Soil Formation: Meaning, Factors and Processes

After reading this article you will learn about:- 1. Meaning of Soil Formation 2. Factors of Soil Formation 3. Stages 4. Processes.

Meaning of Soil Formation:

Soil formation is a process of two distinct phases:

(i) Weathering of rocks and minerals i.e. disintegration (physical) and decomposition (chemical) of rocks and minerals;

(ii) The development or the formation of true soil by some soil forming factors and pedogenic processes. The first phase of soil formation i.e. weathering is considered as a destructive one and helps to change the consolidated rocks and minerals into unconsolidated material (parent material) whereas second phase of soil formation is considered as a constructive process and develops the soil profile.

Factors of Soil Formation:

The transformation of a parent material (end product of weathering to a soil material and to a soil profile is carried out by various factors and that were first put forwarded by Dokuchaiev (1889) in the form of equation:

S = f(cl, o, r, p, t …)

where, S = Soil formation

f=Function

o = Organisms

p – Parent materials

cl = Climate

r = Relief

t = Time

Jenny then emphasized that a soil property is determined by the relative influence of all these factors. Jenny considered temperature and rainfall as climate; flora and fauna as biosphere organisms; elevation, slope/topography and depth of water table as relief.

Jenny (1941) formulated the following equations:

Active factors – Passtive factors

S = f (cl, b, – r, p, t ….)

where, b = biosphere (vegetation, organisms and man), and others are same as Dokuchaiev’s equation.

The soil forming factors are grouped into two as indicated above:

(i) Passive soil forming factors comprising of parent material, relief and time, and

(ii) Active soil forming factors consisting of climate and biosphere.

(i) Passive Soil-Forming Factors:

(a) Parent Material:

It is a most important initial soil material that determines the soil profile development as well as physical properties of soils. The parent material has been defined by Jenny as, “the state of soil system at time zero of soil formation.”

Parent materials influence soil formation with the intensity of weathering, the nutrients they contain for the utilization by the plants and the particle sizes they contain (sandstones- sandy; conglomerates-rocky; shales-clayey).

The less developed a soil is, the greater will be the effect of parent material on the properties of the soil. However, the properties of well- developed soils will also be greatly influenced by the parent material. Clay formation is favoured by parent materials containing a high percentage decomposable dark minerals and less quartz.

The parent materials may be transported from the place of their origin and re-deposited either before they become subject to alteration by soil formers or during the process of modification, or by organic deposits. They are carried from one place to another and deposited by different agencies.

When the material transported and deposited by water is known as alluvium. Colluvium is used for poorly sorted materials near the base of steep slopes transported by the action of gravity. Lacustrine deposits consist of materials that have settled in the stagnant water of lackes.

Glacial drift consists of all the materials picked up, mixed, disintegrated, transported and accumulated by the action of glacial ice or melting of glaciers caused by water. The materials blown by the wind are known as Loess (silty texture) and the Aeolian sand when these are primarily sand.

The mineralogical composition of parent material also leads to the formation of different groups of soil e.g. podzol develops on siliceous, chernozem on calcareous.

(b) Relief or Topography:

The earth’s surface contour is called topography or relief. Topography influences soil formation primarily through its associated water, .temperature, soil erosion and micro-climate relations.

Out of total rainfall or precipitation, a part of it percolates downward and a portion of it is lost as surface runoff. In high hope slope positions like hilly or mountain areas major portion of rain water is lost through runoff and it does not help much for the soil profile formation.

During runoff, the various soluble and insoluble materials are deposited at the base of the slope (at the foot hills). In this way some of the soils formed on the upper portion of a slope are washed down with carrying most of the clay fractioned and get deposited at the base of the slope with increasing clay content in the soil.

Soils on the upper slope (top of the hill or mountain) are different from the soils formed at the foot or base of hill or mountain.

Milne developed the concept of soil catena to represent group complex soils developed from a regular repetition of a sequence of soil properties in association with certain topography; the parent material may or may not be the same.

The catenary sequence shows variable drainage conditions ranging from well-drained soil (upper slope) to soils of impeded drainage conditions (at base of the slope). With the variation of topography the type of vegetation will be different and hence type of vegetation also influences the soil formation.

(c) Time:

The length of time required for soil formation or genetic horizon development depends upon many inter-related factors namely climate, nature of the parent materials and relief etc. This time span from the inception or zero point of soil development to the present stage is called age of soil. Soil formation is a very slow process. Age may vary from a few years to several thousand years.

An approximate age of soil can be assessed by radio carbon dating, pollen analysis or carbonate carbon in Caliche layers etc. Horizons tend to develop faster under warm, humid, forested conditions where there is sufficient water to move colloids.

Factors which cause delay in soil formation are extremes of cold and dry climate, impermeable and consolidated parent material (incomplete weathered rocks and minerals) high in lime and very steep slopes.

