Soil formation Factors - 1 | Agriculture Optional Notes for UPSC PDF Download

Introduction

Soil formation is a two-step process:

• The initial step involves the transformation of rock (R) into regolith.
• The subsequent step entails the development of genuine soil from regolith.

The transition from regolith to true soil occurs through the collaborative influence of soil-forming factors and processes.

• The first stage is achieved through the process of weathering, which encompasses both disintegration and decomposition.
• The second stage is linked to the impact of soil-forming factors.

Factors 

Dokuchaiev (1889) established that the soils develop as a result of the action of soil forming factors
S = f (P, Cl, O)
Further, Jenny (1941) formulated the following equation
S = f (Cl, O, R, P, T, …)
Where,
Cl – environmental climate
O – Organisms and vegetation (biosphere)
R – Relief or topography
P – Parent material
T - Time
…  - additional unspecified factors
The five soil forming factors, acting simultaneously at any point on the surface of the earth, to produce soil.

Two groups

• Passive: i) Parent material, ii) Relief, iii) Time
•  Active: iv) Climate, v) Vegetation & organism

Passive Soil forming factors 

 The passive soil-forming factors are the elements that serve as the origin of the soil-forming material and the conditions that influence it. They establish a foundation upon which the active soil-forming factors operate to facilitate the formation of soil.

Parent Material

It is that mass (consolidated material) from which the soil has formed. 

Two groups of parent material

• Sedentary: This type of parent material is formed in its original location and is considered residual parent material. It can vary widely depending on the types of rocks present.
• Transported: Parent material in this category has been moved from its original location. It is classified based on the primary force responsible for its transportation and redeposition, including:
  a) Gravity-induced - Colluvial
  b) Waterborne - Alluvial, Marine, Lacustrine
  c) Ice-driven - Glacial
  d) Wind-driven - Eolian

Colluvium: This refers to materials that are poorly sorted and found near the base of steep slopes. They are transported due to the force of gravity.

Alluvium: Alluvium consists of materials that are transported and deposited by water. These materials are typically located along major river courses and at the lower portions of mountain slopes. They can also be found along small streams that flow out of drainage basins.

Lacustrine: Lacustrine materials are composed of substances that have settled out of the calm waters of lakes.

Moraine: Moraine is made up of a variety of materials that have been collected, mixed, broken down, transported, and deposited as a result of glacial ice or the water produced primarily from the melting of glaciers.

Loess or Aeolian: These materials are transported by the wind. When the texture is silty, it is referred to as loess, and when it is sandy, it is categorized as aeolian.

Eolian: Soils that develop on such transported parent materials are named after the parent material itself, such as alluvial soils from alluvium and colluvial soils from colluvium. However, in the initial stages, soil properties are primarily determined by the type of parent material.

Endodynamomorphic soils

As soil development progresses and leaching becomes more pronounced, the impact of the parent material on soil characteristics gradually wanes. In some soils, the parent material's composition can temporarily overshadow the influences of climate and vegetation. However, these soils are transient and only persist until chemical decomposition becomes active under the influence of climate and vegetation.

Ectodynamomorphic soils

Formation of a standard soil profile is influenced by climate and vegetation.

The type of parent material significantly impacts profile development and leads to the creation of distinct soil types, particularly in the early stages of soil formation.

• Acid igneous rocks such as granite and rhyolite give rise to light-textured soils, known as Alfisols.
• Basic igneous rocks, like basalt, or alluvial and colluvial materials originating from limestone or basalt, yield fine-textured cracking clay soils called Vertisols.
• Basic alluvial or aeolian materials produce soils with a range of textures, from fine to coarse, resulting in Entisols or Inceptisols.
• The composition of rocks undergoing decay plays a crucial role in soil formation, with elements like silicon (Si) and aluminum (Al) forming the foundation for the production of secondary clay minerals.
• Iron and manganese contribute to the red coloration of soils and influence oxidation and reduction processes.
• Sodium and potassium act as important dispersing agents for clay and humus colloids.
• Calcium and magnesium have a flocculating effect, promoting favorable and stable soil structure that supports plant growth.