AGRICULTURE O LEVEL(FORM TWO) NOTES – SOILS ITS AGRICULTURE UTILIZATION

THEME 5.0 SOILS ITS AGRICULTURE UTILIZATION

SOIL FORMATION

Soil formation can be defined as the genesis or evolution of soil from the parent rock material. This process is continuous and occurs through the action of weathering processes on the parent rock materials, including physical, chemical, and biological activities. As weathering progresses, the parent rock material breaks down into smaller units, which then mix with organic matter, water, air, and living organisms to form soil.

SOIL PROFILE

If a hole about 4 meters deep is dug in the soil, several horizontal layers can be observed. These layers differ in depth, color, and organic matter content. The main horizons are:

1. O horizon – Organic horizon
2. A horizon
3. B horizon – Mineral horizon
4. C horizon

A soil showing such layers is said to be STRATIFIED, and each layer is called a STRATUM or HORIZON. The entire vertical section of these layers is called the soil profile. It represents a vertical section through the soil from the surface down to the underlying unweathered materials (or bedrock), consisting of several horizontal layers differing in physical, chemical, and biological properties.

Characteristics of Soil Profile

Soil horizons found in the soil profile are generally divided into four main groups. The number of horizons depends on the age of the soil; older soils tend to have more horizons.

I. O horizons (Organic horizons)

• Located on the surface of the soil above the mineral soil.
• Formed from remains of dead plants and animals.
• The uppermost part is called the O1 sub-horizon, and below it is the O2 sub-horizon.
• In O1, the original forms of animal and plant material can be seen with the naked eye, while in O2, the organic matter is more decomposed and the original forms are not easily visible.

II. A horizons or horizons of eluviation

These horizons lie below the O horizons and are generally leached (eluviation). They are denoted as A1, A2, and A3.

• A1 is darker in color due to organic matter.
• A2 is composed of iron oxides; iron has been washed away and deposited in the B horizons.

III. B horizons or horizons of illuviation

These horizons lie below the A horizons.

• They contain materials washed from the A and O horizons and have accumulated (illuviated). They are denoted by B1, B2, and B3.
• Iron and aluminum oxides are deposited here after being washed from the horizons above.

IV. C horizon

This is the zone of the soil where very little or no weathering has taken place.

• It consists of unconsolidated bedrock weathered slowly to give rise to soil particles.
• Below this horizon lies the consolidated bedrock.

IMPORTANCE OF SOIL PROFILE

• A good soil profile facilitates healthy plant growth.
• It allows proper drainage, aeration, and penetration of plant roots. For example, a deep and well-structured profile holds more water than a shallow one; deep-rooted plants require deep soil.
• Hard pans and impervious layers in the soil profile slow down water movement, root growth, root distribution, and air movement.

PHYSICAL PROPERTIES OF THE SOIL

This refers to the physical characteristics of soil derived from physical forces acting during soil development. These properties can be described in physical terms such as:

• Soil structure
• Soil texture
• Soil color
• Soil density
• Soil porosity
• Soil water
• Soil temperature

SOIL TEXTURE

Soil texture is the relative proportion of soil separates, i.e., sand, silt, and clay particles, or the relative proportion of soil particles of different diameters in the soil.

• Clay soil particles have diameters less than 0.002 mm.
• Silt particles range from 0.002 to 0.02 mm.
• Sand particles range from 0.02 to 2.0 mm.
• Gravel particles are larger than 2.0 mm.

Importance of Soil Texture

• Determines the rate of water movement, which varies with the fineness of the soil texture.
• Soil fertility: The finer the texture, the greater the fertility.
• Root penetration: Loosely textured surfaces facilitate root penetration.

Soil structure refers to the arrangement of individual soil particles and their aggregation within the soil profile.

Types of Soil Structure

I. Single grained structures

In this type, each soil particle is not cemented to any other particle; an example is sand.

II. Massive structure

In this structure, soil particles are cemented together to form aggregates or pads, which may take different forms. These aggregates are bound by organic matter and substances such as mucilage and gums from decomposition. Different shapes include:

• Plate-like structure (platy): Soil particles form plate-like aggregates arranged horizontally. This structure can reduce penetration of air, water, and roots and is common in horizon A.
• Prism-like structure: Aggregates arranged vertically with sharp pointed tops.
• Columnar-like structure: Prism-like structures with rounded tops.

Both blocky and granular structures are found in horizon B.

• Blocky structure: Made up of flat and upright units that fit together. It is divided into:
  • Angular blocky: Pads with angles close to 90° (right angle).
  • Sub-angular blocky: Pads with angles sharper than 90°.
• Granular structure: Loose aggregates usually 1-2 cm in diameter, characteristic of many surface soils in horizon A, especially those high in organic matter. Types include:
  • Granular-like: Non-porous aggregates.
  • Crumby-like: Porous aggregates.

Importance of Soil Structure

Soil structure influences almost all factors affecting plant growth, such as water supply, nutrient availability, microbial activity, seed germination, and root penetration.

Characteristics of a Good Soil Structure

1. Stability: The soil’s resistance to disintegration by water and physical forces like wind.
2. Facilitates porosity: Allows downward movement of water.

Improving Soil Structure

Soil structure can change under different management practices such as tillage (pouching), liming, fertilizer application, manuring, and improving drainage systems.

