Soil Formation: Factors and Process

After reading this article you will learn about the factors and process of soil formation.

Factors of Soil Formation:

Soil development or soil genesis, is the result of a number of factors known collectively as soil formers, some of which are active and some passive in nature.

These are:

1. Parent material,

2. Relief (topography),

3. Climate,

4. Biosphere (vegetation, organism, man), and

5. Time.

Active Factors:

Which represent agents that supply energy that act upon the mass for process of soil formation. They are climate and biosphere.

Passive Factors:

Which represent the source of the soil-forming mass and conditions affecting it. They are parent material, relief, and time.

(1) Climate:

Climate influences soil formation largely through precipitation and temperature.

(a) Precipitation or rainfall:

Water percolates and moves from one part of parent material to another it carries with it substance in solution as well as in suspension. Rainfall also affects profile development through erosion, producing this soils on slopes and deposition of soil material downhill.

In general, with increasing moisture, nitrogen and carbon content, clay content, aggregation, saturation capacity and exchangeable hydrogen tend to increase. On the other hand, exchangeable bases and pH values show a decrease with increasing moisture. The depth of the calcium carbonate horizon in pedocal (lime accumulating) soils increases with increasing moisture.

(b) Temperature:

It influences the organic matter decomposition and microbiological activities in the soil. In general, with increasing temperature, the depth of weathering and clay content show an increase. Under tropical and sub-tropical conditions, with high rainfall highly leached soils have developed such as laterite soils.

Tropical – soils, especially those developed from igneous and metamorphic rocks, have bright yellow or dark red colours because of intense hydration and oxidation respectively, during their weathering. Tropical climates are generally, favourable to the production of luxuriant vegetation, but high temperature are not much favourable for the accumulation of organic matter in the soil.

Less organic matter is due to rapid decomposition of organic matter because of the increased activities of microorganism high temperature hinder the process of leaching and cause an upward movement of soluble salts (Sodium). Low temperature, on the other hand, increase leaching by reducing evaporation. They favour the accumulation of organic matter by slowing down the process of decomposition.

(2) Biosphere:

Biosphere consists of vegetation and organism.

(a) Vegetation:

It exerts main influence on soil formation through the amount and nature of organic matter it adds to the soil. Vegetation also aids in the control of erosion. They facilitate percolation and drainage and bring about a greater dissolution of minerals through the action of carbon dioxide and other acidic substances.

(b) Organism:

Burrowing animals, rodents, earthworms, ants etc., are highly important in soil formation. Burrowing animals cause constant mixing within the soil profile. The role of microorganisms as soil formers is intimately related to humification and mineralization’s.

(3) Parent Material (or Development of Profile):

As a result of changes the soil develops certain characteristics, which show that it is no longer a uniform body. The soil material at the surface which comes in direct contact with the atmosphere is influenced by climatic agents as well as the growing plants to a much greater extent than the material at lower depths.

The changes mainly consist of the removal or leaching of certain soil constituents from the surface to the lower depths where they are either re-deposited or completely leached out. Due to these changes, the whole soil mass is thrown up into a number of layers. If a vertical section of the soil is taken, it represents a sequence of layers from the surface down to the parent material which has, not suffered such changes.

The vertical section of the soil showing the various layers from the surface to the unaffected parent material is known as a soil-profile (Fig. 20.1) 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. The parent material from which the soil is formed is known as horizon-C.

The surface soil or that layer for 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 eluviation. The intermediate layer in which the materials leached from horizon-A have been re-deposited is known as horizon-B or the horizon of illuviation.

The soil in each of these horizons is usually uniformly developed and presents a more or less homogeneous character. Each layer or horizon develops specific morphological features such as the size and shape of particles, their arrangement, colour, consistence etc., which distinguish one horizon from another.

As the soil body matures, a differentiation of the materials within each horizon of the soil profile takes place. In many cases, horizons are further sub-divided into sub- horizons. The sub-horizons are designated as A₁, A₂A₃; B₁, B₂B₃; C₁C₂ etc. Local relief, local climate moisture and air supply, type of vegetation etc., bring about a differentiation in each horizon.

