizons.

(www.colorado.edu/.../geog_1011_f02/ study2_02.html)

2.1.2. Biotic activity

Bioturbation involves movement of solid material in the soil profile by the actions of soil

fauna (burrowing animals). This is evidenced by presence of soil mounds, worm holes,

worm casts, insect cavities, rodent burrows, snail shells, termite runs and faecal pellets. Soil

fauna activity results in the upward transport of soil components against the gravitational

force and downwards flow of water, biochemical and physical conversion of soil

components, transport of litter and top soil components, facilitation of transport processes

through voids and macropores, and initiation of further microbiological processes

(Bunnenberg and Taeschner, 2000). Termites play a significant role in mixing up of soils

through nest building. They bring up fine material while coarser fractions of the weathered

6

rock are left below. However, the fresh soil material brought to the surface can increase

sediments available for erosion (Cerda`and Jurgensen, 2007). They added that at some point

in time, the moulds collapse and form a layer of fine material at the ground but the new

moulds are built and the process continues. Nest building by termites also contributes to

formation of macropores creating canals and routes for water infiltration. They found a

reduction in soil bulk density, an increase in macropore flow and soil organic matter in ant-

affected soils, compared to soils without ant activity. In a study by Miedema et al. (1998),

channels and infillings, coprogenic granular structure and high porosity were mentioned as

critical evidence for bioturbation by earthworms. They concluded that biological activity was

responsible for a very complex transitional pattern of the AhE and EBt horizons from the

studied profiles. Similarly, Tonneijck and Jongmans (2008) cited bioturbation as a major

process influencing the vertical distribution of soil organic matter (Figure 2). However, they

observed that the influence of bioturbation on vertical SOM transport was complex because

it is a result of interaction between different groups of soil faunal species that redistribute it

throughout the soil profile in distinct ways. Mujinya et al. (2010) mentioned that termite

activities lowered remarkably the actual preferential adsorption of Al on the exchange

complex, actual AEC, the pH0 values and AEC variability; and increased considerably cation

exchange capacities, soil pH, permanent negative charge and CEC's variability.

7

Figure 2. Schematic representation of the effects of bioturbation on pedogenesis (Wilkinson et al., 2009).

2.1.3. Erosion

Mount Elgon region has steep slopes (with slope gradients ranging between 36 and 58%)

making the region erosion prone (Mugagga et al., 2010). They noted that the steep relief

inevitably causes soil materials to be washed downslope during or after a rainfall event. This

leads to truncation of the soils on the upper slope and accumulation of soil materials at the

foot slopes where deposition took place. They further stated that the rate of truncation at

the upper slope and the subsequent deposition at the foot slope entirely depended on the

type of erosion i.e., whether geological (also called natural erosion) or accelerated erosion.

The later removes a considerable amount of soil at rate way above which it’s replenished.

Chaopricha and Marin-Spiotta (2014) noted that soil burial from erosion was globally

important in contributing to the delivery and long-term persistence of substantial SOC

8

stocks to depths beyond those considered in most soil C inventories. This implies that soil

erosion can lead to formation of soils with different properties both on site (through

truncation) and off site (buried horizons). To-date there are no area specific annual soil loss

estimates for Bumwalukani which makes it hard to quantify the contribution of erosion to

soil formation in the area despite presence of erosional features like gullies.

2.1.4. Eluviation, leaching and illuviation

Mount Elgon region receives between 1500 and 2000 mm of rainfall on average annually

(which values also apply to Bumwalukani since it’s found within the same region) which is a

considerable amount of rainfall characteristic of the humid tropics (Mugagga et al., 2012).

As the rainfall infiltrates through the soil, it often carries with it fine clay particles

(eluviation) and chemicals in solution (leaching) which tend to be re-deposited in deeper

horizons (illuviation) (Kjaergaard et al., 2004). They further noted that the clay content of

soil horizons always increased towards the bottom of the studied profiles. They further

added that there was also tendency of acid and base radicles to be leached into deeper

layers. However, they noted a substantial vertical transfer of fine particles from an eluvial E-

horizon to an illuvial B horizon which is referred to as Lessivage (Dultz, 2000; Zaidel’man,

2007). Lessivage has been described as a major or secondary pedogenetic process for many

soil types (Quénard at al., 2011). Additionally, Bockheim et al. (1996) noted that the

progression in soil development in two areas from Spodosols with clay-enriched horizons

and eventually to Ultisols was as a result of leaching, eluviation and illuviation of clay and

organo-metal cations. Translocation of clay is favoured by wet and dry seasons typical of sub

humid and humid tropics. The presence of fine clay coatings in fine pores inside the peds of

the Bt and the presence of clay–humus coatings in large channels through the peds and

planar voids separating the blocky and prismatic peds also can suggest illuviation

(Bagnavets, 1989; Laffan et al., 1989; Blokhuis et al ., 1990; Fedoroff, 1997; ). Moreover, soil

descriptions by Knapen et al. (2006) and Kitutu et al. (2009) of the soils of Bududa, Eastern

Uganda, showed that all the described profiles had a deep Bt horizon therefore confirm

leaching, eluviation and accumulation as major soil forming processes in the area.

9

2.2. Common soil types in the landslide-prone area

Knapen (2003) and Kitutu et al. (2009) classified soils of the landslide-prone area in Eastern

Uganda using FAO et al. (1998) and WRB (2006) respectively. Based on the analytical and

field descriptions, the following soil types were identified;

(a) Nitisols

Nutty shiny peds were observed in the field. They also observed an abrupt increase in clay

content in the Nitic horizon compared to the overlying horizon. In both studies, CEC values

ranged between 24.5-35 cmol (+)/kg clay (<36 cmol (+)/kg clay) in all the profiles. However,

different prefix qualifiers were used i.e. Rhodic and Dystric by Kitutu et al. (2009) and

Knapen (2003) respectively for a detailed classification of the profiles. Rhodic qualifier was

used because the Nitic horizon had a Munsell hue greater than 2.5 YR and a moist colour

value of 2.5. Dystric on the other hand was selected because the Nitic horizon had a low

base saturation of 35% (i.e. less than 50%). In both studies, there were no results on water

dispersible clay (yet it’s an important criterion for classification of a soil as Nitisol) but on the

contrary soils were classified as Rhodic Nitisols and Dystric Nitisols.

(b) Acrisols

In both studies, the soils had a CEC between 20.4-28 cmol (+)/kg clay and a base saturation

of <50%. Profondic and Rhodic prefix qualifiers were used because there was an observed

clay decrease with depth (Profondic), and a Munsell hue greater than 2.5 YR and a moist

colour value of 2.5 (for Rhodic) hence the names; Rhodic Acrisols and Profondic Acrisols.

(c) Ferralsols

Pseudo-sand structure was observed in the field. The soils had low pH values (<5) and a CEC

< 16 cmol (+)/ kg clay. Acric and Rhodic prefix qualifiers were used in the names. This was

because clay increased in the argic horizon relative to the overlying horizons and a base

saturation of less than 50% for the Acric prefix qualifier. Conversely, Rhodic prefix was used

because of a Munsell hue greater than 2.5 YR and a moist colour value of 2.5 in the argic

horizon hence the name Acric Rhodic Ferralsol.

10

(d) Cambisols

Beginning of horizon differentiation was evident in the two profiles described as Cambisols

(Kitutu et al., 2009). This was due to observable changes in structure and texture within the

cambic horizon. The soils were reddish in colour (2.5 YR-5YR 3/3-3/4) with pH of 5-6, CEC

<24 cmol (+)/kg clay and a high base saturation (>50%).

2.3. Soil properties

2.3.2. Morphological properties

Colour

Soil colour is one of the important basic properties which help to identify the kind of soil and

recognize succession of soil horizons within a soil profile. There is therefore a link between

soil colour and the physical, chemical and biological processes in a soil profile. A pioneer soil

characterization study by Ollier and Harrop (1959) showed that the majority of the soils in

the Mount Elgon region (Bududa series) have a reddish brown (5 YR/ 2.5 YR) to dark reddish

brown (2.5 YR) with hue and chroma values ranging between 3 and 4. The reddish brown

colour in soils is attributed to intense leaching of base cation allowing precipitation and

accumulation of iron oxides (Schwertmann, 1985). This explains the use of prefix qualifier

Rhodic in the previous classifications done by Knapen et al. (2006) and Kitutu et al. (2009).

Texture

Soil texture refers to the relative proportion of sand, silt and clay in a given soil. The relative

proportions of the different fractions cannot be easily altered which explains why texture is

considered a basic soil property (Brady and Weil, 2002). A study by Kitutu et al. (2009)

indicated that the soils of Bududa (of which Bumwalukani forms part) dominantly had a clay

texture. They however indicated that there was a small area with a sandy clay texture.

Furthermore, the B horizon was dominantly clay textured (denoted as Bt in many of the

described profiles) which justifies clay illuviation as a soil forming process in most of the

profiles. The texture of the A horizon (usually Ap) varied from sandy clay to sandy clay loam.

Results from a study by Mugagga et al. (2012) about texture varied significantly from those

reported by Kitutu et al. (2009). Their results showed that most of the studied landslide sites

11

had a texture varying from silty clay to sandy clay loam. The discrepancy in the study results

can probably be attributed to the way soil sampling was done. Kitutu et al. (2009) studied

soil profiles dug to a certain depth while Mugagga et al. (2012) studied texture of landslide

scars. Results from the study by Knapen (2003) on texture were intermediate between the

findings of Kitutu et al. (2009) and Mugaga et al. (2012)

Structure

Soil structure is the result of the spatial arrangement of the solid soil particles and their

associated pore space. Aggregation mainly depends on the soil composition and texture, but

is also strongly influenced by other factors such as biological activity, climate and

geomorphic processes. Structure is a typical morphological soil property, which allows

differentiating soil of geological material. Because of its importance, structure is a property

commonly described in soil studies (Brady and Weil, 2002). Knapen (2003) stated that the

soil structure of Bududa series ranged from sub angular blocky to angular blocky. However,

some soils in some places had little or no structural differentiation. Similar notions on

structure of soils in this area were made by Kitutu et al. (2009). They also observed a sub

angular type of soil structure but there was a transition from moderate to strong with

increasing depth of described soil profiles. The angular blocky aggregates are a result of clay

accumulation which is also evidenced by presence of clay cutans on the peds (Brady and

Weil, 2002; Stoops, 2010).

2.3.3. Physico-chemical properties

Particle size distribution

The particle size distribution is a fundamental soil property that affects many processes in

soils. Many empirical relationships have been developed to relate particle size distribution

to other soil properties, such as hydraulic conductivity and water retention characteristics

(Arya et al., 1999).

Mugagga et al. (2012) studied soils at three sites and plotted their particle distribution

curves. From the graphs of the respective studied profiles, they concluded that the soils

were generally fine grained. They argued that the fine nature of soils played a significant

role in making the soils landslide-prone. Kitutu et al. (2009) studied a soil profile along a

12

transect made in the landslide-prone area of Bududa. They observed that there was an

increase in fine grained particles with depth. They concluded that there was clay migration

in the profiles they studied which resulted into an increase in clay content of the sub surface

horizons which was also consistent with the observation of Knapen (2003).