SOIL COLOUR

Soil color refers to the various colors present in the soil mass. Most colors occur in soil except pure blue and green. Common colors include white, yellow, grey, brown, red, and black. Soil color can indicate climatic conditions and parent materials.

Importance of Colour

• Affects soil temperature, which regulates moisture retention.
• Determines soil productivity; for example, black soils tend to be more productive than white or seed-colored soils.

SOIL TEMPERATURE

Soil temperature is extremely important as it directly affects plant growth and influences moisture, aeration, soil structure, microbial activity, decomposition of organic matter, and nutrient availability.

Low Temperature Effects

1. Slows enzyme reactions, decreasing metabolic activities.
2. Reduces plant cell permeability.
3. Retards root elongation, limiting water and nutrient uptake.
4. Affects seed germination of various species.

SOIL WATER

Water plays a vital role in the soil-plant system:

• It acts as a solvent and carrier of plant nutrients in the soil.
• Essential for photosynthesis.
• Constituent of protoplasm.
• Maintains turgidity and body temperature of plant cells.
• Affects weathering of rocks, soil formation, and soil properties.

MOISTURE CLASSIFICATION

The forces with which water is held in soils determine its availability to plants. Adhesion forces hold water strongly to mineral and humus surfaces, while cohesion forces hold water to itself less strongly. Plant roots can initially draw water held by cohesive forces but struggle to extract water held by adhesion forces as the volume decreases. Water held too tightly is unavailable to plants. The amount of available water depends on soil texture, humus content, and plant type. For example, cacti can produce greater suction than many agricultural crops.

In a loam soil with agricultural crops, when a field is flooded, it is saturated, and all air spaces are filled with water. The suction required for roots to absorb water is zero. As water drains due to gravity (called gravitational water), suction increases to about 1/3 bar, reaching field capacity. Plants must produce increasing suction to extract water, up to 15 bars, called the wilting point, where plants cannot sustain water needs and wilt. Air-dry soil occurs at 1000 bars suction, and oven-dry soil at 10,000 bars. Water below the wilting point is unavailable to plants.

SOIL DENSITY

Soil density refers to the relationship between the weight and volume of a given soil mass.

Types of Soil Density

1. Particle density (P.D.)

Particle density is the weight per unit volume of the solid portion of the soil mass. It is not affected by the arrangement of soil particles. For mineral soils, P.D. ranges from 2.40 to 2.75 g/cm³, with 2.65 g/cm³ being the accepted normal value.

2. Bulk density (B.D.)

Bulk density is the weight per unit volume of the total soil, including solids and pore spaces. It is calculated as:

• Bulk density is always less than particle density.
• Sand soils typically have bulk densities between 1.2 and 1.8 g/cm³, while peat soils have about 0.5 g/cm³.
• Bulk density decreases as soil texture becomes finer.

Porosity

Porosity refers to the percentage of soil volume occupied by water and air. Since water and air occupy the non-solid space, the arrangement of soil particles largely determines total pore space.

• Soil porosity (% pore space) is calculated as:

% solid space + % pore space = 100

% pore space = 100 – % solid space

Soil porosity varies with texture, particle shape, structure, organic matter content, and soil compaction. Sand soils have large pores but low porosity due to high particle density, while fine-textured soils have higher porosity due to lower bulk density.

TYPES OF SOILS IN TANZANIA

Common soil types found in Tanzania include:

• Volcanic soil
• Sand soil
• Clay soil
• Loam soil and clay loam soil
• Stony soil
• Floury soil
• Red plateau soils
• Alluvial soil
• Colluvial soil

Sand Soil

Sand soils consist of loosely lying particles that are easily eroded by water or wind.

Characteristics:

• Well aerated
• Easy to cultivate (light soil)
• Low water holding capacity
• Low nutrient supply in upper layers due to high leaching
• Suitable for deep-rooted crops

Common crops grown: Cashew nuts, coconuts, pineapples, etc.

Clay Soil (35% clay)

Characteristics

• Fine pores resulting in moderate to poor aeration and drainage
• High moisture retention
• Difficult to cultivate (heavy soil)

Common crops: Rice and sugar cane.

Sand Clay/Loam Sand

• This soil contains clay and sand in almost equal proportions.
• Composed of approximately 40% sand, 40% silt, and 20% clay.
• These soils are agronomically important as they are ideal for most agricultural crops.

Volcanic Soil

Found in volcanic mountains such as Kilimanjaro, Meru, Monduli, and Rungwe.

Common crops: Bananas, maize, beans, coffee, tea, and sugarcane.

Colluvial Soil

Located at the bottom of mountain slopes such as Usambara and Uluguru mountains.

Crops grown: Bananas, maize, beans, coffee, tea, and soil.

Red Plateau Soil

Found in arid and semi-arid areas, e.g., central Tanzania zones like Dodoma, Singida, and Shinyanga.

Crops grown: Sorghum, millet, groundnut, and grapes.

Alluvial Soil

Located in large river basins such as Ruaha, Rufiji, and Ruvu. These soils are very fertile.

Crops grown: Rice, maize, banana, cotton, sugarcane, tropical fruits, etc.

Floury Sand Soils

Primarily found in western Tanzania, e.g., Kigoma and Tabora. These soils are generally low in fertility or infertile.

Crops grown: Sorghum, millet, maize, groundnuts, cassava, sweet potatoes, and tobacco.