Characterisation and Classification of Soils of Aregedif Watershed, Northwestern Ethiopia

ABSTRACT

To determine the soil's primary potentials and constraints, soil characterization and pedological classification are crucial. However, much of Ethiopia’s documented soil information is scanty and inadequate relative to the large size of the country and the wide diversity of soils and landscapes. The aim of this work was to characterize and classify the morphological, physical, and chemical attributes of soils in Aregedif watershed, Northwestern Ethiopia. Samples were collected from genetic horizons of representative soil profiles to study the morphological and physicochemical characteristics and to classify the soils of the Aregedif watershed, Northwestern Ethiopia. Reconnaissance survey and extensive auger observations (70 points) of the research area were made. Seven representative pedons (2×2×1 m) were opened and described in situ, at a slope of 0-2%,2-5%, 5-10%, 10-15%, and >15%. Seventeen distributed and undisturbed soil samples were collected. Most soil profiles have a depth of >150cm deep and some of them are less than 150cm. Most profiles have granular and subangular blocky structures and textural classes vary from sandy to clayey. The surface bulk density values varied between 1.2 to 1.44 g/cm-3 and the total porosity was between 43 to 55%. The organic carbon content of the soil ranged between low to medium (0.5 -1.83%) and very low to medium (0.09-1.37%) at the surface and subsurface horizons, respectively. The total nitrogen content of the soil ranges from very low (0.04%) to moderate (0.2%) at the surface and subsurface horizon, respectively. Available phosphorous content ranged from 3.74 (low) to 8.86mg kg-1 (high) at the surface horizon and decreased down the depth. The cation exchange capacity of the soil was medium (18.8_cmol (+) kg-1 ) to high (37.8_cmol (+) kg-1 ) for all surface and subsurface horizons. Base saturation also ranged from medium to very high on both surface and subsurface horizons. The soils were found to have low to medium Zn and Cu and moderate to high Fe and Mn contents. Four soil types (Rhodic Luvisol and Pellic Vertisol (0-2%) (Profile 2 & 4), Chromic Vertisol (2-5%) (Profile 7), Vertic Luvisol and Rhodic Nitisol (5-10%), (Profile 1 & 5), Eutric Leptosol (10-15%) (Profile 3), and Skeletic Leptosol (>15%) (Profile 6) were identified based on the classification of world reference base, 2022 in the Aregedif watershed. The varying properties and types of soils identified in the study areas provide adequate information to design soil management options and further study should focus on soil landscape - land management relationships for the sustainable use of the land.

TABLE OF CONTENTS

CONTENTS Page

DECLARATION..................................................................................................... ii

ACKNOWLEDGMENTS ...................................................................................... iv

DEDICATION.......................................................................................................... v

LIST OF ABBREVIATIONS AND ACRONYMS .............................................. vi

TABLE OF CONTENTS ..................................................................................... vii

LIST OF TABLES ................................................................................................... x

LIST OF FIGURES ................................................................................................ xi

LIST OF APPENDIX TABLES .......................................................................... xii

ABSTRACT .......................................................................................................... xiii

CHAPTER 1. INTRODUCTION ........................................................................... 1

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

1.2. Statement of the Problem ............................................. 3

1.3. Objectives of the Study ............................................. 5

1.3.1. General objective .............................................. 5

1.3.2. Specific objectives ........................................ 5

1.4. Research Questions .............................................. 5

CHAPTER 2. LITERATURE REVIEW ............................................................... 6

2.1. An Overview of Soil Formation and Soil Forming Factors ............. 6

2.2. Impacts of Topography on Soil Characteristics ........... 7

2.3. Morphological Properties of Soils ............... 8

2.3.1. Soil depth ........................... 8

2.3.2. Soil color ................................................. 9

2.3.3. Soil structure and soil consistency ...................... 9

2.3.4. Soil texture ........................................... 11

2.3.5. Bulk density and total porosity ..................... 11

2.3.6. Soil moisture ........................................... 13

2.4. Soil Chemical Properties .................................... 13

2.4.1. Soil reaction (pH)................................... 14

2.4.2. Electrical conductivity ................................. 15

2.4.3. Total nitrogen, organic carbon, and C/N ratio ................. 15

2.4.4. Available phosphorous ................................... 17

2.4.5. Cation exchange capacity ................................. 18

2.4.6. Exchangeable bases (Ca, Mg, K, Na) and percent of base saturation (PBS)..... 19

2.4.7. Exchangeable aluminum and exchangeable acidity ................. 20

2.4.8. Micronutrients ............................................. 21

2.5. Major Soils of Ethiopia .............................................. 21

2.6. Soil Characterization, Classification, and Mapping in Ethiopia ............ 22

2.7. Significance of Soils Description and Classification .................. 23

CHAPTER 3. MATERIALS AND METHODS ................................................. 24

3.1. Description of the Study Area ................................. 24

3.1.1. Location .............................................. 24

3.1.2. Climate and agroecology ........................................... 25

3.1.3. Topography and parent materials ................................... 25

3.1.4. Vegetation and land use .................................................. 26

3.1.5. Agriculture and farming system ....................................... 27

3.2 Methods ............................................................... 27

3.2.1 Field Survey, Site Selection, and Sampling...................................... 27

3.3. Soil Sample Preparation and Soil Morphological Description...................... 29

3.4. Laboratory Analysis ............................................................ 30

3.4.1. Soil physical properties .............................................. 30

3.4.2. Soil chemical analysis ......................................................... 31

3.5. Data Analysis ...................................................... 32

CHAPTER 4. RESULT AND DISCUSSION ...................................................... 33

4.1. Morphological Features .................................................... 33

4.1.1. Soil depth and horizon ............................................... 44

4.1.2. Soil color .................................................... 45

4.1.3. Soil structure, soil consistency, and horizon boundary .................. 46

4.2. Soil Physical Properties ........................................ 50

4.2.1. Particle size analysis ...................................................... 50

4.2.2. Silt to clay ratio ............................................................... 51

4.2.3. Bulk density and total porosity ........................................... 52

4.2.4. Soil moisture characteristics of the soils......................................... 54

4.3. Soil Chemical Properties ........................................................... 56

4.3.1. Soil pH and electric conductivity...................................................... 56

4.3.2. Available phosphorus concentrations .................................. 57

4.3.3. Exchangeable acidity ............................................ 59

4.3.4. Organic carbon, total nitrogen, and C: N ratio ......................... 61

4.3.5. Cation exchange capacity ............................................. 64

4.3.6. Exchangeable base and percentage base saturation ........................ 66

4.3.7 Extractable micronutrients ......................................... 71

4.4. Redoximorphic Features .................................................. 75

4.5. Soil classification of the Study Site ..................................... 75

CHAPTER 5. CONCLUSION AND RECOMMENDATION .......................... 79

5.1. Conclusions ................................................ 79

5.2. Recommendations ..................................... 79

REFERENCES ....................................................................................................... 81

APPENDIX ........................................................................................................... 109

Biographical sketch .............................................................................................. 116