Water dispersible clay and organic carbon (O.C)

Water dispersible clay also called natural clay refers to the clay content found when a soil

sample is dispersed with water and without any pre-treatment to remove cementing

compounds (like organic carbon, iron oxides, carbonates) and without use of a dispersing

agent. The proportion of natural clay to total clay is used as an indicator of structure

stability (Brubaker et al., 1992; Seta and Karathanasis, 1996) Clay particles can be dispersed

in soil water by the mechanical energy from traffic and tillage. This is especially pronounced

in soils with low OM content. Dispersed clay particles may be leached to deeper soil

horizons, where they can be deposited as clay skins in macropores (Chenu et al., 2000).

Knapen (2003) indicated that the O.C of soils in the landslide-prone area of Manjiya in

Mount Elgon region ranged between 0.5 to 1.5%. This range of values is below the threshold

of 2% SOC for a soil to be categorized as prone to structural deterioration (Loveland and

Webb, 2003). Soil organic matter is thought to increase aggregate stability by lowering

wettability and increasing cohesion of aggregates (Chenu et al., 2000). This could explain

why the soils have a high erodibility owing to a low amount of cementing agent to bind the

soil particles together and increase cohesiveness (to withstand the dispersive effect of

falling raindrops).

Soil pH

Soil pH or soil reaction is an indication of the acidity or alkalinity of a soil and is measured in

pH units (Brady and Weil, 2002). In separate studies by Knapen (2003) and Kitutu et al.

(2009), the soils in the landslide-prone area had an average pH (KCl) ranging between 4.9

and 6.5 which falls within the acidic to slightly acidic pH range. Acidification can be

attributed to leaching of base cations by percolating water allowing Al3+ and H+ to

accumulate in the soil consequently increasing their respective concentrations thus lowering

pH. Acidification is favoured by high rainfall and good soil porosity to allow downward

movement of base cations (Szott et al., 1991).

13

Cation exchange capacity (CEC) and base saturation (BS)

According to Szott et al. (1991), cation exchange capacity (CEC) is a measure of the quantity

of readily exchangeable cations neutralizing negative charge in the soil. Base saturation on

the other hand is the amount of positively charged ions, excluding hydrogen and aluminum

ion that are adsorbed on the surface of soil particles and is measured and reported as a

percentage. CEC and BS are important parameters that are considered in the identification

of diagnostic horizons and classification of soils. According to Knapen (2003), Cation

exchange capacity values ranged between 15 and 35 cmol (+)/kg clay for the studied soil

profiles in the landslide-prone areas in Mount Elgon region. The base saturation on the

other hand ranged between 25 and 35% for most of the profiles except for one profile

where it was greater than 50%. This justifies the classification of some profiles as Acrisols

because they met the CEC clay requirement of (< 24 cmol (+)/kg clay and base saturation of

< 50% (Deckers et al., 2003).

14

CHAPTER THREE: METHODOLOGY

The details relating to the general description of the study area (in regard to geographical

setting, population dynamics, relief and drainage, geology, soils, climate, natural vegetation

and land use), rock and soil sampling, and the analytical techniques employed in the

investigation are presented in this chapter.

3.1. Choice of study area

Bumwalukani, Eastern Uganda, was chosen for this study owing to its history of landslides.

This is evidenced by fresh scars (Figure 3) left behind after the downslope mass movement

of soil materials. It was with this background that Bumwalukani parish in the Mount Elgon

region of Eastern Uganda was selected for carrying out the study. Soil profile pits were dug,

bulk and Kubiëna soil samples taken for a detailed physico-chemical, mineralogical and

micromorphological analysis in the Laboratory of Soil Science at Ghent University. From

both the field observations and laboratory results, a detailed account on the soil properties

in the landslide-prone area was made.

Figure 3. A fresh scar left behind by the occurrence of 2010 landslide in Bumwalukani (photo by Joseph Tamale, 2013).

15

3.2. General description of the study area

3.2.1. Location

The study was carried out in Bumwalukani parish, Bulucheke Sub County, Bududa district in

Eastern Uganda. The area is geographically bound by latitude 1° 00’N and 1° 05’N, and

longitude 34° 15’E and 34°26’E. The altitude ranges from 1250 to 2850 m a.s.l and is

characterized by steep slopes with V-shaped valleys indicating river incisions (Kitutu et al.,

2011). Bududa district is bordered by Sironko district to the north, Republic of Kenya to the

east, Manafwa district to the south and Mbale district to the west. The district headquarters

of Bududa are located approximately 23 km (14 mi), by road, southeast of Mbale, the

largest city in the sub-region (Figure 4).

Figure 4. A map of Uganda with a window showing the study area

Location of study area

16

3.2.2. Geology

The study area (Bumwalukani) lies on Mount Elgon, a volcano associated with the rift valley.

It’s the oldest solitary volcano in East Africa resting on a dissected peneplain developed on a

Precambrian Basement Complex. The geology of the study area is dominated by fenitised

basement rocks, agglomerates and tuffs (Knapen et al., 2006). These rocks are composed of

extremely fine pyroclasts of potash feldspar and are referred to as potash ultra-fenites

(Reedman, 1973).

3.2.3. Climate

Mount Elgon is a massive solitary volcano in the flat plains of Eastern Uganda with a climate

significantly different from surrounding areas. This is attributed to its geomorphological

setting (Knapen et al., 2006). The climate is dominated by seasonally alternating moist

south-westerly and dry north-easterly air streams (GoU, 1996). The orthographic effect of

the mountain itself and proximity to Lake Victoria are the two influencing factors of rainfall

totals received in this area (Mugaga et al., 2012). Knapen et al. (2006) and Mugagga et al.

(2012) described the general rainfall pattern of the area as bimodal, with the wettest period

occurring from April to November. The mean annual rainfall ranges between 1500 mm on

the eastern and northern slopes and 2000 mm on the southern and the western slopes. The

forest zone receives the maximum rainfall and this makes the mountain an important water

catchment for many people (Synnott, 1968). The mean minimum and maximum

temperatures are 15 and 23 °C and respectively (van Heist, 1994).

3.2.4. Hydrology and drainage

Mugagga et al. (2012) noted that Mount Elgon is a very important water catchment in the

area. This is because it has minor and major rivers cascading from the caldera. They further

noted that the entire mountain is covered by streams that have a radial pattern which flow

all year round. They also mentioned a number of rivers that drain this area and these

include Suam, Sisi, Simu, Lwakaka, Sironko, Manafwa, Sipi and Bukwa. The water from these

rivers serves a good number of people in Bududa district with good quality water.

17

3.2.5. Vegetation

There are three recognisable broad vegetation communities in the area namely mixed

montane forest up to an elevation of 2500 m, bamboo and low canopy montane forest from

2400 to 3000 m, and moorland above 3500 m (Scott, 1994).

3.2.6. Population

Michael et al. (2003) noted that the East African highlands are densely populated owing to

their high agricultural potential and favourable climate for human habitation. Mount Elgon

Region has for long been dominantly occupied by the Bagisu tribe, a faction of the Bantu

speaking people. According to National Environment Management Authority (1997), Bagisu

accounts for 92%, 1.8% for Banyole and 6% for 32 other tribes of the total population of the

Mount Elgon area. According to the last human population census of 2001, the total

population of Mbale was estimated at 710, 980 people but this estimation takes into

account the population of Bududa at that time since it’s was part of Mbale district before

emerging as an independent district.

3.2.7. Land use

Land in the study area is majorly used as a national park and farmland. For the latter,

farming encompasses growing of both food (like beans, bananas, yams and onions) and cash

crops (majorly Arabica coffee). Most of the cultivation is done on steep slopes ranging

between 36 and 58%. Despite cultivation on steep slopes, use of soil conservation measures

in the area is still limited and this has led to severe soil erosion, soil fertility depletion and

reduced crop yields (Mugagga et al., 2010).

18

3.3. Field sampling

3.3.1. General site information

General site information is presented in Table 1 but the detailed profile description

according to FAO guidelines can be found in Appendix I-1, 2 and 3.

Profile 1 (P1), is at an altitude of 1748 m a.s.l located at the crest of a high gradient hill.

Settlement and transport (murram roads) are the dominant land use types with leveling and

ploughing as major forms of human influence. Profile 2 (P2), is at an altitude of 1666 m a.s.l

located at the middle slope of a high gradient hill. The dominant land use type is rainfed

arable cultivation with ploughing as the major form of human influence. Profile 3 (P3), is at

an altitude of 1461 m a.s.l located at the bottom of the valley floor. The dominant land use

type is perennial field cropping with burning and ploughing as the major forms of human

influence.

Table 1. General site information

Profile Elevation (m.a.s.l)

Landform Position Land use Human influence

P1 1748 High gradient hill Crest Settlement, Transport (murram road)

Ploughing, Leveling

P2 1666 High gradient hill Middle slope Rainfed arable cultivation

Ploughing

P3 1461 Valley-floor Bottom Perennial field cropping

Burning, Ploughing

3.3.2. Sampling design

Sampling was done along a transect with bulk and Kubiëna samples obtained from each of

the three profiles for physico-chemical, mineralogical and micromorpholgical analysis. All

the samples were given field identification numbers indicating the exact sampling depth

within the soil profile and then brought to the Laboratory of Soil Science, Ghent University.

The bulk samples were air dried and then passed through a 2 mm sieve to obtain the fine

earth fraction that was later used in the different physico-chemical and mineralogical

analyses. The undisturbed samples were sent to micromorphology section of the lab where

thin sections were prepared.

19

3.4. Methods

3.4.1. Physico-chemical analyses

3.4.1.1. Texture

Organic matter was destroyed by adding H2O2 to about 20 g of soil in a 1 L beaker placed on

a sand bath. Hydrogen peroxide was added until the reaction ceased. Since the soils were

derived from amphibolitic and phonolitic rocks, there was no need for destruction of

carbonates as these rocks are devoid of carbonates. The sand fraction was separated from

the clay and silt fractions by wet sieving (through a 63 µm sieve). The sand fraction was then

dried and sieved by shaking for 10 minutes in a 125, 250, 500, and 1000 µm sieve tower. The

silt and clay fractions were then dispersed using 5% sodium hexametaposphate in 1 L

sedimentation column and shaken overnight in a head-over-head shaker. The next day the

column was transferred to a water bath at a fixed temperature (25°C). The column was

shaken one more time and then 20 mL pipetted immediately, then after 5 and 30 minutes of

sedimentation. The pipetted aliquots were transferred to pre-weighed moisture tins and

dried over night at 105°C. The obtained net weights were then used to calculate the

respective silt and clay contents of the samples (van Reeuwijk, 1993).