In cool, humid, temperate, and moist tropical regions there is usually an accumulation of organic matter on the soil surface. A part of the organic matter decomposes in the course of time and forms a layer of decaying humus on the top of the soil. This layer is designated a horizon-A₀. The un-decomposed material overlying the A₀– horizon consisting of forest litter and leaf mould is known as horizon-A₀₀. The kind of soil that develops depends in part upon the kind of parent rock and parent material which influence the physical and chemical properties of the resultant soil.

Acid igneous rock and sandstones usually weather slowly and give rise to coarse sandy soils with low base status, a kaolinite type of clay and infertile soils. Most of the basic igneous rocks and sedimentary rocks normally weather to finer-textured soils with high base status, montmorillonite type of clay and fertile soils. Hard, pure limestone yield sandy, shallow soils, whereas impure soft limestone’s give rise to deeper and finer- textured soils.

Tamhane et.al., (1959) have shown that black soils in India developed from basalt, limestone, granite, gneiss etc., under tropical climatic conditions did not differ much in general physical and chemical properties.

Horizons in a given Profile:

Each soil is characterised by a given sequence of horizons. Podsols contain well-developed horizons, which develop in cool regions (Fig. 20.1).

Different horizons are given below:

(i) Organic Horizons:

A₀ and A₀₀ groups are organic horizons which form above the mineral soil. They result from litter derived from dead plants and animals. Occurring commonly in forest areas, they are generally absent in grassland regions.

Specific horizons and their description are given below:

A₀: Original forms of the plant and animal residues can be recognized by the naked eye (un-decomposed organic debris).

A₀₀ : Original plant and animal forms cannot be so distinguished (partially decomposed organic matter).

(ii) A-horizons eluvial (outer horizons):

The A group are mineral horizons which lie at or near the surface and are characterised as zones of maximum leaching. Beginning with the surface, these horizons are designated as A₁, A₂, A₃.

A₁: Topmost mineral horizons, dark coloured horizon, usually contains a high proportion of organic matter.

A₂ : Light coloured horizons than Ai, heavy leaching or eluviation of clay, iron and aluminium oxides and corresponding concentration of resistant minerals such as quartz.

A₃ : Transition layer between A and B with properties more nearly like those of A than of the underlying B. Sometimes A3 is absent.

(iii) B-horizons illuvial (inner) horizons:

The B-horizon follow A-horizon. It is the region of maximum accumulation of material such as iron and aluminium oxides and silicate clays. In arid regions, calcium carbonate, calcium sulphate and other salts may accumulate in the low B-horizons.

The B-horizons are sometimes referred to as the sub-soil. This is incorrect however, since the B-horizons may be incorporated at least in part in the plough layer. B-horizons are designated as B₁, B₂ and B₃:

B₁: A transition layer between A and B with properties more nearly like B than A. Sometimes absent.

B₂: Zone of maximum accumulation of clays and hydrous oxides. Organic matter content is generally higher than that of A₂. Maximum development of blocky or prismatic structure or both.

B₃: Transition between B and C with properties more like those of B than those of C below.

C-horizon:

The C-horizon is the unconsolidated material under the B-horizon.

C₁: Partially weathered parent material, contains accumulation of lime and gypsum in some soils.

C₂: Un-weathered parent materials.

Profile of an Ideal Laterite Soil:

A typical laterite soil is dark brownish-red in colour, granular in structure and porous in constitution. It is developed under tropical conditions of high temperature and high precipitation, having alternate wet and dry seasons. It is derived from a red dish coloured clay that has hardened into an indurated crust on exposure to the air.

The lateritic clay or crust is reticulate and mottled and is usually to the air. The lateritic clay or crust is reticulate and very thick extending to nearly 100 ft. in some places. The crust is formed as a result of the sub-aerial weathering of a number of rock formations of which the Deccan trap, granite, gneiss and sandstone are the most common in this country.

The colour of the soil varies from yellowish-brown to reddish- brown, the former being associated with a more humid climate and the latter with drier one. Except for the colour of the surface horizon which is dark owing to a higher proportion of organic matter the other morphological features are almost uniform throughout the profile. Hence, horizon differentiation is not distinct.

The reaction varies from slightly to sometimes very strongly acid. The soil contains a high percentage of colloids, but as the colloidal material consists largely of hydroxides of iron and aluminium, it does not possess the common properties of clay such as swelling, plasticity, shrinkage etc.

