Soil Formation, Profile & Characteristics

This article deals with ‘Soil Formation, Profile & Characteristics ’ This is part of our series on ‘Geography’ which is an important pillar of the GS-1 syllabus. For more articles, you can click here

Introduction

Soil is the uppermost layer of the earth’s crust which can support plant life, and is usually composed of minerals, organic matter, living organisms, air and water.
Naturally occurring soil is influenced by the following factors

Parent rock	Climate
Relief	Physical, Chemical & Biological agents
Land use practice	Time

How is Soil formed?

Soil formation begins with volcanic sedimentary or metamorphic rock material and can be seen in operation at an early stage on a recently formed volcanic island.
Rock is colonized by plants, suited to bare rocks like lichens and mosses, happens.
It is followed by the decay of plant material and the development of a thin organic layer on the rock.
Wind, rain, snow and freezing cause erosion and rock fracturing, leading to more colonization and physical breakdown of the rock materials.
After thousands of years, the upper layer of rocks will be converted into soils of many types.

Soil Profile

Soil development begins with the colonization of parent rock by plants and animals. Due to various factors of erosion and transportation, different layers with different physical and chemical properties are formed. These are known as soil profile or horizons.

O-Horizon

O-horizon is very common to surfaces with lots of vegetative cover. It is the layer made up of organic materials such as dead leaves and surface organisms, twigs and fallen trees.

A-Horizon

A-Horizon is called Top Soil.
It is rich in minerals as well as humus.
Humus is generated by the decay of organic matter in the O horizon is carried downward by percolating water to enrich the A horizon.
A horizon is dark because of a concentration of decomposed organic matter.

E-Horizon

E-Horizon is usually lighter in colour
This layer is rich in nutrients which are leached downwards from A and O horizons.

B-Horizon

It is called Subsoil.
It lies below the E-horizon
Most of the nutrients taken from the A and E horizons are deposited in this zone of accumulation.
B horizon generally has little humus.

C-Horizon

C-Horizon is the weathered parent rock.
This layer is the first step in the development of soil and eventually gives rise to the top two layers.

R-Horizon

It is the unweathered parent rock.

Factors affecting Soil formation

Parent Rocks

Soil is formed by the erosion of the parent rock. Hence, soil inherits many properties from the parent material.
E.g., In Deccan Plateau, Black coloured soil is found as it is derived from lava rocks. At the same time, the alluvial soil of the Northern plains is different from in-situ rocks as its parent rocks are Himalayan rocks.

Climate

Soil formation also depends on how long these have been affected by a particular climate. E.g.,

Precipitation impacts the soil in the following ways
- In regions receiving high precipitation, nutrients leach downward, reducing soil fertility (lateritization and podzolization).
- Salts accumulate in the soil in arid regions due to excessive evaporation and capillary action.
Temperature directly affects the activity of soil microorganisms. Hence,
- In the cool regions (between 0 to 10 °C), there is retarded microorganism activity. Thus, the decomposition of organic material occurs at a lower rate, and thick O-Horizon forms due to the accumulation of organic matter. This organic material turns to humus, which percolates downwards, resulting in organic matter in A Horizon as well.
- In warm & wet tropics, microorganisms rapidly decompose the organic material. Hence, O-Horizon cannot develop properly, and the whole posit profile lacks organic matter.
The density of vegetation depends on the climate of the region, which impacts soil formation.
1. Dense vegetation in a wet climate
  - The dense vegetative cover protects soil from being removed through erosion by running water or wind. Hence, a thick layer of soil develops in these regions.
  - Forests form a protective canopy which prevents heat and rain from directly impacting soil.
  - In areas of dense vegetation, the moisture content in the soil is high due to the action of roots
2. Sparse vegetation in an arid climate
  - In the case of sparse vegetation, soil evaporation will be more than in the case of thick protective vegetation. This evaporation, in turn, increases the movement of capillary water toward the surface leading to higher soil salinity.

Relief

The run-off is fast in hilly areas with steep slopes, and the soil keeps sliding. Hence, soil horizons are thick on gentle slopes and thin on steep slopes.
Along with that, slopes that face away from the Sun have cool-moist soil as they are sheltered from direct insulation. On the other hand, slopes that face the Sun receive direct solar rays, resulting in increased evapotranspiration.

Biota

Vegetation is an important factor as
1. It acts as a source of organic matter and humus in the soil. Hence, some of America’s richest soils developed in the Middle West prairies under a thick grass cover.
2. Roots of vegetation help in breaking the rocks
3. Legumes and certain other plants help in the fixation of atmospheric nitrogen in the soil

Many organisms, from bacteria to burrowing animals, are found in soil. E.g., Earthworms continuously repair the soil by burrowing and moving it through their intestines. They ingest large amounts of decaying leaf matter, carry it down from the surface, and incorporate it into the mineral soil horizons.