3.4.1.2. Soil pH H2O and KCl

The pH in H2O and KCl was measured by mixing 6 g of fine earth material with 15 mL of

distilled water and KCl, respectively. Samples were then left to equilibrate for 2 hours and

then shaken occasionally. A glass-calomel combination electrode was used for the

measurement and readings recorded after values had stabilized (van Reeuwijk, 1993).

3.4.1.3. Organic carbon

The Walkley and Black method was used. The organic carbon was determined through

oxidation with 1N K2Cr7O7 (7%) in the presence of concentrated H2SO4 (96%) and back

titration using ferrous sulphate with ferroin as the indicator (Walkley and Black, 1934).

Calculation;

20

Where;

Vb: volume of FeSO4 in the blank

Vs: volume of FeSO4 in the sample

Ws: weight of sample

C: concentration of FeSO4

0.4: correction factor since only about 75% of total C is detected with this method.

3.4.1.4. Total elemental analysis

Total elemental chemical analysis was done using an alkaline fusion method (DINO ISO

14869, 2002). About 400 mg of finely ground sample was transferred to a platinum crucible.

Each sample was then pre-ignited at 850°C for 30 minutes to avoid damaging the platinum

crucible through reduction of organic matter during fusion. The sample was re-weighed

after heating to obtain the loss (of weight) on ignition (LOI). The pre-ignited sample was

then mixed thoroughly with 2 g of lithium-meta/tetra-borate powder in the platinum

crucible and fused for 15 minutes at 950°C in a pre-heated muffle furnace. The formed flux

is allowed to cool for one night, transferred to a 100 mL Teflon centrifuge tube and

dissolved in 4% nitric acid (HNO3). When the sample was completely dissolved, it was

transferred to a 100 mL volumetric plastic flask and made to volume. Blanks and standard

samples were also added to the sample series and analysed using ICP-OES.

3.4.1.5. Cation exchange capacity and exchangeable base cations

Cation exchange capacity (CEC) was measured by leaching samples with a 1 N NH4OAc at pH

7 using a mechanical vacuum extractor. A 1 g ball of filter pulp was pressed against the

bottom of a syringe tube after which 2.5 g of fine earth material was added. Two blanks

were also included. After saturating the samples with 25 mL of NH4OAc, another 45 mL of

the same solution was added and extracted overnight (16 h period) after which extracts

were transferred to a 100 mL volumetric flask. This extract was used to determine

exchangeable base cations. Excess ammonium acetate in the sample was removed by

washing the samples with ethanol repeatedly. The samples and the pulp were then

transferred to a distillation tube together with 6 g of NaCl. The distillation unit would then

add 10 mL of H2O and 20 mL of NaOH to the tube. The distilled NH4+ was collected in 50 mL

of boric acid solution. This distillate was then back-titrated using 0.05 N HCl (van Reeuwijk,

21

1993). Classification according to WRB (2006, 2014) requires that CEC is expressed per

amount of clay. Klamt and Sombroëk (1986) proposed a graphical method to correct for

organic matter content. A multiple regression analysis is used to separate the effect of the

clay component and organic matter on CEC. This method is only valid if those are the two

only components that contribute to CEC, and that CEC per g O.C and per 100 g clay is

constant. The intercept of the regression line and the y-axis is an estimation of the average

CECclay of the profile. The slope of the curve and the CEC of 1 g carbon can be used to

estimate the relative contribution of O.C to CEC.

Calculation:

They concluded that the formula of Bennema and Camargo (1979) was equally suitable for

this purpose.

The base saturation was calculated using the following formula:

3.4.1.6. Water dispersible clay (WDC)

The water dispersible clay content was determined on 10 g (weight S) of finely ground earth

(< 2 mm) without pre-treatments to remove cementing compounds and without using a

dispersing agent. After weighing the sample in a 1 L bottle, 400 mL of distilled water was

added. The bottles were then placed overnight in an end-over-end shaker at about 30 rpm.

Samples were then transferred to a 1 L sedimentation cylinder and made to volume using

distilled water. After settling for 5 and 1/2 hours, a 20 mL aliquot was pipetted from the

cylinder and transferred to tarred moisture tins. The excess water was then dried overnight

at 105°C. Samples were then removed from the drying oven, closed and cooled in a

desiccator. After cooling, samples are weighed with 0.001 g accuracy (weight R) (van

Reeuwijk, 1993). The water dispersible clay content was calculated as:

Where: r is a correction factor.

22

3.4.1.7. Acid oxalate extraction

The active short range-ordered or amorphous compounds of Fe, Al and Si were determined

using the acid oxalate extraction method. A 0.2 M acid ammonium oxalate solution buffered

at pH 3 was prepared of which 100 mL was mixed with 2 g of fine earth. Samples were

digested by shaking for 4 hours in the dark. After the digestion period, the samples were

centrifuged and supernatant solution decanted, filtered and diluted. The diluted solutions

together with two procedural blanks were analysed using the ICP-OES to determine Fe, Al

and Si concentrations (van Reeuwijk, 1993).

Calculation:

Where:

Cs: measured concentration of the element in the sample (mg/L)

Cb: measured concentration of the element in the blank (mg/L)

df: dilution factor

Ws: weight of the sample (mg)

Vox: volume of oxalate reagent used (mL)

mcf: moisture correction factor (remark: all my samples were air dried which implies that the

mcf is 1)

3.4.1.8. Dithionite-citrate-bicarbonate extraction (DCB)

The method of Mehra and Jackson (1960) was used to determine DCB extractable Si, Fe, Al

and Mn. Approximately 250 mg of fine earth was used to which a solution of sodium-citrate

and sodium-bicarbonate (at pH 7.3) was added. The solution was stirred and brought up to a

temperature of 75°C. The free oxides of Fe, and Mn were reduced using sodium-dithionite

powder added to the samples. After centrifuging, the supernatant solution was decanted

into 205 mL volumetric flasks labelled with respective numbers corresponding to the soil

samples and made up to the mark with distilled water. The concentrations of Si, Fe, and Mn

were determined using the ICP-OES.

Calculation

23

where:

Cs: measured concentration of the element in the sample (mg/L)

Cb: measured concentration of the element in the blank (mg/L)

df: dilution factor

Ws: weight of the sample (mg)

mcf: moisture correction factor (remark: all my samples were air dried which implied that

the mcf is 1)

3.4.2. Mineralogical analysis

Mineralogy was studied using the XRD method. XRD patterns were collected on a Philips

X’PERT SYSTEM with a PW 3710 based diffractometer, equipped with a Cu tube anode, a

secondary graphite beam monochromotor, a proportional xenon filled detector, and a 35

position multiple sample changer. The incident beam was automatically collimated. The

irradiated length was 12 mm. The secondary beam side comprised a 0.1 mm receiving slit, a

soller slit, and a 1° anti-scanner slit. The tube was operated at 40 kV and 30 mA, and the

XRD data were collected in a theta, 2-θ geometry from 3.00° 2θ onwards at a step of 0.020°

2θ, and a count time of 1 sec. per step. XRD patterns of oriented samples on glass slides

were recorded, before and after specific treatments. The Na+-saturated silt (2-63 µm) and

clay (0-2 µm) fractions were first recorded as powder patterns (non-concentrated).

Saturation with Ca2+ and K+ was obtained by repeated washing with 1N solutions of CaCl2

and Ca(AOc)2 or KCl and KOAc respectively. The excess of the saturating solution was

washed twice with distilled water, after which dialysis was used, until free of Cl-. Samples

were then dried using freeze-drying. Glycol solvation of the Ca2+ saturated samples was

carried out in vacuum with glycol vapour during 24 hours. The different heat treatment

(350° and 550°C) of the K+ saturated samples were always made during 2 hours (van

Reeuwijk, 1993).

3.4.3. Micromorphological analysis

Undisturbed soil samples were air-dried and impregnated with a polyester resin. After

hardening, a slice of more or less 30 µm was then polished and fixed on a glass slide

according to standard methods (Murphy, 1986). Thin sections were then analysed according

to the concepts and terminology of Stoops (Stoops, 2003).

24

CHAPTER FOUR: RESULTS

Results of morphological, physico-chemical, mineralogical and micromorphological analyses

of rock samples and selected soil profiles in the landslide-prone area are presented as

follows;

4.1. Rocks

The total elemental composition of rock samples from the respective bedrocks of the

studied soil profiles (P1 and P2) is presented in Table 2. Rock 1 is dominantly composed of

Si, considerable amounts of Al and Fe, and small amounts of Mg, Ca and Ti. Rock 2 (sampled

from the vicinity of profile 1) has a different elemental composition from that of rock 1

despite its proximity to profile 1. It is dominated by Si, Al, and Fe with low Mg, Ca, and Ti.

Rock 3 (bedrock of profile 2) has a similar elemental composition to that of rock 1. It has

small amounts of Ca and Mg with an appreciable amount of K and Na. All the rocks have

high LOI values (i.e. over 8.5%).

Thin sections of rock samples from profile 1 and 2 are presented in Figure 5 and 6

respectively. They show cracks in weathered grey amphibolitic matrix filled with unknown

material, laminated illuvial clay infillings, advanced weathering stage of amphibolitic rock

with fragments of goethite, anatase and quartz, and pseudomorph after nepheline replaced

with clay. There is evidence of weathering of rock 3 as nepheline had been replaced by

feldspars, quartz and zeolites (Figure 6). The XRD patterns (Appendix IV-1, 2 and 3) of the

rock powders show that rock 1 is dominated by amphiboles (0.841 nm peak) and quartz

(0.334, 0.426, 0.182 nm peaks); rock 2 has mica (0.988 nm), kaolinite (0.705 nm peak); rock

3 has mica, kaolinite, and quartz.

.

25

Table 2. Total elemental composition of rock samples related to the selected soil profiles

% Rock 1

(for P1)

Rock 2

( related to P1)

Rock 3

( for P2)

SiO2 44.05 34.75 45.88

Al2O3 16.52 18.82 15.17

Fe2O3 17.34 23.43 14.15

MnO 0.13 0.02 0.55

MgO 3.92 0.09 2.36

CaO 4.46 0.09 0.68

Na2O 0.51 0.00 3.06

K2O 0.40 0.08 5.73

TiO2 1.47 2.33 2.04

P2O5 0.09 0.30 0.61

LOI 10.22 19.81 8.84

TOTAL 99.14 99.72 99.08

26

Figure 5. (a) Cracks in weathered amphibolitic matrix (plane-polarised light); (b) cracks in weathered amphibolitic matrix

(crossed-polarised light), (c) grey amphibole matrix with cracks filled with unknown material (crossed-polarised light), (d)

laminated illuvial clay infilling in amphibolitic matrix (plane-polarised light), (e) advanced weathering stage of amphibolitic

rock, with fragments of goethite, anatase and quartz (plane-polarised light), (f) advanced weathering stage of amphibolitic

rock, with fragments of goethite, anatase and quartz (crossed-polarised light) for the bed rock of profile 1.