Notwithstanding the abundance of vegetation, there is no great accumulation of organic matter in soil. It is rapidly decomposed and mineralized. In some cases, a humus horizon may be present on the surface.

4. Relief (Topography):

Relief influences soil formation primarily as a factor affecting erosion and as a modifier of climate and water-air relationship in the soil. With the increasing altitude, the climate becomes cooler and sometimes more humid.

Topography largely determines the drainage condition and the ground water level in the soil. Milne’s catena concept (Milne, 1936), which, represents a group of complex soils developed from similar parent materials, is primarily dependent upon topographical and hydrological conditions. Catenas are of common occurrence in tropical regions.

With the same kind of climate and parent material, soils that have developed on steep hill sides have thinner A and B-horizons. This is because the surface erodes quite rapidly and less water moves downward within the profile.

Soil materials on gently sloping topography have more water passing through them and the profile is generally deeper, the vegetation more luxuriant and the organic matter level higher, than in soils on steep topography (Fig. 20.2)

Materials lying in land locked sessions receive run-off waters from above. Such conditions favour a greater production of vegetation but a slower decomposition of the dead remains; the result is the existence of soils with large amounts of organic accumulation’s. If the area is wet at the surface for nearly the whole year, a peat or muck soil may develop.

In India where black and red soils occur in close proximity, it is observed that red soils occupy higher levels while black soils are at lower levels. Black soils in India have developed on broad plains with gently undulating slopes usually not exceeding 10 per cent.

5. Time:

The length of time required for a soil to develop horizons depends upon many interrelated factors, such as climate, nature of the parent material, burrowing animals and relief. Certain soils are termed mature or immature which give some idea of the time factor. Mohr (1959) has suggested five stages of weathering that are dependent on the mineralogical features of the soil.

Soil Forming Processes:

The collective interaction of various soil-forming factors under a different set of conditions sets a course to certain recognised soil-forming processes. The ultimate result of soil formation is profile development. Specific profile features develop under particular soil-forming processes.

The fundamental processes that develop a profile are described as follows:

Humification:

Helps in the formation of the surface humus layer, called A₀-horizon. The percolating water passing through this humus layer dissolves certain organic acids and affects the development of the lower A-horizon and the B-horizon.

Eluviation and Illuviation:

Development of profile horizons is mainly dependent on the amount and nature of movement of water in the soil. Eluviation (meaning “wash out”) is the process of removal of constituents by percolation from upper layers to lower layers. This layer of loss is called eluvial and is designated as the A-horizon.

The eluviated products move down and become deposited in the lower horizon which is termed as the illuvial (meaning “wash in”) or B-horizon. Mechanical eluviation removes finer suspended fractions of soils, producing textural profiles characterised by a coarser-textured A-horizon and a finer-textured B-horizon that sometimes develops into a hard pan.

Some important soil formation processes are described as follows:

Podsolisation:

It is a type of eluviation in which humus and sesquioxides become mobile, leach out from upper horizons and become deposited in the lower horizons. This process is favoured by cool and wet climate. It requires high content of organic matter and low alkali in the parent material. The process increases the proportion of silica, sesquioxide in A-horizon and accumulation of clay, iron and aluminium in B-horizon.

Laterisation:

In this process, silica is removed while iron and alumina remain behind in the upper layers and usually there are no well-defined horizons. Laterisation is favoured by rapid decomposition of parent rocks under climates with high temperature and sufficient moisture for intense leaching, such as found in the tropics. The soil formed in this process is acidic in nature.

Podsolisation and laterisation produce soils that belong to the pedalfer (iron accumulating) group.

Calcification:

In this process, there is usually an accumulation of calcium carbonate in the soil profile Such soils belong to the group called pedocal (calcium accumulating). This process is favoured by scanty rainfall and alkali in parent material.

Hydromorphic Profile Development:

Such soil forming process occur under impeded drainage conditions when certain horizons become saturated and percolation is restricted. Anaerobic conditions develop and more chemical reduction process set in. Marsh, bog, swamp, muck and peat soils are produced.

Under fluctuating ground water level and under monsoon climatic conditions soils develop under alternating oxidation and reduction conditions leading to the formation of yellow brown, or rusty mottlings.

Soil forming processes: