Soil classification

Soils are classified based on various criteria:

1. Maturity

   • Zonal soils (mature soils): Fully developed soils with distinct horizons
   • Azonal soils (immature soils): Young soils lacking developed profiles, e.g., alluvial and volcanic soils
   • Intrazonal soils: Locally influenced soils, e.g., calcimorphic and hydromorphic soils

2. Climatic disparities

   • Humid tropics soils: Lateritic soils
   • Cold region soils: Tundra soils

3. Soil forming factors: Classified based on parent rock materials and time of development

4. Soil formation processes

   • Gleying: Produces peat
   • Podsolization: Produces podsols

5. Morphological characteristics: Texture, structure, color, depth, and porosity are used to group soils

6. Presence of diagnostic horizons: Diagnostic features indicating soil formation processes

1. Morphological approach: Focuses on the physical properties of the soil profile
2. Genetic approach: Based on soil formation processes and origin
3. Integrated (morpho-genetic) approach: Combines morphological and genetic aspects

1. USDA Soil Taxonomy: Developed for interpreting soil surveys. Includes 12 soil orders, 64 sub-orders, and 300 great groups
2. FAO Legend: Represents global soil distribution and geography
3. World Reference Base (WRB): Established in 1980 to unify soil classification systems globally

Early soil classification

1. Originated from ancient societies
2. Chinese classification (Yugong): Based on texture, color, and drainage

Modern soil classification (1882–1900)

1. Initiated by Vasily V. Dokuchaev (Russian scientist)
2. Introduced global zonation of soils based on temperature, precipitation, and vegetation

Zonal soils

Mature soils influenced by climate and vegetation.

Examples:

• Arid and semi-arid soils: Sandy, light-colored, may be saline
• Cold climate soils: Tundra soils with frozen subsoil
• Tropical climate soils: Latosols and lateritic soils

Azonal soils

Young soils lacking developed profiles.

Examples:

• Alluvial soils: Found in river valleys, fertile
• Volcanic soils: Rich in nutrients, e.g., andisols

Intrazonal soils

Locally influenced soils with well-developed profiles.

Examples:

• Calcimorphic soils: Found in limestone areas
• Hydromorphic soils: Found in waterlogged areas

The USDA Soil Taxonomy is a hierarchical system that classifies soils based on diagnostic horizons, properties, and processes. It was first published in 1975 and is organized into 12 soil orders, 64 sub-orders, and approximately 300 great groups.

Alfisols

Key features:

1. Ochric or umbric epipedons (surface horizons)
2. Argillic or natric horizons (accumulation of clay)
3. High water availability for plants

Uses: Suitable for agriculture due to water and base supply.

Distribution: Found in low and middle latitudes.

Other characteristics: High clay content.

Andisols

Key features:

1. Formed from volcanic materials
2. High phosphorus retention and cation exchange capacity
3. Dominance of short-range-order minerals or Al-humus complexes

Uses: Fertile and suitable for agriculture.

Distribution: Found in volcanic regions with sufficient moisture.

Aridisols

Key features:

1. Lack of water availability for most of the year
2. Found in arid regions
3. Sparse vegetation (xerophytic plants)

Uses: Limited for agriculture without irrigation.

Distribution: Deserts and semi-arid regions.

Entisols

Key features:

1. Absence of distinct horizons due to minimal soil development
2. Found in diverse climates

Uses: Varied; depends on location and parent material.

Distribution: Recent deposits, steep slopes, or areas with slow soil formation.

Gelisols

Key features:

1. Contain permafrost within 2 meters of the surface
2. Features related to freezing and thawing

Uses: Limited agricultural potential due to frozen ground.

Distribution: Polar regions and high altitudes.

Histosols

Key features:

1. High organic matter content (>20-30%)
2. Found in waterlogged environments

Uses: Fertile if drained; supports peat or muck formation.

Distribution: Wetlands, bogs, and marshes.

Inceptisols

Key features:

1. Weakly developed horizons
2. Found in various climates except arid regions

Uses: Can support a wide range of crops.

Distribution: Diverse landscapes.