27

Figure 6. (a) Pseudomorph after nepheline replaced by clay (plane-polarised light); (b) pseudomorph after nepheline

replaced by clay (crossed-polarised light), (c) plagioclase feldspar (plane-polarised light), (d) plagioclase feldspar (crossed-

polarised light) for the bed rock of profile 2.

28

4.2. Soils

4.2.1. Soil morphological properties

Selected soil morphological properties for the three soil profiles are presented in Table 3 but

a detailed morphological description of all the profiles in the field according to FAO is

presented in appendix I.

Profile 1 (P1) was dug up to a depth of 225 cm (depth of saprolite). The colour of the surface

horizon is brown (5YR 2/3) and bright brown (7YR 5/6) in moist and dry conditions

respectively. In contrast, subsurface horizons have a dark reddish brown (5YR 3/6) and

orange (7YR 5/6) colour in moist and dry conditions respectively. The soil structure is

granular close to the surface and angular blocky in deeper horizons (Appendix I-1). The

texture is silty clay for surface horizons and clay for underlying horizons.

Profile 2 (P2) was dug up to a depth of 250 cm (depth of rock substrate). The colour of

surface horizon is brown (7.5YR 4/4) both in moist and dry conditions. However, a dark to

dull reddish brown colour (7.5YR 4/6) is observed in underlying horizons. The structure is

granular in the whole profile except for the saprolite which is massive (Appendix I-2). The

dominant texture is clay with silty clay in the BC horizon.

Profile 3 (P3) was dug up to a depth of 64 cm (depth of the water table). The colour of the

surface horizon is dark (7.5YR 4/1) and dull brown (7.5YR 5/4) in moist and dry conditions

respectively but it changes to dark yellowish brown (10YR 5/6) as depth increases. The soil

structure is granular for the entire depth of the soil profile (Appendix I-3). Texture is loam in

the surface horizon and dominantly sandy loam in the subsurface horizons.

4.2.2. Soil physico-chemical properties

Physico-chemical properties of soil samples from the selected profiles are summarized in

Table 3 and 4.

Particle size distribution

Profile 1 has an appreciable amount of clay and silt with a small sand fraction. Clay, silt and

sand contents range between 33 and 52%, 39 and 61%, 2 and 19% as lower and upper limits

of the respective fractions for the entire profile. Therefore the corresponding textural

classes are silty clay, clay and silty clay loam. The B horizon has a higher clay content

29

compared to overlying and underlying horizons. In contrast, the subsurface horizons have a

low sand content compared to surface horizons.

Profile 2 has a high clay, moderate silt and small sand content. Clay, silt and sand contents

range between 42 and 72%, 24 and 41%, 4% and 17% as the lower and upper limits of the

respective fractions for the whole soil profile. Therefore, the corresponding textural classes

are dominantly clay (for A and B horizon) and silty clay for BC horizon. The Bt horizon has a

low sand and high clay content compared to overlying and underlying horizons. Profile 3 has

a high sand content, considerable amount of silt and small proportion of clay. Clay, silt and

sand contents range between 10 and 26%, 23 and 36%, 47 and 65% as the lower and upper

limits for the respective fractions for the entire profile. Therefore, the corresponding

textural classes are loam (Ap), sandy loam and sandy clay loam (C3). The silt content

decreases with depth, but there is no systematic trend of increase or decrease in clay and

sand content with depth.

Water dispersible clay (WDC)

Water dispersible clay values range between 2 and 19%; 2 and 26%; 3 and 10% for P1, P2,

and P3, respectively. The B horizons of P1 and P2 have relatively higher WDC values

compared to overlying and underlying horizons. There is no systematic trend of increase or

decrease of WDC in P3. On average, P2 has the highest WDC contents, followed by P1 with

moderate and then P3 with the lowest contents.

Bulk density

Bulk density is between 1.04 and 1.41 Mg m-3 for the two profiles (P1 and P2) where it was

determined. Bulk density for P3 was not determined because the profile pit was buried by

the owner of the land before soil samples could be taken with Kopecky rings. In P1 and P2,

surface horizons have low bulk densities compared to underlying horizons thus there is an

increase in bulk density with depth.

30

Table 3. Morphological and physical properties of selected profiles

Profile Depth

(cm)

Color

(dry)

Color

(moist)

Particle size (%) Texture

Classa

WDC

(%)

BD

(Mg m-3) Clay Silt Sand

P1

C1 0-13 7.5YR 5/6 7.5YR 4/6 43 45 12 SiC 9 1.04

Apb 13-35 7.5YR 4/3 5YR 2/3 48 39 13 C 10 1.04

BC 35-140 7.5YR 5/6 5YR 3/6 52 40 8 C 19 1.32

CB 140-225 7.5YR 6/8 5YR 4/4 33 61 6 SiCL 2 1.24

P2

Ap 0-15 7.5YR 5/6 7.5YR 4/4 54 37 9 C 17 1.22

A 15-70 7.5YR 4/4 5YR 2/3 56 36 8 C 22 1.26

B1t 70-155 7.5YR 5/6 2.5YR 3/4 71 25 4 C 26 1.34

B2t 155-250 7.5YR 5/6 5YR 4/4 72 24 4 C 2 1.28

BC 250+ 7.5YR 7/6 7.5YR 4/6 42 41 17 SiC 2 1.41

P3

Ap 0-15 7.5YR 5/3 10 YR 3/4 17 36 47 L 6 n.d.

C1 15-29 7.5YR 5/4 7.5YR 4/1 10 28 62 SL 3 n.d.

C2 29-54 7.5YR 6/6 7.5YR 3/4 14 27 59 SL 4 n.d.

C3 54-64 7.5YR 4/6 10YR 5/4 26 26 48 SCL 10 n.d.

C4 64+ 7.5YR 4/6 10YR 5/6 12 23 65 SL 5 n.d.

SiC: Silt clay; C: Clay; L: Loam; SCL: Sandy clay loam; SL: Sandy loam; SiCL: Silt clay loam

aUSDA Soil texture classes

n.d: not determined

31

Soil pH

The average pH (H2O) and pH (KCl) for the entire transect is 6.4 and 5.24 respectively for the

horizons of P1, P2, and P3 (Table 4). Generally, soil pH of the studied profiles is slightly acidic

to neutral. pH (H2O) of P1 ranges between 5.4 and 7.08, 5.77 and 6.20 for P2; 6.66 and 7.46

for P3. Conversely, pH (KCl) ranges between 4.33 and 5.57 for P1, 4.79 and 5.09 for P2; 5.23

and 6.17 for P3. There is a general increase in pH with depth observed for all the profiles

both in water and KCl (Figure 7 b).

Organic carbon (O.C)

Organic carbon content of P1 is about 2.9% in the surface horizons and drastically decreases

with depth. P2 has slightly low organic carbon content (ca. 1.4%) in surface horizons and a

remarkably low O.C% in subsurface horizons compared to P1 and P3. P3 has the lowest

O.C% of the three profiles (P1 and P2). All the subsurface horizons have a low O.C content

(Figure 7a).

Cation exchange capacity (CEC), exchangeable base cations and base saturation

Cation exchange capacity of the profiles at the shoulder (P1) and middle slope (P2) is high

(>16 cmol (+) kg-1 soil) compared to that of P3 at the valley bottom (Figure 7c and Table 4).

CECsoil ranges between 20 and 30 cmol (+) kg-1 soil for P1; 17 and 26 cmol (+) kg-1 soil for P2;

14 and 18 cmol (+) kg-1 soil for P3. CECclay ranges between 26 and 64 cmol (+) kg-1 clay for P1;

36 and 40 cmol (+) kg-1 clay for P2; 74 and 132 cmol (+) kg-1 clay for P3.

Calcium is the dominant exchangeable cation with relatively higher amounts on the soil

exchange complex compared to magnesium, potassium and sodium (Table 4). Magnesium

(Mg2+) values on the other hand range between 1.29 and 3.70 cmol (+)/kg soil. All profiles

are almost devoid of exchangeable K+ and Na+ since their concentrations are below 1 cmol

(+)/kg soil . Base saturation of P1 and P2) is low (<50%) compared to that of P3 (BS>50%)

(Table 4). Base saturation for P1 ranges between 20 and 43%, 23 and 39% for P2 and 49 and

66% for P3.

32

Figure 7. Evolution of O.C (a), pH (b), and CEC (c) with depth for the selected soil profiles.

-300

-250

-200

-150

-100

-50

0

0 0.5 1 1.5 2 2.5 3 3.5

soil

dep

th (

cm)

O.C (%)

P1

P2

P3

-300

-250

-200

-150

-100

-50

0

4 5 6 7 8

soil

dep

th (

cm)

pH

P1

P2

P3

-300

-250

-200

-150

-100

-50

0

10 15 20 25 30 35

soil

dep

th (

cm)

CEC soil (cmol(+)/kg soil)

P1

P2

P3

(a)

(b)

(c)

33

Table 4. Chemical properties of the selected soil profiles

Profile pH H2O (1:2.5)

pH KCl (1:2.5)

O.C (%)

Exchangeable complex(cmol(+)/kg BS (%)

Ca2+ Mg2+ K+ Na+ CECsoil CECclay

P1

C1 6.35 4.95 2.93 5.53 2.48 0.88 0.08 24.46 26.22 37

Apb 6.58 5.28 2.95 7.99 2.91 1.57 0.07 30.10 35.05 42

BC 7.08 5.57 0.91 4.27 3.10 1.09 0.13 19.94 30.47 43

CB 5.40 4.33 0.02 0.08 3.70 0.57 0.09 21.06 63.55 21

P2

Ap 5.77 4.81 1.41 4.53 1.56 0.17 0.05 26.14 36.66 24

A 5.93 4.81 1.43 7.49 1.36 0.16 0.09 25.60 34.22 36

B1t 6.20 4.92 0.34 6.00 1.77 0.24 0.05 20.69 26.99 39

B2t 6.00 5.09 0.10 4.32 3.21 0.37 0.04 25.80 35.21 31

BC 5.86 4.79 0.05 1.78 1.29 0.86 0.09 17.14 40.27 23

P3

Ap 6.73 5.68 0.76 8.42 1.47 0.49 0.12 16.00 74.00 66

C1 7.28 6.03 0.86 5.88 1.43 0.43 0.08 13.51 96.00 58

C2 7.46 6.17 0.43 6.55 2.46 0.46 0.06 15.82 99.18 60

C3 7.10 5.65 0.39 7.52 2.47 0.38 0.08 17.56 60.79 59

C4 6.66 5.23 0.23 6.51 1.47 0.16 0.13 16.81 131.46 49

34

4.2.3. Selective dissolution composition

The results of selective dissolution analyses are presented in Table 5. The oxalate-

extractable Fe, Al and Si (Feo, Alo and Sio) contents are low (< 1%) for all the profiles

regardless of their respective topographic location. The dithionite extractable values for Al

and Si (i.e. Ald, Sid) are also (< 1%) low except for the dithionite extractable iron (Fed) which

is relatively high (between 3 and 7%) compared to Ald and Sid for all the profiles.

