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Soil Profile: Meaning
The vertical face of the horizontal layers of the entire soil system located between the earth’s surface and base rock is collectively called the soil profile. The soil profile represents the vertical distribution of soil components. An ideal soil profile has the following characteristics:
- Generally, from top to bottom in the soil profile, there is a gradual decrease in organic matter, number of living organisms, activities of living organisms, etc.
- The amount of air present in soils decreases with depth.
- The number of minerals and their types increases from top to bottom in a soil profile.
- There is no definite trend of increase or decrease in the amount of water/moisture present in the soil from top to bottom in the soil profile because there is considerable variation in the amount of water present in the soil at different depths.
Soil strata are the levels or strata or horizons in the soil profile having specific characteristics of physical and chemical composition, organic matter, soil structure, etc. It is known that there is considerable variation in the physical, chemical, biological, abiotic, and structural characteristics of different layers of a soil profile. In the development of soil layers, the climate, vegetation, animals, and elevation & the internal processes occurring in them play an important role. It is noteworthy that time is also an important factor in the development of soil profiles.
Soil Strata or Horizons
The horizons in the soil profile are mainly divided into two categories-
- Organic horizon and
- Mineral horizon
1. Organic horizon
The uppermost layer or strata of any soil profile is called the organic horizon or carbonic horizon which is characterized by the dominance of living or dead organic matter. This layer or strata is represented by the letter O in English (O = organic). This organic layer is formed due to the accumulation of organic matter obtained from plants and animals. The organic layer (O layer) is further subdivided into two sub-layers or substrata –
- O1 layer and O2 layer.
The O1 layer or substratum is the uppermost part of the organic layer, and it is formed by the original plant material. These materials are of two types— (1) pure or fresh leaf pile and (ii) partially decomposed leaf pile. The O1 layer with fresh litter is also called the litter layer and is denoted by the letter L and the layer with partially dissociated litter is denoted by the letter F.
Just below the O1 layer is the layer O2 which contains the modified remains of plants and animals. In this O2 layer, the dead parts of plants and animals are so transformed by decomposition and pulverization that it becomes difficult to detect their (plants and animals) original forms without the help of microscopic instruments. The fermented organic matter of the O2 layer is called humus and the process of formation of the humus and development of the O2 layer is called humification.
2. Mineral horizon
It is made up of inorganic substances or minerals. These minerals are of two types – skeletal minerals like quartz and clay. Apart from these, some organic substances are also found in limited quantity in the upper part of this horizon. Substantial variations are also found in the mineral strata from top to bottom, so this horizon consists of three layers or strata A, B, and C.
There are enough variations in these three strata so that many sub-divisions are made for each layer or stratum like- A1, A2, A3: B1, B2, B3, etc. There are substantial differences in the characteristics of the A and B layers of soil profiles from one soil type to another and from one climate region to another. Different sub-layers have the following features-
A1 substratum
This is the uppermost substratum of the mineral strata which is located below the organic layer (O layer). It is a mixture of organic matter and minerals. It has more biological functions and its color is black or dark.
A2 substratum
It is a light-colored substratum. In this, minerals such as silicate, clay, iron oxide, aluminum, etc. are eluviated downwards. This sub-layer is also called the eluviation zone.
A3 substratum
This substratum is a transitional zone between the A2 and B strata, but its characteristics are more similar to those of the A stratum than the B stratum. This layer is not always found in all soils.
B1 substratum
This stratum is an interlayer between the A and B strata, but the characteristics of this substratum are more similar to those of the B stratum than those of the A stratum. This layer is not always developed in all types of soils.
B2 substratum
This stratum is characterized by the dominance of silicates, clay minerals, and organic matter. That is why this substratum is called the alluviation zone.
B3 substratum
This substratum is an interlayer between B and C strata, but its characteristics are more similar to those of the B stratum than the C stratum. This layer may or may not be present in all the soil profiles.
C Stratum
The weathered materials of the bedrock are called regolith. These substances are loose and unorganized. This layer is also called the subsurface horizon. This layer lacks the properties of A2, A3, or B2 layers. The characteristics of this layer depend on the characteristics of the base rock. This layer is also called the clay layer.
D or R stratum
The group of organized, firm, and hard base rock located below the soil is called the D or R stratum. It is known that due to differences in the characteristics of soil constituents, differences are also found within individual substratum (such as A1, A2, A3, B1, B2, etc.) of soil profiles. Therefore, small letters of the English alphabet (such as a, b, c, d, etc.) are used to indicate the specific characteristics and properties of the major strata and sub-strata of any soil section.
For example, part of the A horizon can be subclassified Aa, Ah, or Ae according to whether it is bleached (an ashy), humus rich (Ah), or eluviated (Ae). Likewise, if the B horizon has an iron pan caused by the accumulation of iron leached down from above, this can be labeled Bir. If the B or C horizons have an accumulation of calcium carbonate, they can be labeled Bca or Cca.
| Strata/Substratum | Naming | Characteristics |
| O1 | Organic Layer | Loose leaves and organic debris, largely undecomposed. |
| O2 | Organic Layer | Organic debris, partially decomposed. |
| A1 | Mineral Layer | A dark-colored horizon of mixed mineral and organic matter and with much biological activity. |
| A2 | Mineral Layer (Eluviation Zone) | A light-colored horizon of maximum eluviation. |
| A3 | Transitional Zone | Transitional to B but more like A than B, may be absent. |
| B1 | Transitional Zone | Transitional to A but more like B than A, may be absent. |
| B2 | Illuviation Zone | Maximum accumulation of silicate clay minerals or sesquioxides and organic matter. |
| B3 | Transitional Zone | Transitional to C but more like B than C, may be absent. |
| C | Regolith | Weathered parent material, occasionally absent, formation of horizons may follow weathering so closely that the A or B horizon rests on a consolidated rock. |
| R | Base Rock | A layer of consolidated rock beneath the soil. |
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