The soil on the surface of earth is found in layers and that layers of the soil is arranges during the soil formation. This layers of the soil is called horizons and the sequence and arrangement is called soil profile.
Soil profile is the vertical section of soil which has different layer of soil according to their formation.
There are total five layers if soil in which three are main layers called organic, topsoil and subsoil.
Each soil is characterized by a given sequence of horizons. Combination of this sequence is known as a soil profile which may be defined as a vertical section of soil.
It represents sequence of horizons differentiated from one another but genetically related and included to the parent material from which the soil profile is developed. These horizons above the parent material are collectively referred to as the Solum (latin word solum meaning soil).
It is a histrionic record of all the soil forming process and it forms the unit of study in pedological investigations.
Practically, a study of soil profile is important both from the standpoint of soil formation and soil development (pedology) and crop husbandry (edaphology). In deep soils, it is generally studied up to a depth of 150 cm and in others up to the parent material.
Basic concepts of soil profile
1.) The soil profile is an important tool for soil classification which is applicable for through understanding the soils.
2.) It is generally studied up to a depth of 150 cm and in others up to the parent material.
3.) The layers of soil in the soil profile which vary in thickness have different morphological characteristics which include colour, texture, structure, etc.
4.) The soil profile is an important tool in nutrient management. By examining a soil profile, soil fertility can be assessed.
Basic components of soil profile
• Organic matter
Different layers of soil
The soil profile is divided into five horizons viz. organic horizons (OL1, OL2), A, B and C horizons and all these horizons in combination are called Master Horizons. The soil profile extends from the soil surface to the parent rock material.
This is the top layers of soil. The ‘O’ groups are horizons dominated by organic material. These horizons normally occur in forest areas where decomposition of plant debris and dead animals products are found.
It can be divided into two specific horizons as follows –
OL1 – In this first layer of organic horizons (O horizon), the original forms of plants and animals can be easily recognized or identified by the naked eye.
OL2 – In this specific layer of organic horizons, the original forms of plants and animals cannot be recognized visually.
This A horizon is present at or near the surface and it consists of sun-horizons rich in organic matter intimately mixed with mineral matter. This horizon is characterized as zones of ‘washing out’ or eluviation.
This horizon can be divided into three specific horizons as follows –
A1 – It is the uppermost mineral soil horizon rich in organic matter which imparts darker colour compared to the lower horizons.
A2 – It is the horizon of washing out or eluviation of clay, iron and aluminum oxides and corresponding accumulation or resistant minerals, such as quartz. It imparts light colour than that of the upper horizons (A1).
A3 – It is transition horizon between A and B having properties more alike those of A1 and A2 that of B horizons. Sometimes it may be absent.
The B horizon is below ‘A’ horizon showing characteristics dominated features of the concentration of clay, iron, aluminum or humus alone or in combination.
This horizons are the zone of ‘washing in’ or accumulation of materials such as iron and aluminum oxides and silicate clays, from the above horizons or even from the below horizon in arid conditions as calcium carbonate (CaCO3), calcium sulphate (CaSO4) and other salt in arid zones.
This horizons is known as zone of accumulation or the illuvial horizon. This horizons sometimes are referred to as the ‘Sub-soil’ where it may not be considered as the plough layer. It can be divided into three specific horizons as follows –
B1 – It is a transition horizon between A and B having properties more nearly like B than A. It may be sometimes absent.
B2 – This horizon considers as the zone of maximum accumulation of clays and hydrous oxides. In this horizon organic matter content is generally higher than that of A2 horizon. Development of blocky or prismatic structure are found.
B3 – It is the transition horizon between B and C having properties more like those of B than C horizon.
The C horizon is the where major biological activities are minimum and which is least affect by soil forming processes. It is the unconsolidated material underlying the solum (A plus B horizon). The upper portion of the C horizon may sometimes be considered as the solum since continually weathering the erosion are going on.
- When a virgin soil is put under cultivation, the upper horizons become the furrow slice.
All the profiles may not show the sequence of horizons mentioned above. Profiles developed due to in place weathering over a sufficient period show presence of horizons ‘A’, ‘B’ and ‘C’.
Since the formation of a soil and the development of profile are dependent on the varying genetic and environmental factors, variations in horizonation are frequent and common.
Presence of well development ‘B’ horizon may be an evidence of a relatively mature soil which is commonly formed in the red and black soil showing all the ‘A’, ‘B’ and ‘C’ horizons.
Difference between surface soil and sub soil
|It is completely weathered.||It is partially weathered.|
|Surface soil is dominated by finer particles like silicate clays.||Sub soil is dominated by quartz particles and other coarse fragments of minerals.|
|The number and activity of soil microorganisms is very high.||The microbial population and their activity is very low.|
|Surface soil is porous and friable.||The sub soil is more massive and compact.|
|Cation exchange capacity is very high.||Cation exchange capacity is low.|
|Surface soil has no hard pan.||Sub soil sometimes has hard pan.|
|It has good physical management condition.||It has poor physical condition.|
|It is fertile. Most of the essential plant nutrients are present.||It is less fertile, very few essential plant nutrients are present.|
|Due to presence of high organic matter content the colour of surface soil is deep brown or dark.||The colour of sub soil is light and sometimes may be light yellowish colour depending on nature and kind of unweathered materials.|
The solum in soil science consists of the surface soil and sub soil layers that have undergone the some soil forming conditions. Solum and soils are not synonymous. Some soils include layers that are not affected by soil formation. These layers of soil are not part of the solum.
A soil that consists only of recently deposited alluvium or recently exposed soft sediment does not have a solum. Solum mainly includes, A and B horizons.
Importance of soil profile
Study of the soil profile is important because it indicates surface and subsurface characteristics and qualities, texture, structure, namely depth, drainage conditions and soil moisture relationships, which affect plant growth.
Therefore, the soil profile is taken as a unit of study whinch helps investigators not only to classify soils but also to understand soil-moisture-plant relationships.