The equation ((Fed - Feo) /Fed)*100 is an estimation of the degree of crystallization of free

iron oxides (Schwertmann and Taylor, 1989). The values derived using this equation are high

(> 50%) ranging from 61 to 98%, with most of the soil horizons having values > 75%. A

general decrease in the degree of crystallinity of free iron oxides is noted with a decrease in

elevation. Very high values of over 85 and 95% are observed in P2 (middle slope) and P1

(shoulder) respectively. The dithionite extractable iron content is more (>3%) than the

active or short range order-iron (1%) content determined by acid oxalate extraction.

Additionally, an increase in iron content with depth is observed. The Alo + 1/2 Feo values

range between 0.2 and 1.0 % but overall, the majority of the values are below 1% (thus very

low).

The extracted Sid and Sio are both low (below 1%). P2 and P3 have a negative to zero

difference between the values of Sid and Sio while P1 has almost near to zero difference

between the two extractions (dithionite and oxalate). Estimated ferrihydrite contents on the

other hand range between 0.3 and 4% which values are significantly lower than those of

crystalline Fe oxides extracted by dithionite. Values for the profile at the shoulder (P1) are

higher than those of P2 (middle slope) and P3 (valley bottom).

35

Table 5. Selective dissolution analysis of the selected soil profiles (expressed in %)

Profile Depth Alo Ald Feo Fed Sio Sid ((Fed-Feo)/

Fed))*100

Alo+ 1/2Feo

Fed -

Feo

Sid -

Sio

Ferr.

P1

C1 0-13 0.1 0.4 1.5 6.4 0.1 0.1 75.9 0.9 4.9 0.1 2.6

Apb 13-35 0.2 0.4 2.4 6.2 0.1 0.1 61.5 1.4 3.8 0.0 4.0

BC 35-140 0.2 0.6 0.5 10.1 0.1 0.2 95.3 0.4 9.6 0.2 0.8

CB 140-225 0.1 0.7 0.2 9.7 0.0 0.2 98.4 0.2 9.6 0.2 0.3

P2

Ap 0-15 0.2 0.5 1.1 5.7 0.0 0.0 80.1 0.8 4.5 0.0 1.9

A 15-70 0.2 0.5 1.4 6.5 0.1 0.1 77.8 1.0 5.1 0.0 2.5

B1t 70-155 0.2 0.6 1.0 7.1 0.0 0.1 86.6 0.7 6.1 0.0 1.6

B2t 155-250 0.2 0.6 0.8 7.2 0.0 0.1 88.8 0.6 6.4 0.0 1.4

BC 250+ 0.2 0.3 0.4 3.9 0.0 0.1 89.8 0.4 3.5 0.0 0.7

P3

Ap 0-15 0.2 0.2 0.7 3.2 0.1 0.1 77.7 0.5 2.5 0.0 1.2

C1 15-29 0.3 0.2 1.1 4.4 0.3 0.1 74.9 0.9 3.3 -0.2 1.9

C2 29-54 0.3 0.3 1.4 4.8 0.2 0.1 70.6 1.0 3.4 -0.1 2.4

C3 54-64 0.2 0.3 1.1 4.0 0.1 0.1 72.1 0.7 2.9 0.0 1.9

C4 64+ 0.1 0.3 1.3 5.2 0.1 0.2 75.4 0.8 3.9 0.0 2.2

Alo, Feo, Sio= acid-oxalate-extractable Al, Fe and Si; Ald, Fed= dithionite-extractable Al and Fe;

Ferr=Ferrihydrite(%)=1.7*Feo

36

4.2.4. Soil mineralogical properties

Silt fraction

The XRD patterns of silt fractions of the different horizons from the respective soil profiles

(P1, P2, and P3) show presence of quartz (0.426, 0.334, 0.182 nm peaks), kaolinite (0.72,

0.36 nm peaks), feldspars (0.640, 0.328-0.312 nm peaks), amphiboles (0.850-0.840 nm

peak), mica (1.0, 0.5, 0.33 nm peaks) and open 2:1 phyllosilicates (1.4, 1.25 and other nm

peaks) (Figure 8, 9 and 10). The silt fraction of all profiles is dominated by quartz with very

few to few kaolinites (Table 6). P1 is devoid of feldspars but very few feldspars are present

in P2 and P3. There are no amphiboles in P2, very few in the BC horizon of P1 and few in P3.

Mica is absent in the silt fraction of P1 but a few are present in P2 and P3. The profile at the

shoulder (P1) is completely devoid of open 2:1 phyllosilicates but some are identified in P2

and P3.

Clay fraction

The XRD patterns of clay powder from the three profiles (Appendix II-1, 2 and 3) and

oriented clay samples from profile 2 (Appendix III-1, 2, 3, 4, 5) show quartz (0.426, 0.334,

0.182 nm peaks), kaolinite (0.72, 0.360 nm peak) and mica (1.0, 0.5, 0.33 nm peak). Kaolinite

and mica are dominant while quartz is common in the clay fraction of all the profiles (Table

7).

4.2.5. Soil micromorphological features

Thin sections of the B horizon of P1 (Figure 11) are characterised by illuvial clay features,

fragmented fibrous goethite coating, angular feldspar dominated rock fragments, planar

voids, quartz-amphibole dominated rock fragments and coarse kaolinite aggregates, while

the Bt horizon of P2 (Figure 12) is characterised by laminated clay infillings and infillings

related to termite activity with a crescent structure.

37

Figure 8. XRD patterns of silt powders for the respective horizons for profile 1 (all d spacing of the peaks are in nm)

38

Figure 9. XRD patterns of silt powders for the respective horizons for profile 2 (all d spacing of the peaks are in nm)

39

Figure 10. XRD patterns of silt powders for the respective horizons for profile 3 (all d-spacing of the peaks are in nm)

40

Table 6. Qualitative mineralogical composition of the silt fraction based on XRD analysis

Horizon Open 2:1 phyllosilicate

Mica Amphibole Feldspars Kaolinite Quartz

P1 C1 - - - - + +++

Apb - - (+) - + +++ BC - - - - + +++ CB - - - - + +++

P2 Ap (+) - - - (+) +++ A + - - (+) (+) +++

B1t + + - (+) (+) +++ B2t (+) + - - (+) +++ BC (+) - - (+) (+) +++

P3 Ap + + + (+) (+) +++ C1 (+) - - (+) (+) +++ C2 + + + (+) (+) +++ C3 + + + (+) (+) +++ C4 + + + (+) (+) +++

+++: many; +: few; (+) very few; -: absent.

Table 7. Qualitative mineralogical composition of the clay fraction based on XRD analysis

Horizon Open 2:1 phyllosilicate

Mica Kaolinite Quartz

P1 C1 - +++ +++ ++

Apb - +++ +++ ++ BC - +++ +++ ++ CB - +++ +++ ++

++ P2 Ap (+) +++ +++ ++ A + +++ +++ ++

B1t + +++ +++ ++ B2t (+) +++ +++ ++ BC (+) +++ +++ ++

P3 Ap + +++ +++ ++ C1 (+) +++ +++ ++ C2 + +++ +++ ++ C3 + +++ +++ ++ C4 + +++ +++ ++

+++: many; ++: common; (+): very few; -: absent

41

Figure 11. (a) Clay illuviation in the saprolite (plane-polarised light); (b) illuvial clay with incorporated coarse kaolinite

aggregates (plane-polarised light), (c) illuvial clay along planar voids in soil matrix (plane-polarised light), (d) quartz (white)-

amphibole (green) dominated rock fragment (plane-polarised light), (e) fibrous goethite coating (plane-polarised light), (f)

goethite coating consisting of radial aggregates (plane-polarised light) from Profile 1.

42

Figure 12. (a) Laminated clay infillings (plane-polarised light); (b) infilling with crescent structure related to termite activity

(plane-polarised light), (c) angular feldspar dominated rock fragment (crossed-polarised light), (d) angular feldspar

dominated rock fragment (plane-polarised light), (e) fragmented fibrous goethite coating (plane-polarised light), (f)

fragmented fibrous goethite coating (plane-polarised light) from Profile 2.

43

CHAPTER FIVE: DISCUSSION

Discussion of results is in two-fold with respect to rocks and soils. The results on soils are

discussed in relation to identifiable soil-forming processes, evolution of soil properties,

classification of selected soil profiles and susceptibility to landslides.

5.1. Rocks

All the rock samples have a low silica content (<50%), therefore can be categorized as basic

igneous rocks according to the geological classification of rocks (Table 8). However, if the

mineralogical composition level of this classification is considered, then only rock 1 qualifies

to be basic since XRD patterns of its powder (Appendix IV-1) and thin sections (Figure 5)

confirm presence of amphiboles. Additionally, its total elemental composition shows

presence of some calcium feldspars which are also a prerequisite for classification of a rock

as basic. Mineralogical classification of rock 2 and 3 is very complex since they both have

mica (Appendix IV-2, Appendix IV-3), but rock 3 additionally has a considerable amount of

sodium and potassium (Table 2) which is rare for basic rocks.

The high loss on ignition (over 8%) in all the rocks is entirely attributed to loss of water since

all of them are devoid of carbonates. This (high LOI) suggests that the rocks are weathered

since fresh rock samples are expected to have low values for LOI. According to thin section

observations and loss on ignition values, all the rocks are weathered. Based on these

observations, rock 1 can be called amphibolitic whereas rock 2 and 3 are phonolitic.

The high Na and K content in the phonolitic rocks (rock 3) suggest abundance of Na- and K-

feldspars, and mica in the rock. On the other hand, the relatively high amount of Mg and Ca

in the amphibolitic rock (rock 1) suggests a high amount of amphiboles. The presence of

relatively high values of TiO2 (>1.5%) for all the rocks can be attributed to the nature of the

parent rock (Cheng et al., 2000).

Table 8. Geological classification of rocks (Tilley, 2010).

Rock Mineralogical composition

Basic Calcium feldspar, pyroxene, olivine, amphibole, iron ores

Intermediate Sodium/potassium feldspar, some calcium feldspar, amphibole,

pyroxene, mica

Acid Sodium/potassium feldspar, quartz, amphibole, mica

44

5.2. Soils

5.2.1. Soil-forming processes

From the physico-chemical results and thin sections of the selected profiles, the following

soil-forming processes can be identified;

5.2.1.1. Leaching and clay illuviation

Leaching refers to loss of mineral and organic solutes together with percolating water in the

soil. It is often predicted by fraction of clay that disperses in water (WDC). High WDC values

indicate high susceptibility to leaching (Kjaergaard et al., 2004). Additionally, evidence of

leaching can be reflected by the amount of exchangeable base cations present on the soil

exchange complex. Results of the study show the relative abundance of respective base

cations on the soil exchange complex as follows: Ca2+ >Mg2+> K+> Na+. This implies that

divalents are more common than monovalents. Abundance of exchangeable base cations on

the exchange complex is largely affected by their solubility. In humid tropics, solubility is

linked to hydration energy (heat energy released when new bonds are formed between ions

and water molecules) and lattice energy (forces holding the crystals together) of the cations.

