Soil Formation, Factors influencing Soil Formation and Soil Profile Formation

 

Soil Formation

Soil minerals form the basis of soil. They are produced from rocks, the parent material, through the processes of weathering and natural erosion. Water, wind, temperature change, gravity, chemical interaction, living organisms and pressure differences all contribute to the breakdown of parent material to form soil.

Stages of Soil Formation:

A.  Decomposition of Parent Material

B. Decomposition of Organic Materials

C. Formation of Organo-Mineral Complex

 

A. Decomposition of Parent Material

Soils are formed by disintegration and decomposition of rocks and vegetables and animal fossils imbedded in them. The conversion of parent material into soil is known as soil formation.

Decomposition or weathering of parent materials are by three types of agents and accordingly the processes are termed as:

1. Physical or mechanical weathering

2. Chemical weathering

3. Biological weathering

1. Physical or Mechanical Weathering:

The soil formed by physical weathering is known as “skeletal soil”.  The principal agents of physical or mechanical weathering are

Heat and Cold:

Rocks are composed of various mineral crystals which possess different coefficient of expansion and contraction. The expansion and contraction due to variation of temperature of adjacent unlike minerals loosen the crystals causing the rocks to crumble. In this way, rocks are weathered and finally reduced to soil material by other agents. This type of weathering is most common in dry climate.

Freezing and Thawing:

When water freezes, its volume increases by about nine per cent and exerts force. In the crevices of rock, water frequently freezes and the crevices are enlarged by breaking of fragments and when thawing occur they may roll down the slopes.

Glaciers:

The moving of ice in Glacier has a great grinding power.

Water:

Water have both erosive and transporting action.  Rain dislocates solid particles from rock and puddle the surface of some parent materials. The streams of water with their loads of various materials have grinding capacity. They also transport and deposit soil in low lying areas. Water frozen in rock crevices in cold regions breaks them open. Flowing water, waves on the shores of the seas and large lakes and moving ice-glaciers in high mountainous region wear off rocks and cliffs thus helping in soil formation.

Wind:

Sand storms in deserts, semi-arid and dry river beds and high winds on sea shore have erosive and transporting action. Wind makes the action of sea waves stronger in causing weathering along the coasts.

2. Chemical Weathering:

The physical disintegration produces a greater surface area of rock which then gets exposed to the chemical weathering agents. Chemical weathering is a complicated process. In chemical weathering, the minerals in the parent rocks are decomposed and new substances are originated. Chemical weathering involves two steps, Disappearance of certain minerals and Formation of secondary products.

The chemical weathering of rocks is brought about by

Water:

Gypsum and calcareous rocks which are easily soluble in water make the soil after dissolving in water.  Most of minerals in the parent rocks are more or less soluble in water. The decomposing action of water is increased by the presence of carbon dioxide and organic acid formed from the oxidation of sulphur compounds or iron sulphide.

Hydration:

Hydration is the chemical combination of water with other chemical substances.  The union of water with minerals softens the rocks which then result in the weathering of rocks and ultimately brings about their decomposition. Hydration is also accompanied with increase in volume and thus the physical disintegration of rocks.

Example of hydration

Hydrolysis:

Hydrolysis involves exchange of constituent part between water and mineral. The minerals which are affected by hydrolysis are the complex silicates of calcium, magnesium, potassium, sodium, aluminium and iron.  After the decomposition, the oxides of aluminium and silicon may again combine under suitable condition to form new silicate minerals like kaolinite or they may remain in the soil or they may be leached down by water.

Oxidation:

Oxidation mainly affects ferrous iron which is a constituent of many minerals such as olivine, hornblende and augite. Oxygen combining with various minerals produces soluble oxides which dissolve in water and weaken the rock and thus help its disintegration.

Reduction:

Under anaerobic conditions, when soil pores are saturated with water, oxygen will be absent and reduction may occur. Under certain conditions, organisms causing decay of organic matter may cause reduction by taking up oxygen.

Carbonation:

When carbon dioxide (CO₂) combines with water, it forms carbonic acid which increases the solvent power of water. Carbon dioxide or carbonic acid may also react with other chemical compounds resulting in soluble carbonates which dissolves in water and weakens rock and help its decomposition.

