Various modes of soil formation

Soil Formation

The transformation of rocks into soil may be termed as soil formation. It starts with the weathering of rocks. The weathering processes are primarily destructive in nature. It helps to change the consolidated rocks into unconsolidated material. The soil forming processes are constructive and result in a soil profile that has been developed from the surface few feet of parent material.

Factors of soil formation

Five factors, at any specific location on the surface of the earth, act simultaneously to produce soil. They are (1) Climate (2) Parent material (3) Relief /Topography (4) Biosphere (Vegetation, organism, man, etc.) (5) Time. Their relationship to the soil properties has been expressed by Jenny in the following equation.

s = f (cl, b, r, p, t)

Where, ‘s’ = Any soil property e.g. clay content. ‘f’ = function of or dependent upon ‘cl’ = climate, ‘b’ = Biosphere, ‘r’ = relief, ‘p’ = parent material, ‘t’ = time (age)

Therefore, any soil property is the function of collective efforts of all the soil forming factors.

1. Climate and soil formation: Climate is dominant factor in soil formation, climate influence soil formation largely through precipitation and temp and air.

a. Precipitation:

i. It primarily regulates the moisture air regime of the soil and determines the leading trends in soil profile depending upon available percolating water.

ii. Rainfall also affects profile development through erosion producing thin soils on steep slopes and deposition of soil material down hill.

iii. The intensity, frequency and distribution of precipitation influence the course of soil formation.

iv. With increasing moisture, nitrogen and carbon content, clay content, aggregation, saturation capacity and exchangeable hydrogen tend to increase.

v. Exchangeable base and pH value tends to decrease with increasing moisture.

b. Temperature: Temperature affects the velocity of chemical reactions, which approximately doubles for every 10⁰C increase in temperature. It influences the organic matter decomposition and microbial activities in soil though the evapotranspiration phenomenon. Temperature also determines the efficiency of rainfall. In general, with increase in temperature the depth of weathering and clay content show increase on the contrary nitrogen, organic matter, silica, alumina and base alumina ratio tends to decrease with rising temperature.

2. Parent material and soil formation: Rock on the surface of earth is weathered until the essential elements become available to support lichen and other lower forms of plant life. As continuing generations of lichens grows, die and decay, they leave increasing amounts of organic matter. Organic acid further has tendency to decay the rock. Different parent material affect profile development and produce different soil especially in initial stages. The nature of the elements released during decaying of rocks has a specific role in soil formation.

3. Relief / Topography and soil formation: The prominent types of topography designations as given in FAO guidelines are:

i. Flat or almost flat: Land surface with slope less than 2%

ii. Undulating: Land surface with slope between 2-8%

iii. Rolling: Land surface with slope between 8-16%

iv. Hilly: Land surface with slope between 16-30%

v. Steepy dissected: Land surface with slope greater than 30%

a. Soil formation on slopy land: The soils on steep slopes are generally shallow, storey and have weakly developed profiles with less distinct horizonation. It is due to

i. Accelerated erosion, which removes surface material before it has time to develop.

ii. The reduced percolation of water through the soil because of surface runoff.

iii. Lack of water for the growth of plants, which are responsible for checking erosion and promoting soil formation.

b. Soil formation on level topographic position: On level topographic position, almost then entire water received through the rain percolates through the soil. Under such condition the soil formed may be considered as representative of the regional climate. They have normal solum with distinct horizons.

4. Biosphere and soil formation: Vegetation, microbes, animals and man all greatly influence the soil formation processes. Vegetation exerts its main influence on soil formation through the amount and nature of O.M. that it adds to the soil. Soil developed under biosphere of forest vegetation have more horizon a more highly leached. Vegetation also aids in control of erosion. Burrowing animals causes constant mixing within soil. Man through his land use cause both beneficial and harmful effect on soil.

