Soil Formation

Soil formation

Soil formation is the process through which rocks are transformed into soil. This process involves both physical and chemical changes that alter the parent material, resulting in the development of soil. Soil formation, also called pedogenesis, begins with the weathering of parent rocks and progresses through a combination of biological, chemical, and physical processes.

Soil formation equation

Soil formation can be described by the equation:

$\text{Soil} = f(\text{parent rock material} + \text{climate} + \text{topography} + \text{organisms} + \text{time})$

Where:

• f represents the function of the interaction between these factors.
• The soil that forms depends on how these factors work together in a given environment.

Factors affecting soil formation

Soil formation is influenced by several interrelated factors. The main factors are:

Parent rock materials

The material from which soil forms is called parent rock material (regolith). It is the source of soil's mineral components. Parent materials include:

• Igneous, sedimentary, and metamorphic rocks.
• Weathering: Parent materials undergo chemical, physical, and biological breakdown processes (weathering) to form soil.

Types of parent materials

1. Mafic rocks (rich in iron and magnesium) form fertile soils with higher calcium, magnesium, and phosphorus content.
2. Felsic rocks (rich in silica) form soils with more clay and a reddish-brown color.
3. Quartzite (pure quartz) is resistant to soil formation and yields infertile soil.
4. Granite gneiss: Forms acidic soils with low base cation content.
5. Sandstones: Coarse-textured soils that are often deep and well-drained.
6. Limestone and dolomites: Create alkaline soils with carbonates.
7. Volcanic ash: Creates fertile soils known as Andisols, especially in regions with favorable temperature and moisture, such as Java, Japan, and East Africa.

Climate

Climate significantly influences soil formation by regulating temperature, precipitation, and moisture, which affect physical, chemical, and biological processes in the soil.

1. Precipitation and Temperature: These factors control weathering rates, leaching, and organic matter content.
2. Leaching: Water carries away soluble materials from the soil, which can enrich the lower layers.
3. Temperature: Warmer temperatures promote faster weathering and organic decomposition.
4. Desert and Semi-desert Areas: In such climates, temperature variations lead to rock disintegration, and wind or gravity transports fine particles.

Topography (relief)

Topography refers to the physical features of the land, including slope, elevation, and orientation.

1. Steep Slopes: These are prone to soil erosion and shallow soils due to rapid runoff, which prevents deposition.
2. Gentle Slopes: Allow for better water infiltration and deposition of materials, leading to deeper soils.
3. Influence on Soil Formation: Topography determines water movement, runoff, and soil depth. Soils on steep slopes tend to be thin and poorly developed, while those on gentle slopes are deeper and more fertile.

Living organisms (biota)

Organisms such as plants, animals, and microorganisms play a crucial role in soil formation.

1. Plant Decay: Plants decompose, enriching the soil with organic matter (humus). Microorganisms break down plant materials and convert them into nutrients.
2. Animal Activity: Larger animals (e.g., moles, gophers) burrow into the soil, mixing organic and mineral materials and aiding water and air circulation in the soil.
3. Humus Formation: Humus acts as a glue that binds soil particles together, improving soil structure.
4. Human Activities: Activities like tilling, fertilizing, and irrigation influence soil development and stability.

Time

Time is a critical factor in the development of soil profiles. It takes a long time for soil to develop fully, ranging from a few centuries to thousands of years.

1. Soil Maturity: Over time, soils evolve and develop multiple horizons (layers). The greater the number and thickness of horizons, the older and more mature the soil is.
2. Rate of Formation: Soil formation is slow; for example, it may take 400 years to form 10 mm of soil and up to 1,000 years to form 1 mm under extreme conditions.
3. Environmental Conditions: Warm and moist environments facilitate faster soil development than cold and dry regions.

Simple processes of soil formation

1. Weathering: Breakdown of rocks into smaller particles by mechanical, chemical, or biological means. Releases primary minerals and produces secondary minerals, determining soil texture, depth, and drainage.
2. Leaching: Downward movement of dissolved minerals from the topsoil (A-horizon) to the subsoil (B-horizon). Prominent in areas with high precipitation.
3. Eluviation: Movement of soil particles (organic and mineral matter) from the upper horizons to lower ones. Differs from leaching as it involves colloidal material.
4. Illuviation: Deposition of materials like clay, organic matter, and minerals in the B-horizon, often forming layers like hardpan.
5. Humification: Decomposition of organic matter to form humus, enriching the soil with nutrients and contributing to soil structure.
6. Cheluviation: Movement of soluble chelates (formed by organic acids) downward in the soil, dissolving and redistributing minerals.
7. Organic Sorting: Reorganization of mineral and organic particles by soil organisms like earthworms, improving soil structure.
8. Mineralization: Conversion of organic compounds into inorganic forms through microbial activity, releasing essential nutrients for plant growth.

Complex processes of soil formation

1. Podsolization: An intense form of leaching, common in cool, moist climates with acidic soils. Results in distinct horizons and often an iron-rich pan.
2. Laterization: Occurs in tropical climates, leading to the formation of laterite soils, characterized by high iron and aluminium content and low organic matter.
3. Calcification: Accumulation of calcium in the B-horizon in semi-arid and sub-humid climates. Soils formed through this are known as pedocals.
4. Salinization: Deposition of salts at or near the surface in arid regions due to evaporation exceeding precipitation.
5. Gleying: Formation of waterlogged, poorly drained soils, often in depressions or areas with impermeable rock. Characterized by bluish-grey soils due to reduced iron.

Soil catena

Soil catena refers to the variation in soil type along a slope due to changes in topography, leaching, erosion, and deposition. It demonstrates how soils on a slope transition from eluviation zones at the top, through translocation zones in the middle, to illuviation zones at the base. This variation also affects soil depth, pH, and drainage.

Soil as a system

The soil system is an open system with interconnected components, including inputs, processes, outputs, and recycling mechanisms:

1. Inputs: Water, mineral nutrients, organic matter, gases, and solar energy.
2. Processes: Mechanisms like weathering, leaching, humification, and mineralization that shape soil development.
3. Outputs: Losses such as water through evapotranspiration, soil nutrients through leaching and runoff, and organic matter through harvesting.
4. Recycling: Nutrients and organic matter are returned to the soil through decomposition, forming a self-sustaining nutrient cycle.