Soil Formation: Profile, Morphological Characteristics (With Diagram)

Soil Formation: Profile, Morphological Characteristics!

The surface rocks of the earth are weathered and as a result of weathering, small particles of parental materials are formed.

The weathering process continues until all the essential elements locked up in the rocks become available to all sorts of organisms.

Simultaneously with weathering process, pedogenesis or soil forming process also runs which leads weathered materials to develop into mature soil with well differentiated soil profiles and biological system.

Soil Profile:

Weathering of parental rocks results in the development of several loose layers or horizons of weathered materials. Biological system, addition of organic matter (humus) and interaction between organic and mineral compounds make the horizons more distinct. The complete succession of horizons down to the level of undifferentiated parent materials is called soil profile. In other words, it is vertical section of earth crust showing different layers or horizons of soil.

The soil profile may be divided into the following three zones or horizons (Fig. 22.3):

(1) A horizon (Top soil or the zone of extraction),

(2) B horizon (Subsoil or subsurface zone), and

(3) C horizon (Regolith).

Some workers have recognized D horizon below C horizon:

1. A horizon:

It is the top soil which consists of mineral matter and organic residues. The soluble chemicals are leached away by water. Upper zone or A horizon is very loose and it supports vegetation. Because it is full of organic elements and minerals, the top soil is very fertile. The thickness of A horizon varies from one to ten feet. A horizon is differentiated into several sub layers which are Aoo, Ao, A₁, A₂, A₃. These can be distinguished by their distinct colours and textures.

2. B horizon:

A zone of soil present below the top soil or A horizon is called B horizon. It is also known as subsoil. In this zone, the weathered substances or minerals are deposited and organic residues are present in very small quantities. This horizon is lighter in colour than A horizon. B horizon can also be differentiated into several subzones, as B₁, B₂, B₃ and so on, on the basis of colour and texture. Upper subzone of B horizon, which is just below the top soil (A horizon), is looser than lower subzones.

3. C horizon (regolith):

Below B horizon and above the surface of weathered rock, there is a third horizon, the C horizon or zone of regolith. It is formed of freshly weathered parental materials.

4. D horizon:

It is also called R. horizon (rock in active weathering state). Below the C horizon, there may be found rock in active weathering state. This is recognized as a fourth zone or D horizon by some workers.

Morphological Characteristics of Soil Profile:

The following morphological characteristics are recorded in respect of each of the horizons:

Colour, texture, structure, consistence, concretions, calcareousness, roots, etc.

Colour:

Colour is one of the most noticeable characteristics of a soil. As we examine a soil profile, we are likely to see definite changes in colour from the surface through the subsoil and underlying parent material.

The colours of the surface and subsoil may be classified as follows:

Surface soil — Dark, moderately dark, light, very light.

Subsoil — Dull, mottled, bright.

As a rule, the surface soil is darker than the subsoil since it contains more organic matter. Subsoil colour is less affected by organic matter than surface soil colour.

Texture:

The soil particles which resulted from weathering vary greatly in size. They are classified on the basis of size into gravel, sand, silt, and clay. Most soils are mixtures of the various sized particles except that many contain no gravel and some very little sand. If a soil contains considerable quantities of at least two sizes of particles, it is known as loam. It may be called a gravelly loam, sandy loam, silty loam, or clay loam, depending on the size of particle that predominates in the mixture. If however, a mixture consists mostly of one size of particle, it is not called as loam.

Structure:

Structure means the aggregation or arrangement of the soil particles into clusters or shape of various sizes. Structure does not change soil texture. The structure of soil prides may be crumb, granular, angular, sub-angular, blocky, prismatic, platy and columnar type.

Consistence:

Consistence is a term expressing the degree of cohesion of the soils and the resistance opposed to forces tending to deform or rupture the aggregates. Consistence of soil materials is determined at dry, moist and wet conditions.

The terms used are:

Dry — Loose, soft, slightly hard, hard, very hard, extremely hard. Moist — Loose, very friable, firm, very firm, extremely firm.

Wet — Non-plastic, slightly plastic, plastic, very plastic, non-sticky, slightly sticky, sticky, very sticky.

Concretions:

Iron and manganese concretions alone have no definite meaning, although their presence usually indicates that conditions of alternate oxidation and reduction have taken place. Presence of calcium carbonate concretions throughout the profile and on the surface may be the result of resistance action. These concretions at deeper layers may indicate the usual depth of penetration of rainwater.

Calcareousness:

Soil horizons are tested with dilute HCI and based upon the briskness of the effervescence; they are classified as weakly, moderately and strongly calcareous. Calcareousness has an effect on the availability of plant nutrients.

Roots:

The abundance of roots in each horizon is normally recorded in terms of few, common, abundant etc. Other features like presence of clay; pores etc. within each horizon are also recorded.