Soil profile

Soil profile

Definition of soil profile:-

The vertical section of the soil showing the various layers from the surface to the unaffected parent material is known as a soil profile.

The various layers are known as horizons. A soil profile contains three main horizons. They are named as horizon A, horizon B and horizon C.

Ø The surface soil or that layer of soil at the top which is liable to leaching and from which some soil constituents have been removed is known as horizon A or the horizon of eluviations.

Ø The intermediate layer in which the materials leached from horizon A have been redepositing is known as horizon B or the horizon of illuviation.

Ø The parent material from which the soil is formed is known as horizon C.

A study of soil profile is important as it is historic record of all the soil forming processes and it forms the basis for the study in pedagogical investigations. Soil profile is the key for the soil classification and also forms the basis for the practical utility of soils.

A hypothetical minerals soil profile will include O, A, B, C and R master horizon and all the possible sub-horizon,

Master horizon and sub-horizon.

O horizon -  it is called as organic horizon. It is formed in the upper part of the mineral soil, dominated by fresh or partly decomposed organic materials.

Ø This horizon contain more than 30% organic matter if minerals fraction has more than 50% clay (or) more than 20% organic matter if mineral fraction has less clay.

Ø The organic horizons are commonly seen in forest areas and generally absent in grassland, cultivated soils.

Ø O1 – organic horizon in which the original forms of the plant and animal residues can be recognized through naked eye.

Ø O2 – organic horizon in which the original plant and animal matter cannot be recognized through naked eye.

Ø A Horizon – horizon of organic matter accumulation adjacent to surface and that has lost clay, iron and aluminum.

Ø A1 - top most mineral horizon formed adjacent to the surface. There will be accumulation of humified organic matter associated with mineral fraction and darker in color than that of lower horizons due to organic matter.

Ø A2 – horizon of maximum elevation of clay, iron and aluminum oxide and organic matter. Loss of these constituents generally results in accumulation of quartz and other sand and silt size resistant minerals. Generally lighter in color than horizon above and below.

Ø A3 – a transitional later between A and B horizon with more dominated properties of A1 and A2 above than the underlying B horizon. This horizon is sometimes absent solum.

Ø B Horizon – horizon in which the dominant features are accumulation of clay, iron, aluminum or humus alone or in combination. Coating of sesquioxide will impart darker, stronger of red color than overlying or underlying horizons.

Ø B1 – a transitional layer between A and B. more like A than B.

Ø B2 – zone of maximum accumulation of clay, iron and aluminum oxide that may have moved down from upper horizons or may have formed in situ. The organic matter content is generally higher and color darker than that of A2 horizon above.

Ø B3 – transitional horizon between B and C and with properties more similar to that of overlying B2 than underlying C.

Ø C horizon – it is the horizon below the solum (A + B), relatively less affected by soil forming processes. It is outside the zone of major biological activity. It may contain accumulation of carbonates or sulphates, calcium and magnesium.

Ø  R horizon – underlying consolidated bed rock and it may not be like the parent rock from which the solum is formed.

Beside, lower case letters are used to indicate the special features of master horizons. This case letter follows the subdivisions of master horizons. E.g. Ap-ploughed layer e.g. B2t- illuvial clay. When two or more genetically unrelated (contrasting) materials are present in a profile as in the case of alluvial or colluvial soils then the phenomenon is known as lithological discontinuity. This is indicated by the use of Roman letter as prefixes to the master horizons.