(ii) Active-Soil Forming Factors:

(a) Climate:

Climate includes rainfall, temperature humidity, aridity and wind. Different climate elements or components determine the water supply through rainfall and evapotranspiration, temperature and total amount of heat supplied through solar radiation.

Climate is perhaps the most important factor that can influence the soil formation, climate affects soil formation directly (through the supply or water and heat) and indirectly (different types of flora and fauna forming different organic matter of different natures).

For an example high temperature and moderate to high rainfall (Arid and semi-arid areas) favours the formation of lateritic acid soil (due to intense weathering and leaching of basic cations) which occurs in some parts of Midnapore, Purulia, Bankura, Burdwan and Birbhum districts of West Bengal. Soils of these regins exhibit only slight organic matter accumulation.

On the other hand, humid and cool condition (temperature climate condition) prevailing in the Northern districts (Darjeeling, Jalpaiguri, Coochbehar) of West Bengal also favours the formation of other groups of soil podzols, forests soils etc. (containing large amount of un-decomposed organic matter).

So it may be concluded that different climatic conditions also determine the different pedogenic processes and thereby helps for the formation of different groups of soil like zonal, intra-zonal and azonal soils.

(b) Biosphere:

The activity of living plants and animals and the decomposition of their organic wastes and residues have profound influence on soil formation as well as soil profile development. The different types of vegetation like forest trees and grasses having different root systems, kinds of root exudates etc. also influence the soil formation.

Vegetation particularly grasses reduce the runoff loss of water and increase soil water content which also influences the soil formation. The micro-organisms play their role in the decomposition and transformation of organic matter. The organic matter content of soils is also taken into consideration for the determination of different diagnostic soil horizons.

Burrowing animals (fauna) such as moles, earthworms, ants, termites and rodents etc. are highly important in soil formation, when they are present in large numbers. They also contribute biomass to the soil. Ants, termites and rodents carry material from lower depths upwards and even to the surface.

Such constant mixing within the profile nullifies the organic colloid and clay movements downward. Besides interactions of all these soil forming factors like topography × parent material, climates × biosphere etc. influence the soil formation.

Stages of Soil Formation:

Mohr and van Baren recognized the following five stages of soil development:

1. Initial Stage:

Un-weathered parent material.

2. Juvenile Stage:

Weathering just started, but much of the original material is still un-weathered.

3. Virile:

Easily weatherable minerals have been decomposed for the greater part, the clay content has increased and certain mellowness is discernible. The content of soil components less susceptible to weathering is still appreciable.

4. Senile:

Decomposition arrives at a final stage, and only the most resistant minerals to weathering have survived.

5. Final:

Soil developed has been completed and the parent material is completely weathered. The term young soil means the soil where the soil forming factors and pedogenic processes are still in operative conditions and changing the properties of soil in the profile and the processes have not made a prominent impression on the soil profile.

A mature soil represents a steady state of parent material (no change of parent material). So time has to relevance on the soil maturity.

In mature soil, the different soil layers or horizons are distinctly discernible. In young soil, clay from primary minerals is still being formed whereas in mature or old soil the clay is more or less in equilibrium with the primary minerals. In young soil, clay content decreases with depth whereas in case of mature soil, clay content increases with depth, accumulating at an intermediate depth.

Processes Involved in Soil Formation:

The soil forming processes or pedogenic processes are very complex because of simultaneously operating various chemical, mechanical and biological reactions. There are generally two categories of pedogenic processes involved in the formation of well-developed or mature soil i.e. basis or fundamental and specific pedogenic processes.

A. Basis or Fundamental Processes:

The basic or fundamental processes of soil formation are:

(i) Addition of water, organic and mineral matter to the soil,

(ii) Losses of these materials from the soil,

(iii) Movement or translocation of soil materials from one point to another within the soil profile, and

(iv) Transformation of the mineral and organic matter in the soil and formation of definite layers or horizons.

B. Specific Pedogenic Processes:

The basis or fundamental pedogenic processes bring about certain specific processes by means of a variety of reactions like Calcification. Decalcification, Podzolization, Laterization, Salinization, Gleization and Alkalization etc. The fundamental processes along with some important specific pedogenic processes and reactions are given in table 10.1.

Humification is the process of decomposition of organic matter and synthesis of new organic substances.

Eluviation is the mobilization and translocation of certain constituents namely clay, Fe₃O₃ , Al₂O₃, SiO₂, humus, CaCO₃ and other soluble salts etc. from one area of the soil body to the other area.

Illuviation is the immobilization and accumulation of the eluviated constituents at a depth below the soil surface.

Calcification and Gypsification are the forming processes of arid and semi-arid regions and refer to the formation and accumulation of calcium carbonate and gypsum respectively.

Argillation is a process by which the dispersed clay particles are migrated from upper to the lower soil horizons resulting a textural horizon.