The extent to which hydration energy is greater than lattice energy determines whether or

not the cation will be soluble. Hydration energies of base cations are in the order; Na+>

K+>Mg2+>Ca2+. This implies that Ca2+ is sparingly soluble (low hydration energy) while Na+ is

highly soluble (high hydration energy) and this explains the high amount of Ca2+ on the

exchange complex compared to Na+ in all the profiles (Table 4). Additionally, the base

saturation of P1 and P2 located at middle and upper slope positions (where drainage is

good) is low (<50%) compared to high BS values (>50%) in P3 located at the valley bottom

(where drainage is impeded). The difference in BS is good indicator about the soil leaching

regime. Profile 1 and 2 are highly leached compared to P3 owing to low and high BS values

respectively.

There is clay illuviation in the Bt (B1t and B2t) horizons of profiles 2 evidenced by presence

of clay coatings along planar voids (Figure 11 and 12) and an increase in clay content with

depth (Table 3). The mechanical migration of clay from surface to deep horizons in a soil

profile is attributed to mobilisation of clay by percolating atmospheric precipitation and

subsequent infiltration in form of suspensions through macro soil voids. When the

45

suspensions reach deep horizons where the soil is dry, water in them is suctioned out

through micro voids of surrounding areas leading to formation of fine clay skins/argillans

having particles arranged parallel to each other and also parallel to the walls of voids. It is

these clay skins that coat walls of macro voids (Rawling, 2000). Similarly, when the

suspensions reach aggregates, water deposits clay particles on their surface as it goes

towards the interior of edaphic aggregates forming illuviation argillans that cover the

aggregate (Fedoroff, 1997). Clay migration is favoured by slightly acidic to neutral pH typical

of the studied profiles (Table 4). Under these pH conditions, clay particles are easily

dispersed irrespective of the concentration of Ca2+ on the exchange complex (Greene et al.,

1988). Furthermore, the bimodal climate (characterised by distinct dry and wet seasons) in

the area highly favours precipitation and translocation of clay during dry and wet periods

respectively. In P2, surface horizons have low bulk densities compared to subsurface

horizons (Table 3) which could as well indicate clay accumulation in the latter. The high bulk

density in subsurface can be linked to filling of pores with illuvial clay which consequently

clogs fine pores leading to compaction and reduction in pore space. All these observation

indicate that clay illuviation is an important soil forming process in the study area.

5.2.1.2. Formation of goethite and hematite

Thin sections of subsurface horizons of P1 and P2 show presence of goethite in form of

fibrous and radial coated aggregates (Figure 11 e, f; and 12 e, f). Goethite (α-FeOOH) is a

crystalline Fe oxide formed by weathering of iron bearing minerals (van der Zee et al., 2003).

The formation and transformation of Fe oxides is based on numerous chemical reactions but

the most pronounced is hydrolytic and oxidative decomposition of lithogenic containing

primary minerals mainly Fe (II) silicates (Sposito, 1989) illustrated by the equation below;

Fe2SiO4 + 1/2O2 + 3H2O → 2FeOOH + H4SiO4

In the soil environment this reaction is irreversible. The degree to which Fe (II) silicates are

transformed to goethite or hematite reflects the degree of weathering of a soil and varies

widely between weakly and strongly developed soils. The extent of transformation of Fe (II)

silicates to Fe (III) oxides can be measured using ratios of Fe in oxides and total Fe through

selective dissolution analysis (Schwertmann, 1989). The formation of crystalline Fe oxides

(usually goethite and hematite) is not practically achieved in a single step instead follows a

46

chain of reactions which yield intermediate non-crystalline Fe oxides (ferrihydrite) before

formation of crystalline Fe oxides. Ferrihydrite (a poorly crystalline Fe oxide) is the initial

product of weathering of iron rich minerals but re-crystallizes under favourable conditions

forming goethite (Schwertmann, 1985). The high amount of ferrihydrite and oxalate-

extractable Fe in P3 than dithionite-extractable Fe (Table 5), suggests that the soils have

more non-crystalline Fe oxides than crystalline Fe oxides. This is because soils of P3 are

young (Fluvisol) owing to fresh sediments received from the seasonal flooding of River

Tsutsu. However, an opposite trend is observed in P1 and P2 since their ferrihydrite values

are low probably because most of it has been converted to goethite/hematite owing to a

higher degree of weathering. Moreover, there is more dithionite-extractable Fe than

oxalate extractable Fe (Table 5) in P1 and P2 implying that crystalline Fe (e.g.

goethite/hematite) is more abundant than amorphous Fe in these soils. Goethite formation

is favoured by warm temperatures (typical for this area) and low organic carbon

(characteristic of BC and Bt horizons of P1 and P2 respectively). Additionally, the low

Feo/Fed ratio (< 0.1) in the two profiles also suggests high abundance of crystalline Fe oxide

(Torrent, 1994).

Both hematite and goethite are formed from ferrihydrite. However, the mechanism of

formation of these two Fe oxides is significantly different. Goethite crystals form in solution

from dissolved Fe (III) ions produced by the dissolution of ferrihydrite, whereas hematite

forms through an internal dehydration and rearrangement within the ferrihydrite

aggregates (Schwertmann, 1985). Therefore, goethite formation is favoured by an increase

in the concentration of Fe (III) ions in equilibrium with ferrihydrite while hematite is

favoured by a decrease in concentration. The concentration and form of Fe (III) ions in

equilibrium with ferrihydrite are strongly dependent on pH. Goethite formation is favoured

by pH values ranging between 4 and 7 while hematite is favoured by pH (>8) (Ebinger and

Schulze, 1990). Indeed the results of soil pH for the respective profiles (Table 4) are within

the pH range favouring formation of goethite and thus the yellowish brown colour of the

soils.

47

5.2.1.3. Bioturbation

Infillings with a crescent structure present in profile 2 are attributed to termite activity

(Figure 12 c) and thus illustrate the influence of soil fauna activity on soil properties.

Termites and other soil fauna play an important role in transporting and altering soil

components like soil structure formation and organic matter decomposition thereby

influencing physical and chemical processes in the soil (Castellanos-Navarrete et al., 2012).

However, effects of soil fauna on soils can be multifold and complex e.g. one group alone for

example earthworms, termites or molluscs can produce over 50 different features (Stoops,

2010). Preferential transportation of fine materials from deeper horizons to the surface by

termites during nest building creates fine textured soils at the surface and coarse textured

soils in the subsurface horizons (Van Wambeke, 1992). He further noted that termite

activity could lead to formation of well sorted soil materials, free of gravel and with a

mineralogical composition similar to that of the underlying rock. Generally, the contribution

of termite activity to soil textural evolution in the studied profiles is complex to quantify

despite the evidence of crescent infillings in the thin sections.

5.2.2. Evolution of soil morphological and physico-chemical properties in relation to

weathering

Soil morphological properties

Colour

There is a clear distinction between colour of surface and subsurface horizons of the studied

profiles. Surface horizons are darker owing to a higher O.C content than subsurface horizons

(Table 4). Conversely, later (subsurface horizons) are more yellowish brown in colour than

the former (surface horizons) (Table 3). Yellowish brown colour in soils is associated with

liberation and accumulation of free iron oxides (goethite) upon chemical weathering. This is

predominant in areas which receive high precipitation totals with alternating dry and wet

periods (typical of the study area) (Van Wambeke, 1992). Additionally, the increase in both

oxalate and dithionite-extractable-Fe contents with depth (Table 5) can also be associated

with precipitation and accumulation of Fe oxides after soluble cations have been leached to

deeper horizons. Translocation of soluble elements is enhanced by a good micro and macro

pore network system, and deep drainage typical of P1 and P2. In the past, several

48

hypotheses were suggested to account for yellowish brown and reddish colour in soils e.g.

Van Wambeke (1992) suggested that red colour in soils was strongly related to hematite

content but the difference in intensity of redness was attributed to different

hematite/goethite ratios. Additionally, he hypothesised that the red colour was linked to

preferential dissolution of hematite from existing hematite/goethite mixtures.

Soil physico-chemical properties

Particle size distribution

Textural evolution in soils is largely dependent upon the type of parent material and stage

of weathering. Parent materials are often characterized by their clay producing capacity. A

basic rock will produce more clay upon weathering than an acid rock. Soils (derived from

basic rocks) of P1 and P2 are characterized by a clayey texture (in A and B horizons) and silt

clay (in BC or CB horizon). However, soils of P3 (young) have a significant amount of sand

which is linked to high amount of coarse grained weatherable primary minerals

(amphiboles) in the profile. The abrupt decrease in clay content close to the saprolite is

attributed to high weatherability of primary minerals in the parent rock.

Soil pH

According to Landon (2014), the pH of the three profiles can be described as near to neutral.

There is a general increase in pH with depth (Figure 7b) since upper horizons receive more

precipitation than deeper horizons thus are more leached (i.e. the former has low amount

of base cations compared to the later) (Sposito, 1989). The amount of acidity in soils

depends on the stage of evolution and mineralogy of soils (Van Ranst, 1991). Conversely, in

soils with high variable charge, amount of acidity depends on ‘point of zero charge’ (PZNC)

i.e. pH at which the net total surface charge is zero. The difference (∆pH) between pH

determined in a KCl solution and distilled water (pH KCl - pH H2O) is used to estimate PZNC.

∆pH is either positive or negative but the smaller the difference, the lower the amount of

negative charge. Variation in ∆pH is a good indicator of the stage of weathering in tropical

soils. Baert (1995) suggested the following pH ranges for soils derived from basic rocks in

relation to weathering; recent stage of weathering: high pH-H2O values (6.4 to > 7) and a

∆pH ranging from -1.0 to -2.1 in the subsoil; intermediate stage of weathering: pH-H2O

49

values range from 5.5 to 6.5 and a ∆pH between -0.4 to -0.9; ultimate stage of weathering:

pH-H2O values range from 4.2 to 5.5 and a positive ∆pH. If these pH and ∆pH ranges are

considered in the determination of the stage of weathering of the studied soils, all soil

profiles would be classified as young soils but this is not true since they are deeply leached

and weathered with significant amount of Fe oxides typical of old soils . Therefore the stage

of weathering of the soils in the study area can’t be adequately explained by the obtained

pH and ∆pH values.

5.2.3. Mineralogical composition of silt and clay fraction

Clay fraction

Profile 2 (close to the scar left behind by the occurrence of 2010 Bududa landslide) was

selected for a detailed qualitative mineralogical analysis to identify specific clay minerals in

this profile. XRD patterns of oriented clay samples (from the respective horizons of P2)

saturated with K+ (and after heating at 350 and 500 °C) and Ca2+ (air dry and glycolated)

(Figure 14 and Appendix III) were used in the confirmation of constituent minerals. The

following minerals were identified;

Kaolinite

Kaolinite is commonly identified by the 0.72-0.360 nm XRD maxima if neither chlorite nor

Mg-saturated vermiculite is present. All diffractograms show rational series of OOI

reflections starting with 0.72 nm (1st order), 0.360 nm (2nd order), 0.240 (3rd order). After

heating at 550 °C, all these peaks disappear because the lattice is completely destroyed as a

result of dehydroxylation and therefore this confirmed presence of kaolinite in this soil

profile.

