The Genesis of soils

The Genesis of soils or pedogenesis is the study of the origin and formation of soil

There are two aspects to soil Genesis

• Soil forming factors
• Soil forming process

Soil forming factors

The Russian scientists are considered pioneers in soil Sciences. The study of soil is called pedology. Russian scientists identified the following factors as the decisive soil formation factors

1. Type of rocks
2. Rock structure and relief topography
3. Biotic factors
4. Time
5. Processes - Climate

Type of rocks

• It is the composition and hardness of rocks.
• Rock type decides the type of mineral constituents.
• Micronutrients are called as base content of the soil
• Cations - Base content, the soil is fertile if it has base content
• Rock type decides PH value soil, color, and soil texture
• Hard Rocks are responsible for coarse textured soil (Granitic)
• Soft rocks - shale limestone, etc form fine-textured soil
• The texture of soil decides the soil porosity and permeability
• Granite has low porosity and high permeability

Climate

• It decides the type, intensity, and effectiveness of weathering and denudational process in the rock
• It decides what will be the Agents of denudation.
• Humid conditions and warm conditions are ideal for fast and thick soil formation
• Cold conditions inhibit chemical processes like oxidation and reduction and mechanical processes predominate.

Example 1

In periglacial condition, actions like frost action called the congeli-fraction result in Angular fragments and soil with coarse texture. Whereas the oxidation process & hydrolysis process in warm conditions create fine-textured soil.

Example 2

Feldspar rich rocks can undergo hydrolysis to form a type of clay called kaolin. The process is called kaolinization

Rock structure and relief topography

• Relief topography - it decides the effectiveness of agents of denudation. For example, steep slopes have soil vulnerable to erosion. Plains and Flat topography have thicker soils
• Basin topography - Ill drained soils control soil formation
• Slopes and elevation also control climate and therefore also influence soil formation

Biotic factors

• It decides the mixing of air in the soil
• It decides soil texture
• It adds humus to the soil
• Nitrogen-fixing bacteria rhizobium is responsible for the enrichment of soil
• Responsible for weathering of rocks

Time

• The duration for which soil formation act
• It decides soil maturity and thickness of soil
• Older soils and mature soils have a thicker profile than younger soils and generally mature soils have better soil Horizons

The maturity and thickness of soil do not always decide soil fertility. In fact, older soils can be more leached. Because poor soil like oxisols of equatorial region where as younger soils are without horizons. Eg. Indian Alluvial soils are some of the most fertile soils of the world.

Soil forming process

Weathering process

• In situ Rock disintegration
• Gives the soil its sediments
• Decides the type of texture of soil fine course angular etc
• Indirectly decides soil porosity and permeability
• Course soil well-drained example red soil soils of India Telangana Madhya Pradesh Jharkhand not vulnerable to salinity formed due to weathering of granite gneissic rocks
• Weathering in cold condition due to Frost action angular fragments
• Chemical weathering oxidation of basaltic rocks black cotton soil problem of salinity

Capillary action

• Upward movement of water because of dry warm condition that induces evaporation
• Can result in Salt accumulation in the soils and therefore problems of soil salinity can occur
• It's more pronounced for soil that is ill drained
• Capillary action of dry regions like in Punjab, Haryana, UP, Northern Rajasthan (Sri Ganganagar), and parts of Maharashtra & Karnataka in black cotton soil track has problems of salt accumulations

Leaching

• Capillary action and leaching leads to the process called translocation i.e movement within the soil system
• Translocation is the relative displacement of nutrients within the soil profile
• Upward movement - capillary action
• Downward movement - leaching
• In leaching, the extremely fine nutrients move down with percolating waters.
• Leaching is of the following types
  • Leaching in humid warm conditions
  • Podsolization
  • Gleization

lateralization - Leaching in humid warm conditions

• Leaching is also called
• It happens in alternate wet and dry conditions and is more pronounced for the uplands.
• Therefore laterite soil is found in mountain slopes
• In lateralization, silica is more mobile so it moves down oxide of aluminium and iron are concentrated in the above superficial layers
• Laterites are poor soils and not good for agriculture and forest growth

Podsolization

• Leaching in cool and humid conditions happen in the Taiga belt in coniferous forests
• But they happen in the presence of a chemical called chelating agents
• chelating agents are the catalysts that make reactions faster are found in the needles of Pine leaves
• Here Iron and Aluminium oxides become more mobile and are leached down forming impermeable layers of sesquioxides of iron and Aluminium
• The layers are also called iron pans and silica remains behind
• These types of soils are greyish in color. Podsols - silica-rich
• Therefore they are acidic in nature and hence not fertile soils

Gleization

• It is a type of reduction reaction which happen in waterlogged condition or swamp condition
• It produces potassium and iron glauconites which give the soil the greenish-blue tinge
• The greenish-blue patches are called glei patches and the soil is called Glei soils

In soil-forming factors, and climate is considered the most important and decisive factor whereas Rock topography, relief, and time are considered as passive factors.