Mollisols

Key features:

1. Thick, dark surface horizons (mollic epipedons)
2. High calcium and base supply

Uses: Highly fertile, ideal for grasslands and crops.

Distribution: Temperate grasslands (steppes).

Oxisols

Key features:

1. Highly weathered soils with kaolin and free oxides
2. Low fertility

Uses: Limited without fertilization.

Distribution: Tropical and subtropical regions.

Spodosols

Key features:

1. Accumulation of humus, aluminum, and iron (spodic horizon)
2. Low nutrient availability

Uses: Limited for agriculture; commonly forest soils.

Distribution: Cool, moist environments with sandy parent material.

Ultisols

Key features:

1. Highly leached soils with argillic horizons
2. Low base saturation and cation exchange capacity

Uses: Suitable for cultivation with fertilizer application.

Distribution: Warm, humid regions.

Vertisols

Key features:

1. High clay content with shrinking and swelling properties
2. Formation of cracks and slickensides

Uses: Challenging to manage due to poor drainage but fertile.

Distribution: Tropical and subtropical regions with clay-rich parent material.

The soil classification system identifies major soil groups globally, out of which 11 major groups are found in Tanzania. These soils vary based on geological, hydrological, and meteorological factors.

Major soil types in Tanzania

Histosols

1. Formation: Decomposed organic matter in poorly drained areas
2. Regions: Kagera, Kigoma, Shinyanga, Kilimanjaro
3. Uses: Forestry, horticulture, arable farming

Andosols

1. Formation: Volcanic parent materials like ash and pumice
2. Regions: Kilimanjaro, Arusha, Mara
3. Uses: Tea, coffee, vegetables, wheat

Arenosols

1. Formation: Sandy, quartz-rich materials, prone to leaching
2. Regions: Tabora, Iringa, Mbeya, Rukwa
3. Uses: Cereal crops, groundnuts

Vertosols

1. Formation: Clay-rich parent materials, prone to shrink and swell
2. Regions: Mwanza, Tabora, Kigoma, Mara, Lindi, Coast
3. Uses: Rice, maize, cotton, sugarcane

Fluvisols

1. Formation: Alluvial deposits in flood plains
2. Regions: Rufiji, Ruaha, Ruvu, Kilombero basins
3. Uses: Annual crops, grazing, mangrove forests

Gleysols

1. Formation: Poorly drained, fluvial and lacustrine sediments
2. Regions: Mbeya, Rukwa, Morogoro
3. Uses: Dairy farming, horticulture

Leptosols

1. Formation: Thin, shallow soils on steep slopes
2. Regions: Dodoma, Lindi, Mbeya, Iringa
3. Uses: Forestry, conservation

Ferralsols

1. Formation: Deeply weathered, red to yellow soils in warm climates
2. Regions: Kigoma, Morogoro, Tanga, Kilimanjaro
3. Uses: Agriculture in humid areas

Acrisols

1. Formation: Acidic, weathered clay in hilly areas
2. Regions: Tanga, Singida, Dodoma, Mbeya
3. Uses: Limited agriculture

Solonetz

1. Formation: Sodium-rich, fine-textured soils in dry areas
2. Regions: Arusha, Dodoma, Rukwa, Mbeya
3. Uses: Limited cultivation due to salinity

Luvisols

1. Formation: Stratified deposits in gentle slopes
2. Regions: Dodoma, Arusha, Tanga, Kilimanjaro
3. Uses: Agriculture and grazing

The total area covered by these soils varies, with Cambisols (35.64%) being the most widespread, followed by Acrisols and Leptosols.

1. Facilitates global collaboration among soil scientists
2. Supports land-use planning and identifying appropriate uses (e.g., forestry vs. agriculture)
3. Aids in understanding soil management techniques like controlling erosion or improving fertility
4. Links soil properties with disciplines like geology and meteorology
5. Helps evaluate soil productivity and value

1. High costs: Requires significant resources and expertise
2. Complexity: No clear soil boundaries, leading to challenges in categorization
3. Foreign origins: Many systems don't fit local Tanzanian conditions
4. Criteria overload: Can be confusing due to multiple parameters