5. Time and soil formation: The length of time required for a soil to develop horizons depends upon many unrelated factors such as climate, nature of the parent material, burrowing animal and relief. It has been seen that rocks and mineral disintegrate and /or decomposes at different rates, the coarse particle of limestone are more resistant to disintegration than those of sandstone. There are five stages of weathering that are dependent on minerological features of soil.

Weathering stages in soil formation:

1. Initial – Unweathered parent material.

2. Juvenile – Weathering started but much of the original material still Unweathered.

3. Virile – Easily weatherable mineral fairly decomposes, clay content has increased.

4. Sensile – Decomposition reaches at final decomposition stage only most resistance minerals survive.

5. Final – Soil development completed under prevailing conditions.

Processes of soil formation: The basic processes involved in soil formation are

1. Gains or addition of water, organic and inorganic minerals to the soil.

2. Losses of the above material from the soil.

3. Transformation of mineral and organic substances with the soil

4. Translocation or movement of soil material divided into

i. Movement in solution (leaching).

ii. Movement in suspension (eluviation) of clay, organic matter etc.

A. Fundamental processes of soil formation:

1. Humification: Helps in formation of surface layer, called Ao horizon. Its characteristics depends upon the nature of vegetational residue and the way it becomes decomposes and synthesized into new organic compounds. The percolating water passes through this humus layer dissolves certain organic acids affect the development of lower A and B.

2. Eluviation: Elevation means washing out. It is a process of removal of constituents in suspension or solution by the percolating water from the upper to the lower layer. Mechanical eluviation removes finer suspended fraction of soils, producing textural profiles by a coarse texture.

3. Illuviation: The process of deposition of soil material (Removed from the eluvial horizon) in the lower layer is known as illuviation. The horizons formed by this process are termed illuvial horizons.

B. Specific pedogenic processes:

1. Calcification: It is the process of precipitation and accumulation of calcium carbonate (CaCO₃) in some part of the profile. The accumulation of CaCO₃ may result in the development of acidic soil. Such soil belonging to group called pedacal.

2. Decalcification: It is the reverse of classification i.e. the process of removal of CaCO₃ or Ca ions from soil by leaching.

3. Podsolization: Podzolization is negative of calcification where as calcification tends to concentrate calcium in the lower part ‘B’ horizon podsolization reaches the entire solum. Apart from the calcium the other bases are also removed and the whole soil becomes distinctly acidic. Process is mainly acid leaching.

4. Laterization: In this process, silica is removed while iron and alumina remain behind in the upper surface / layers and usually there are no well-defined horizon. Laterization is favoured by rapid decomposition of parent rocks under climates with high temperature and sufficient moisture for intense leaching. Podsolization and latrization produce soils that belong to the pedalfer (iron accumulating) group.

5. Gleization: It is a process of soil formation resulting in the development of glei (or gley horizon) in the lower part of soil profile above the parent material due to poor drainage condition and where water logged conditions prevail such soils are called hydromorphic soils. Gleying may be observed at any depth depending on the depth of ground water.

6. Pedoturbation: Inversion of soil takes place in deep black cotton soils which contain montmorillonite clay colloid (vertisol) has max swelling shrinkage capacity. As alternate weting and drying, expansion and contraction takes place due to which cracks is formed. Due to crack formation A horizon goes down and soil B horizon comes up. These are also known as self ploughed soils. Therefore in dry farming technique ploughing once in three years is recommended to conserve the soil moisture and since the inversion of soil takes place naturally there is no need of ploughing every year.

7. Salinization: Accumulation of soluble salts under dry climate or no rainfall. Salts accumulate on soil surface. Na⁺, K⁺⁺, Ca⁺⁺, Mg⁺⁺. Soil becomes saline due to high rate of evaporation, reclaimed by leaching, flooding. (pH8.5) Grow salt tolerant crops, shevri, Dhaincha.

8. Alkalinization: Accumulation of sodium salts only like Na₂Co₃ and Na₂ (CO₃)₂.