Melka Sadi-Amibara Proposed Irrigation Project Feasibility Study

Part II: Soil Science (Volume 2)

1. Introduction

The study was conducted between December 1968 and March 1969, involving:

• Preliminary photointerpretation using 1:10,000 aerial photographs
• Detailed soil classification at 1:5,000 scale
• Detailed land classification at 1:5,000 scale

Methodology: 1,682 soil profiles examined (400×400 m grid), 14 piezometers drilled, and 400 test holes sunk.

2. General Features of the Study Area

2.1 Geomorphology

Three main geomorphological units identified:

1. Recent alluvial plain: Very recent deposits with variable textures
2. Old fluviatile terraces: Two visible terraces with gravel and sand
3. Volcanic formations: Lava flows and outcrops of various ages

2.2 Drainage

Characterized by temporary streams draining to the Awash River, with uncertain drainage patterns in some areas and a swamp in the northern part.

2.3 Vegetation

Three parallel strips from river to terraces:

1. Tunnel forest along river banks
2. Scrub vegetation on alluvial plain
3. Sparse vegetation on upper terraces

2.4 Soil Formation Factors

Key factors influencing soil development:

• Parent material: Volcanic origin with variable textures
• Climate: 500-600 mm annual rainfall, mean temperature 25-26°C
• Relief: Significant impact on soil moisture and erosion
• Time: Different rates of weathering in various landscape positions
• Human influence: Overgrazing, vegetation clearing, and compaction

3. Soil Classification

Based on the U.S. Soil Survey System (7th Approximation), 12 soil mapping units identified:

Mapping Unit	Description	Area (ha)	Classification
1	Soils on very recent alluvium	2,076	Ustifluvents (Entisols)
3	Vertisols on recent alluvium	3,362	Vertic Usthortents (Entisols)
4	Non-calcareous vertisols	2,035	Chromusterts/Pellusterts (Vertisols)
5	Hydromorphic vertisols	3,710	Udic Chromusterts (Vertisols)
6	Soils with frequent texture alternations	8,251	Ustifluvents (Entisols)
9	Soils on terrace alluvium	5,803	Calciorthids (Aridisols)
10	Saline and alkali soils	1,899	Saline/alkali phases
11	Calcareous soils on pebbly terraces	451	Calciorthids (Aridisols)
12	Lithosols on volcanic rocks	193	Lithosols

4. Land Classification

Based on U.S. Bureau of Reclamation criteria, assessing suitability for sustained irrigation:

Land Class	Description	Area (ha)	Characteristics
1	Prime arable land	969	Best soils, gentle slopes, good drainage
2	Good arable land	13,124	Moderate limitations, most of alluvial plain
3	Marginal arable land	6,230	Significant limitations, restricted crop options
4	Limited arable land	457	Severe limitations, special management needed
6	Non-arable land	7,768	Unsuitable for irrigation (terraces, rocky areas)

5. Key Findings

• Most arable lands (Classes 1-3) are located on the alluvial plain (20,323 ha)
• Vertisols and Entisols dominate the alluvial plain
• Aridisols dominate the terraces
• Main limitations are salinity, soil reaction, and drainage
• About 27% of the area (7,768 ha) is classified as non-arable

6. Conclusions

The study provides detailed soil and land classification information essential for planning the irrigation project. The alluvial plain contains the most suitable lands for irrigation, while the terraces are largely unsuitable. Soil characteristics vary significantly across the landscape, requiring different management approaches for successful agricultural development.