Notes on Soil Profile (With Diagram)

Soil profile may be defined as a vertical section through a soil. It represents sequence of horizons or layers differentiated from one another but genetically related and included to the parent material from which the soil profile is developed. A soil profile is an historic record of all the soil forming processes and it forms the unit of study in pedological investigations.

Practically, the soil profile is an important tool for soil classification which is applicable for thorough understanding the soils. The soil profile is an important tool in nutrient management. By examining a soil profile, soil fertility can be assessed. As the soil weathers and/or organic matter decomposes, the profile of the soil changes.

For instance, a highly weathered infertile soil usually contains a light coloured layer in the sub-surface soil from which nutrient have leached away. On the other hand, a highly fertile soil often has a deep surface layer that contains high amounts of organic matter. In fact, the soil profile is made up of distinct layers, known as horizons.

The soil profile is divided into five (5) horizons viz. organic horizons (OL₁, OL₂), A, B and C horizons. Soil scientists the capital letters O, A, B, C and E (elurial horizon) to identify the Master horizon and lower case letters for distinctions of these horizons. Such five most common horizons are collectively known as the “Master Horizons” the sub­surface horizons.

A soil horizon makes up a distinct layer of soil having different properties and characteristics. The soil profile extends from the soil surface to the parent rock material. A theoretical soil profile showing different horizons or layers are shown in the figure 1.2.

1. Organic Horizons:

The ‘O’ group is the organic horizons which are formed at the very surface of the mineral soil resulting from the decayed plant and animal products. This horizon is commonly found in forest areas where sufficient plants are present.

It can be divided into two specific horizons as follows:

OL₁ — In this first layer of organic horizons, 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.

2. ‘A’ (Eluvial) Horizons:

This ‘A’ horizon is present at or near the surface and it is characterized as zones of ‘washing out’ or maximum leaching.

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 as compared to the lower horizons.

A₂ — It is the horizon of washing out or eluviation of clay, iron and aluminium oxides and corresponding accumulation or resistant minerals, such as quartz. It imparts light colour than that of the upper horizon (A₁).

A₃ —It is transition horizon between A and B having properties more alike those of A₁ or A₂ that of B horizons. Sometimes it may be absent.

3. ‘B’ (Illuvial) Horizons:

This horizons are the zone of ‘washing in’ or accumulation of materials such as iron and aluminium oxides and silicate clays, from the above horizons or even from the below horizons in arid conditions as calcium carbonate, calcium sulphate and other salts during evaporation. These 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. Developments 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.

4. ‘C’ Horizon:

It is the unconsolidated material underlying the solum (A plus B horizon). It may or may not be the same as the parent material from which the solum formed. This horizon also considers as outside the zones of major biological activities and is very little affected by solum forming processes.

The upper portion of the C horizon may sometimes be considered as the solums since continually weathering the erosion are going on.

The solum (plural—sola) in soil science consists of the surface and sub-soil layers that have undergone the some soil forming conditions. The base of the solum is relatively un-weathered parent material. Solum and soils are not synonymous. Some soils include layers that are not affected by soil formation.

These layers 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. The solum of a soil is not necessarily confined to the zone of major biological activity. A solum does not have a maximum or a minimum thickness.

5. Regolith:

The layer of rock and mineral fragment that rests on bed rock and is produced by the weathering of rocks. Regolith constitutes the surface of most land. It includes all of the weathered material within the profile. It has two components—”most weathered the solum and the least weathered saprolite.” Saprolite lies directly above the solid, consolidated bed rock but beneath the regolith. It includes A, B and C horizons.