Mica

Micas are characterized mainly by two intense peaks in the region of 1.0 and 0.33 nm and a

relatively weaker peak at 0.5 nm. All XRD patterns show a strong first order reflection (1.0

nm) and weak second order at 0.5 nm and these are unaltered upon glycerol solvation, K

saturation and heating up to 500 °C. The weaker second order reflection might suggest the

presence of some Fe-for-Al substitution in the crystal lattice of a phyllosilicate. Fe influences

50

the relative intensities of the (OOI) diffraction lines particularly the 0.5 nm line. Most likely

the mica is type of a muscovite.

Quartz

XRD patterns show well defined, sharp and symmetric peaks at 0.334 and 0. 426 nm

confirming presence of quartz in the profile. All the profiles have high amounts of quartz

(Table 6 and 7) and there is uniform distribution with depth.

Silt fraction

XRD patterns of the silt powder (Figure 8, 9 and 10), show quartz as the dominant non-clay

mineral in all the profiles. However, profile 1 and 2 have appreciable amounts amphiboles

with traces of anatase (weak peak of 0.350nm) in P1. Anatase is usually common in strongly

weathered soils because it easily accumulates due to its low solubility.

5.2.4. Evolution of mineralogy of silt and clay fractions in relation to weathering

The presence or absence of various minerals in the silt and clay fractions of the selected

profile (P1, P2, and P3) can be attributed to weathering. It involves disintegration of primary

minerals and their reformation into new (secondary) minerals. The latter are usually formed

by dissolution or chemical alteration of minerals. The weathering process is favoured by

availability of water (medium for chemical and physical reactions), oxygen (for oxidation-

reduction reaction) and presence of cleavage planes and fissures (allow translocation of

dissolved minerals). All these factors are in abundance in the environment where the soil

samples were obtained from. The silt fraction is essentially dominated by inherited primary

minerals while the clay fraction is dominated by secondary minerals either formed in-situ or

transported from other environments. However, it is not unusual to find feldspars, mica and

quartz in the clay fraction noted Karathanasis (2006) (Table7) since they are relatively stable

to chemical weathering but can be broken down into clay sized fractions by physical

weathering. Bowen (1956) suggested a schematic series to explain evolution of minerals

(Figure 13). The minerals at the top (also referred to as the zone of rock formation) of the

illustration are first to crystallize. Similarly, the temperature gradient can be read to be from

high to low with the high temperature minerals being on the top and the low temperature

51

at the bottom. The chart can be used as an indication for the stability of minerals i.e. the

ones at the bottom being more stable and the ones at top being more susceptible to

weathering. This is because minerals are most stable in the conditions closest to those

under which they were formed i.e. minerals formed at high temperatures are more stable at

high temperatures and those formed at low temperatures are more stable at low

temperatures, and because of differences in composition i.e. minerals containing more base

cations (like Ca2+, Mg2+, K+) are more susceptible to weathering compared to minerals

having relatively more silica and or aluminium. Aside from these general principles, the

crystal structure obviously plays a very important role. The high stability of quartz and mica

(i.e. most likely muscovite) explains why they are present in the silt and clay fraction of the

studied profile. The presence of both minerals in the silt and clay fractions suggests that

they are susceptible to physical weathering but more resistant to chemical weathering (Van

Ranst, 1991). Very few feldspars are seen in the silt fraction and completely absent in the

clay fraction since they are quickly weathered before arriving in the clay fraction.

Amphiboles are present in profile 3 (Fluvisol) probably because it receives fresh sediments

rich in these minerals from seasonal flooding of Tsutsu River during rainy periods, as this

minerals should normally weather faster compared to feldspars or micas.

There are noticeable mineralogical differences between the composition of silt and clay

fractions in the studied profiles and are as follows; silt fraction; quartz > kaolinite > feldspars

> amphiboles > mica > open 2:1 phyllosilicates; clay fraction; mica, kaolinite > quartz > open

2:1 phyllosilicates. Presence of open 2:1 phyllosilicates in both silt and clay fraction suggests

chemical alteration of primary minerals. The swelling minerals seen both in profile 2 (Nitisol)

and 3 (Fluvisol) (Table 6 and 7) could be as a result of transformation of mica (biotite) to

trioctahedral smectites or weathering of amphiboles and ferromagnesian minerals in the

respective profiles.

52

Figure 13. Bowen reaction series (1956)

53

Figure 14. XRD patterns of oriented clay samples of the B2t horizon of profile 2 after K+

and Ca2+

saturation.

54

5.2.5. Classification of the selected soil profiles

Classification of all the profiles was based on the guidelines proposed in the World

Reference Base for Soil Resources (IUSS Working Group WRB, 2014).

Profile 1

Profile 1 has a deep (105 cm deep) diagnostic B (BC) horizon starting from 35 to 140cm.

Horizon BC contains some characteristics of the saprolite (C) but the properties of B

dominate over those of C. It has a clay content of 52%, CEC clay of 30.47 cmol (+) kg-1clay and

a base saturation of 43%. The silt to clay ratio is lower than 0.4. Accordingly, horizon BC is a

cambic horizon owing to evidence of alteration relative to underlying horizons.

Furthermore, the cambic horizon has silty clay to clay texture, shows evidence of

pedogenetic alteration, does not form part of another typical horizon and has a thickness of

more than 15 cm. Therefore Profile 1 is a Cambisol with dystric and rhodic suffix qualifiers.

The suffix qualifier dystric is used because the BS is less than 50% in the major part between

20 and 100 cm from the soil surface. Rhodic is used because the Munsell hue is redder than

2.5YR and the value is less than 3.5 i.e. the hue and value for the BC horizon is 5YR 3/6.

Therefore soil profile P1 is a Cambisol (Dystric, Rhodic).

Profile 2

Profile 2 has a deep (180cm deep) diagnostic Bt horizon (comprising of B1t and B2t) starting

from 70 to 250cm. The horizon has higher clay content (72%) than the overlying and

underlying horizons. Clay skins, shiny peds and strong angular blocky structure were

observed in the field indicating clay accumulation in this horizon. There is a 1% (< 20%)

change in clay content over 20 cm depth to layers immediately above and below Bt, a WDC

to total clay ratio of 0.03, a silt to clay ratio of 0.33 (<0.4), 7.2% (>4%) citrate-dithionite

extractable Fe in the fine earth fraction, 0.8% (>0.2%) acid oxalate extractable Fe in the fine

earth fraction, and 0.11 (>0.05%) ratio between active and free Fe. Accordingly, horizon Bt

is a nitic horizon since it meets all the criteria for a nitic horizon as per the guidelines of WRB

(2014). Profile 2 is therefore a Nitisol with dystric and rhodic suffix qualifiers. The suffix

qualifier dystric is used because the BS is less than 50% in the major part between 20 and

100 cm from the soil surface. Rhodic is used because the Munsell hue is redder than 2.5YR

55

and the value is less than 3.5 i.e. the hue and value for BC horizon is 5YR 3/6. Therefore soil

profile P2 is a Nitisol (Dystric, Rhodic).

Profile 3

Profile 3 has a high sand content (>45%) in the fine earth fraction because the area is

seasonally flooded when water spills over the banks of River Tsutsu during rainy seasons

thus allowing deposition of fresh coarse sediment. There is clear horizonation, gleyic color

pattern in the subsurface horizons, very high CECclay (>60 cmol (+) kg-1 clay), and high base

saturation (>50%) in all the horizons of P3. The fluctuating water table (found at 64cm)

normally creates reducing conditions seen in the profile. Profile 3 is a Gleyic Fluvisol (Eutric,

Loamic). The prefix qualifier gleyic is used due to evidence of reducing conditions at some

depth with in the profile and presence of a gleyic colour pattern. The suffix qualifier eutric is

used because the base saturation on average throughout the entire depth of the profile is

greater than 50%. Loamic is used due to a sandy loam texture in a layer ≥ 30 cm thick, within

≤ 100 cm of the mineral soil surface.

5.2.6. Implications for landslide susceptibility

Soil mineralogy

Downslope movement of slope materials (soils, rocks and debris) normally happens when

the force of gravity (major driving forces)/ shear stress exceeds the slope shear strength

(Highland and Bobrowsky, 2008). Shear strength (sum of all forces resisting downslope

movement of materials) can be attributed to frictional resistance and soil cohesion which

are both influenced by the mineralogy of the soil. Presence of swelling minerals in a soil

influences the stability and susceptibility of slope materials to sliding. Indeed XRD results

(Appendix II) for P2 (close to fresh scar from occurrence of landslide) and P3 in the valley

bottom reveal the presence of swelling minerals. Discussion of influence of mineralogy on

shear and frictional resistance of the soils is made in reference to observations of Fall et al.

(2006). Presence of swelling clay minerals in soils imposes a high plasticity on soils lowering

their resistance to deformation. Since plasticity is a result of adsorption of water in clayey

soils, heavy rainfalls can trigger higher plasticity and consequently lower shear strength.

56

Soils with swelling minerals undergo cyclic seasonal swelling and shrinking which is favoured

by wet and dry periods respectively (typical of the climate of the study area). The alternate

swelling and shrinking during wet and dry seasons creates slip planes (when moist) and

cracks (when dry). These cracks usually provide preferential infiltration of water and thus

saturation is easily reached in areas with such features. In addition, the sensitivity to

formation of erosional features (e.g. rills and gullies), becomes higher due to channelling of

runoff water. Once the soil materials become saturated, the cohesion is lowered and in case

of any trigger like an earthquake, the water pressure will increase to a point where soil

particles can readily slide relative to each other causing a landslide. This should make it clear

that landslides are complex phenomena involving an interaction of many factors and

therefore cannot be solely explained by mineralogy of slope materials. However, the

presence of swelling minerals can be a good indicator for the susceptibility of slopes to

failure.

57

CHAPTER SIX: CONCLUSIONS AND SUGGESTIONS FOR FURTHER RESEARCH

6.1. General conclusions

Soils in the landslide-prone area of Bumwalukani, Eastern Uganda, were investigated for

their physico-chemical, micromorphological and mineralogical properties to determine the

dominant soil-forming processes, and their sensitivity to landslides. The studied soil profiles

are within the same geographical setting, therefore the significant differences in their

properties cannot be linked to differences in climate. Younger soils were found close to river

in the valley bottom while well-developed soils were found in the middle and upper slope

topographic locations.