Time

Time is of great importance in the development of soil. Humus and organic matter are added to the soil every year, and soil fertility continues to increase. However, the process continues for thousands and thousands of years before good-quality soil is formed.

Transportation of Minerals in Soil

Laterization

Laterization is a soil-forming regime that occurs in humid tropical areas with high temperatures and abundant precipitation.
The soil in this type of climate does not have an O-horizon (because of the rapid decomposition of organic material by microorganisms), and most of the minerals and silica of A horizon are leached downwards except for iron and aluminium compounds, which are insoluble primarily because of the absence of organic acids.
B horizon of Laterite soils is enriched in all the leached materials like silica.
Lateritic soils are generally reddish in colour due to the presence of iron oxides

Podzolization

Podzolization occurs mainly in the high middle latitudes where the climate is moist and cool.
The soil in this type of climate has a thick O-horizon because, due to low temperatures, microorganism activity is reduced enough that humus accumulates. When water percolates downwards through the O-Horizon, it becomes acidic due to the formation of an Organic Acidic solution.
Leaching by organic acidic solutions removes the soluble bases and aluminium and iron compounds from the A horizon leaving behind silica. The remaining silica gives a distinctive ash-grey colour.

Calcification

In contrast to laterization and podzolization, which require humid climates, this occurs in arid regions with high temperatures where evaporation significantly exceeds precipitation.
In arid regions, B horizon has high levels of Calcium. When water is evaporated from the upper layers of soil in these regions, water from the lower layers of soil mixed with Calcium comes up by capillary action. Hence, the proportion of calcium salts increases and soil salinity increases.

Characteristics of Soil

1. Colour

Soil colour might not be the most important attribute of soil, but it is certainly the most visible.
A soil’s colour offers a clue to its physical and chemical characteristics
E.g.:
1. Humus is black or brown, and soils with a high humus content tend to be dark. As soil’s humus content decreases because of either low organic activity or its loss by high microbiological activities, soil colours gradually fade to light brown or grey.
2. Reddish soil usually indicates that iron is present in the soil.
3. In cold-moist climates, a light grey soil indicates that iron has been leached out, leaving oxides of silicon

2. Texture

oil texture refers to the size of grains that make up soil. Soil grains can be of the following types

Clayey	Grains have a diameter of less than 0.002 millimetres
Silt	Grains have a diameter between 0.002 to 0.05 millimetres
Sandy	Grains have a diameter between 0.05 to 2.0 millimetres
Gravel / Pebble	Grains have a diameter above 5 millimetres

Soil texture helps determine a soil’s capacity to retain the moisture and air necessary for plant growth.

Soils with a higher proportion of larger grains (i.e. sandy soils) have many small passages between touching mineral grains, which allow water to seep downwards. They are well aerated as the space between the grains is occupied by air. But they allow water to seep through so quickly that plants cannot use it.
Clay soils present the opposite problem. They have fine particles, resulting in smaller passages and spaces, so water penetrates slowly, and soils become waterlogged. But at the same time, they are deficient in air

Soils can be classified based on their proportion of sand, silt, and clay. Loam soils occupy the central areas of the triangular diagram. Loam is soil that contains a proportion of each of the three grades and, as a result, is most effective at storing moisture and air. This mixture is often added to gardens because it is particularly suitable for plant growth. Depending on which grade is dominant, loams can be further classified as sandy, silty, or clay-rich.

3. Soil Structure

oil structure refers to the way soil grains are clumped together into larger masses called peds.
Along with soil texture, soil structure also determines the permeability of water. Soils with the same texture but different structures can have different water permeability.
Soil structure can be of the following type

Granular structure	– Small peds, shaped roughly like spheres, give the soil a granular structure. – They are loosely packed and found in the surface layers, along with organic material and roots.
Blocky Structure	– Larger peds form an angular, blocky structure – These are typical of B-horizons
Palty structure	Platy structure tends to indicate compaction of the soil by animals or human activities
Columnar structure	A columnar structure can occur in arid or sandy soils.

4. Soil Acidity and Alkalinity

Soil pH is an important indicator of soil fertility as soil acidity, or alkalinity helps determine nutrient availability for plant growth.

Why is soil acidity important for the fertility of soil?

Since plants receive virtually all their nutrients in solution, they can only absorb nutrients dissolved in liquid. However, if the soil moisture lacks some degree of acidity or is basicity, soil water has little ability to dissolve these nutrients. As a result, even though nutrients are in the soil, plants may not have access to them.
To correct alkalinity, which is common in the soils of arid regions, farmers can flush the soil with irrigation water.
Strongly acidic soils are also detrimental to plant growth. In acidic soils, soil moisture dissolves nutrients, but they may be leached away before plant roots can absorb them. Soil acidity can be corrected by adding lime to the soil.