Soil Profile

The vertical and sequential arrangement of soil layers superimposed one above the other from the top surface to the unaffected rock below is termed as soil profile. The individual layers constituting the soil profile are called soil horizons.

For instance, if we cut a vertical section of soil at a particular place, we clearly see different soil layers or horizons from top mature soil to the rock below.

These horizons are formed through the differential downward movement of various materials such as organic matter, clay particles etc. moreover soil horizons vary in properties like colour, texture, structure, thickness, chemical composition, mineral content, consistency and reaction.

In general soil profiles tend to become thicker, deeper and more altered over time.

Soil profile can be as little as 10cm thick in immature soils and as deep as several meters in tropical areas.

Soil profile is considered as one of the most important key factor for understanding the process of soil development and is the basis for their classification.

 Characteristic features of soil profile

• Upper most layer of the soil profile contains maximum amount of organic matter, living organisms and life activities.
• The quantity of minerals keeps on increasing from the top to the bottom in a profile.
• The amount of soil air goes on decreasing by increasing the depth.
• The amount of soil water is variable in different layers.
• Profiles of different types of soils differ in in physico-chemical and biological properties.
• The differences are developed from the interaction of soil farming factors as parent material, slope, weathering, native vegetation, climate etc.

  

  Soil Horizons

Soil Horizons

According to German terminology the different layers or horizons of a soil profile were grouped under three headings, A, B, and C. But according to present terminology which is in general use the soil profile consists of five main horizons O, A, B, C, and R horizon. Depending upon further variations within these O₁, O₂, A1, A2, B₁, B₂, B₃, C and D also form a part of the soil.

The ‘O’ horizons

These are the topmost horizons of soil profile commonly known as organic horizons or carbonic horizons consisting of dead and leaving organic matter. These horizons are found in forested areas specially in temperate regions but absent in grassland ecosystem. ‘O’ horizon consists of two sub-layers:

O₁ region: This is the uppermost region of the ‘O’ horizon consisting of freshly fallen leaves, twigs branches, fruits, seeds, dead remains of plants and animals. Due the presence of these undecomposed organic material this layer is also known as litter layer. This layer is present in forest soils but absent in grassland soils.

O₂ region: The layer below the O₁ or the second layer of the ‘O’ horizon is names as O₂ region.In this region the decomposition of organic matter is just begin under the action of various microorganisms such as bacteria, fungi, slime molds, actinomycetes. metabolic process such as fermentation and humification occur in this layer that is why this region is also known as F (fermentation) or H (humified zone).

• The ‘A’ horizon

‘A’ horizon is often referred to as topsoil and is considered as the mineral horizons which is formed by the accumulation of organic matter and the products of decomposition. ‘A’ horizon is also known as eluviation zone because there occurs a downward loss of soluble salts, clay, iron or aluminum due to leaching and bleaching mechanism. ‘A’ horizon provide the best environment for the growth of plant roots, microorganisms and other life. This horizon is further divided into two sub-layers.

A₁ region (‘A’ horizon): Typical dark colour is the characteristic feature of this layer due to the presence of organic matter which mixes with inorganic minerals formed as a result of weathering. This mixing phenomenon of organic matter with the inorganic product of weathering is referred to as humus which is dark brown or black in colour therefore this region is also known as humic or melanized region. This region is quite dominant in grassland as compared to forest soils.

A₂ region or ‘E’ horizon: This layer is not present in all profiles. In comparison to A₁ region this region is of light colour with less amount of organic matter. The organic matter and mineral elements are rapidly lost downward as a result of eluviation because of rapid eluviation this region is also known as E-horizon or eluvial zone or podsolic zone or zone of leaching.

• The ‘B’ horizon

These are the mineral horizons lies just beneath the ‘A’ horizon the characteristic feature of this horizon is process of illuviation that is accumulation of downward moving fine material consisting of clay minerals, iron, silicates, aluminum, manganese, and humus. Sesquioxide coating of mineral grains imparts an intense dark colour in this horizon as compared to the zones above or below it.

The colour of this zone is dark and the texture is coarse due to accumulation of above mentioned inorganic minerals. Together ‘A’, ‘E’ and ‘B’ horizons are known as solum. The ‘B’ horizon is also known as zone of illuviation or illuvial zone. The ‘B’ horizon is also sub divided into B₁, B₂ and B₃ layers.

• The ‘C’ horizon

This layer often referred to as sporolite or regolith or weathering rock and lies just beneath the ‘B’ horizon consisting of partly weathered parent material. It is devoid of organic matter.

• The ‘R’ horizon

This zone is characterized by the presence of un-weathered parent material or bed rock such as limestone, sandstone or granite. It is also known as ‘D’ horizon.