Leaching, clay eluviation and illuviation, goethite/hematite formation, and bioturbation are

the dominant soil forming processes. Accumulation of iron oxides in the middle and upper

slope profiles is an indicator of intense weathering. Additionally, the yellowish brown to

reddish brown colour of the soils can be attributed to high amounts of iron oxides (e.g.

goethite/hematite). Kaolinite and mica are the dominant minerals (typical of older soils) in

the clay fraction. Conversely, the silt fraction is dominated by quartz. Soils also have a high

clay content compared to silt and sand contents which indicates an advanced degree of

physical and chemical weathering.

Open 2:1 phyllosilicates observed in profile 2 (close to the scar of a recent land slide) and P3

suggest susceptibility of these soils to sliding following moderate to heavy rainfall events.

XRD peaks for open 2:1 phyllosilicates were not sharp in P2 possibly indicating a low amount

of these minerals in the soils. However, the fact that there is a scar from the occurrence of a

landslide in the vicinity of P2 suggests that soils can still be susceptible to sliding even at low

amounts of swelling minerals provided other contributing factors are present. Finally, the

presence of different soil types e.g. Cambisol (P1), Nitisol (P2), and Fluvisol (P3) within a

small geographic setting implies high spatial variability in soil properties within the study

area and therefore calls for caution when making generalized conclusions based on limited

soil data sets.

58

6.2. Recommendations for further research

Future studies should try to focus on the determination of area specific threshold values for

soil physical parameters (e.g. shear strength, plasticity index, and infiltration capacity) that

determine the risk of slope failures in relation to their mineralogy. Furthermore, materials

above, on, and below slip plane scars should be characterized for their physico-chemical and

mineralogical properties to gain insight on the factors leading to their formation.

59

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I

APPENDICES

Appendix I: FAO soil profile description

Appendix I-1: Profile P1

General site information

Author(s): Joseph Tamale and Amaury Defrère

Date: 07/09/2013

Profile number: UG/BU/BUM-P1

Sprofile description status: 2

Location: Bumaraka site is found in Uganda, Bududa district, Bulucheke subcounty, Bumwalukani parish

Coordinates: 01,06060°N, 034,38072°E

Elevation: 1748 m.a.s.l.

Map reference: BUDADIRI , sheet 54/4, scale 1:50.000

Soil formation factors

Climate

Atmospheric climate: The mean maximum and minimum temperatures are 23°C and 15 °C respectively. The wettest period of the year is from March till October, while the dry season starts from November till February with a short dry period around July. The mean annual rainfall is 1500 mm.

Present weather conditions: Sunny/clear (SU) at the time the profile description was made.

Former weather conditions: Rainy without heavy rain in the last 24 hours (WC4)

Landform and topography

Major landform: High gradient hill (TH)

Position: Crest (summit, CR)

Slope gradient and orientation: Flat

Land use and vegetation

Land use: Settlement, transport (ST)

Human influence: Ploughing (PL), Levelling (LV)

Vegetation: Short grass (HS)

II

Soil horizon description for profile 1

Horizon Description

C1 (0-13 cm) 7,5YR 4/6 (brown) clay loam, fine to medium sized granules, very friable when moist, soft consistence when dry, moist soil water status, common very fine to medium roots, many biological activity (earthworms and ants), medium to high porosity, very fine voids (vughs), many very fine to medium pores, clear and wavy boundary

Apb (13-35 cm) 5YR 2/3 (dark brown) sandy clay loam, few rock fragments (coarse gravel), fine to medium sized granules, friable when moist, soft consistence when dry, moist soil water status, many very fine to medium roots, common biological activity (ants and other insects), medium to high porosity, common very fine to fine pores, very fine voids (vughs), some brick pieces and charcoal artefacts, clear and wavy boundary

BC (35-140 cm) 5YR 3/6 (dark reddish brown) clay, common rock fragments (stones+coarse gravel), angular-blocky structure, firm when moist, hard consistence when dry, moist soil water status, common very fine to fine roots, common biological activity (termite nests and ant barrels), many clay coatings, compacted not cemented, fine voids (fauna-channels), low porosity, very few very fine pores, diffuse, broken and discontinuous boundary

CB (140-225 cm) 5YR 4/4 (dull reddish brown) sandy clay loam, dominant rock (saprolite) fragments, rock structure intermingled with angular blocky structured soil, extremely hard consistence when dry, very firm when moist, moist soil water status, no roots, very low porosity, fine voids (channels), very few fine pores, few clay coatings, broken compactation, abrupt, irregular boundary

R (225+ cm) Rock layer, clear and broken boundary

Profile P1 with corresponding horizons and exact location of Kubiëna box sampling

III

Appendix I-2: Profile P2

General site information

Author(s): Joseph Tamale and Amaury Defrère

Date: 17//08/2013

Profile number: UG/BU/BUN-P2

Soil profile description status: 2

Location: Bunamurembe site is found in Uganda, Bududa district, Bulucheke subcounty, Bumwalukani parish

Coordinates: 01,06089°N, 034,37907°E

Elevation: 1666 m.a.s.l.

Map reference: BUDADIRI , sheet 54/4, scale 1:50.000

Soil formation factors

Climate

Atmospheric climate: The mean maximum and minimum temperatures are 23°C and 15 °C respectively. The wettest period of the year is from March till October, while the dry season occurs from November till February with a short dry period around July. The mean annual rainfall is 1500 mm.

Present weather conditions: Sunny/clear (SU) at the time the profile description was made.

Former weather conditions: Heavier rain for some days (WC5)

Landform and topography

Major landform: High gradient hill (TH)

Position: Middle slope (MS)

Slope form: Concave + convex (CV)

Slope gradient and orientation: 35%

Land use and vegetation

Land use: Rainfed arable cultivation (AA4) and non-irrigated cultivation (AP1)

Crops: Coffee (LuCo), avocado , beans (PuBe)

Human influence: Ploughing (PL)

IV

Soil horizon description of profile 2

Horizon Description

Ap (0-15 cm) 7,5YR 4/4 (brown) clay, fine to medium sized granules, friable when moist, soft consistence when dry, moist soil water status, many very fine to medium roots, many biological activity (earthworms and ants) , medium to high porosity, common very fine to medium pores, clear and wavy boundary

A (15-70 cm) 5YR 2/3 (very dark reddish brown) clay, fine to medium sized granules, friable when wet, slightly hard consistence when dry, moist soil water status, common fine to medium roots, common biological activity, medium to high porosity, common fine to medium pores, some plastics and charcoal artefacts, clear and wavy boundary

B1t (70-155 cm) 2,5YR 3/4 (dark reddish brown) clay, fine to medium sized granules, friable when wet, slightly hard consistence when dry, moist soil water status, common fine to coarse roots, common biological activity (termite nests), medium to high porosity, common fine to medium pores, some charcoal artefacts, smooth and diffuse boundary

B2t (155-250 cm) 5YR 4/4 (dull reddish brown) clay, fine to medium sized granules, friable when wet, soft consistence when dry, moist soil water status, common fine to coarse roots, charcoal artefacts, low porosity, few fine pores, compacted and not cemented, clear and wavy boundary

BC (250+ cm) 7,5YR 4/6 (brown) and clay, granular to massive structure, hard when dry, very friable consistence when wet, moist soil water status, very few fine to coarse roots, termite nests, very low porosity, very few very fine pores, compacted and not cemented, clay coatings, clear and wavy boundary

Profile P2 with corresponding horizons and exact location of Kubiëna box sampling.

V

Appendix I-3: Profile P3

General site information Author(s): Joseph Tamale and Amaury Defrère

Date: 19/08/2013

Profile number UG/BU/MAT-P3

Soil profile description status 2

Location Mataya site is found in Uganda, Bududa district, Nakatsi subcounty, Bumwalukani parish, on the boundary with Bulucheke subcounty, 3 meters from Zuzu-river.

Coordinates 01,05234°N

034,37167°E

Elevation 1461 m.a.s.l.

Map reference BUDADIRI , sheet 54/4, scale 1:50.000

Soil formation factors

Climate

Atmospheric climate The mean maximum and minimum temperatures are 23° and 15 °C respectively. The wettest period of the year is from March till October, while the dry season occurs from November till February with a short dry period around July. The mean annual rainfall is 1500 mm.

Present weather conditions Sunny/clear (SU) at the time the profile description was made.

Former weather conditions Heavier rain for some days (WC5)

Landform and topography

Major landform Valley-floor (LV)

Position Bottom (BO)

Land use and vegetation

Land use Perennial field cropping (AP1)

Crops Sugar cane (OtSc)

Human influence Burning (BR) and Ploughing (PL)

Parent material Unconsolidated fluvial material (UF1)

Age of the land surface Young (Yn)

VI

Soil horizon description of profile 3

Horizon Description

Ap (0-15 cm) 10 YR 3/4 (dark brown) sandy clay loam, fine to coarse gravel, abundant small to big rocks, granular structure, friable when moist, soft consistence when dry, moist soil water status, many very fine to medium roots, many biological activity, charcoal artefacts, medium to high porosity, fine to coarse voids (vughs), common very fine to medium pores, smooth and clear

C1 (15-29 cm) 7,5YR 4/1 (brownish gray) sandy loam, dominant rock fragments, fine to medium sized gravel, granular structure, loose when moist, loose consistence when dry, moist soil water status, few very fine to fine roots, few biological activity (earthworm channels), high porosity, many fine to medium pores, fine voids (vughs), continuous compaction, smooth and clear boundary

C2 (29-54 cm) 7,5YR 3/4 (dark brown) sandy loam, fine to medium gravel, granular structure, loose when moist, loose consistence when dry, moist soil water status, very few, very fine roots, very high porosity, many fine to medium pores, fine voids (vughs), clay coatings on gravels, smooth and diffuse boundary

C3 (54-64 cm) 10YR 5/4 (dull yellowish brown) sandy clay loam, granular structure, friable when miost, soft consistence when dry, moist soil water status, common fine to coarse roots, very fine to fine voids (vughs), high porosity, common pores, common clay coatings, wavy abrupt boundary

C4 (64+ cm) 10YR 5/6 (yellowish brown) sandy loam, dominant rock structure, fine to medium gravel, granular structure, loose when dry, loose consistence when moist, wet water status, very few very fine roots, high porosity, many fine to coarse pores, fine to coarse voids (vughs), clay coatings, clear and wavy boundary

Profile P3 with the corresponding horizons and exact location of Kubiëna box sampling.

VII

Appendix II: XRD patterns of clay powders for the respective horizons with all d-spacings indicated in nm

Appendix II-1: Profile P1

VIII

Appendix II-2: Profile P2

IX

Appendix II-3: Profile P3

X

Appendix III: Oriented XRD patterns of the K+ and Ca2+-saturated clay samples of profile P2 (All d-spacings are indicated in nm)

Appendix III-1: Horizon Ap of P2

XI

Appendix III-2: Horizon A of P2

XII

Appendix III-3: Horizon B1t of P2

XIII

Appendix III-4: Horizon B2t of P2

XIV

Appendix IV: XRD patterns of rock powders (all d-spacings are indicated in nm)

Appendix IV-1: Rock 1

Appendix IV-2: Rock 2

XV

Appendix IV-3: Rock 3