3. Biological Weathering:

Biological weathering is physical and chemical weathering by biological agents. Physical and chemical weathering brings about physical disintegration and chemical decomposition of rocks in which organic matter are added and thus soil is formed. Pants and animals are responsible for the further reduction of rock minerals’ particles into soil. The activities of plant and animals change mineral composition as well as the physical structures of rocks and their growth may cause further weathering.

Plants:

Plants such as mosses and lichens grow on bare rocks and cause gradual disintegration. Grasses, shrubs and trees grow in rock crevices and extend the cracks of the rocks by their root growth. Plants accelerates weathering process by producing carbon dioxide during respiration and by providing the organic materials for humus and humic acid formation.

Animals:

The soil organisms convert the organic matter into humus. Animals like earthworm, termites, ants, moles, rats and rodents burrow underground and loosen hard soil and change its character by mixing upper and lower layers of soil.

B. Decomposition of Organic Materials:

The decomposition of organic material may progress through two distinct stages. As dead vegetation accumulates on the soil surface, it provides food for soil organisms, and these organic materials are fragmented and chemically degraded by them to become the humus. The production of humus is largely controlled by soil flora and fauna and physical and chemical agents also contribute to the formation of humus.

 

C. Formation of Organo-Mineral Complex:

The mechanisms involved in the formation of organo-mineral complex are of two types

1.       Electro-Chemical Bonding - Aggregation of negatively charged colloidal clay and humus particles is brought about through electrostatic bonding.  Water molecules and metallic ions such as calcium are involved.

2.       Cementing - Substances adsorbed on the surface of soil particles effectively glue or cement them together.

 

Factors that influence Soil Formation

1. Climate:

Climate is the most influential and determine the nature of weathering. The precipitation or rainfall and temperature are the important elements of weather that affects various physical and chemical process in soil formation.

The climate influences the process of soil formation directly and indirectly

Direct influence of Climate on soil formation

Rainfall and temperature supply water and heat respectively to react with parent materials. Rainfall primarily determines moisture which affect the decomposition of minerals. The soluble products of decomposition are removed along with percolating and run-off water. The increase in temperature also increases the rate of weathering of primary minerals and clay content and cation exchange capacity young soil. Temperature influences the decomposition of organic matter and microbiological activity.

Indirect influence of Climate on soil formation

Climate control vegetation of a particular area, the vegetation is the source of organic matter. This acts on rocks and mineral material by means of acid and salts released during the process of organic matter decomposition. The vegetative cover protects the soil from wind and water erosion.

2. Living Organisms or Biosphere

Living organisms render an indirect effect on soil formation.  There are two components of biosphere which influences the soil formation – plants and animals.

Plants / Phytosphere

Mosses and lichens grow on bare rocks and respire to produce carbon dioxide which react with water to form carbonic acid. This carbonic acid dissolves primary minerals and releases the nutrient contained in them for the growth of plant.

Algae fix atmospheric nitrogen which is released in the soil upon the death of algae. 

Plant grow in an environment having nutrients and water. After the death of these plants, the organic matter of rocks increases. Carbon dioxide produced from the decomposition of organic matter and respiration of microorganism and plant roots combines with primary minerals to form more clay and convert insoluble minerals to soluble ones.

Plant roots penetrate into the rocks and minerals and thus open channels for the movement of water and air and creates favourable environmental conditions for biological activity. As the roots die and decompose, a good number of organic and inorganic acids are released which may initiate different chemical reactions.

Vegetation is also the basic supplier of organic remains.

Animals / Zoosphere

The contribution of animal kingdom to soil formation is primarily mechanical. Burrowing animals like rodents, termites, earthworm etc. when present in large number help to change the character of the soil as they harbour in soil, dig into soil body and mix the materials of different horizons. Man through his land use activities causes both deleterious and beneficial effect on soil.  Man converts the forest areas into agricultural land. The human interference accelerates erosion of soil through some activities such as burning of forests, shifting cultivation and indiscriminate grazing. The agricultural practices such as cultivation, puddling, cropping system, use of manures, fertilizers, pesticides, drainage, irrigation etc. alter the general character of soil profile.

3. Parent Materials:

All rocks from which soils are formed are called soil forming materials or parent materials. A rock is an aggregate of one or more (usually more) minerals and solid materials which form the crust of the earth. Rocks may be classified according to the mode of origin and composition of rock.

The rocks are classified on the mode of origin as follows:

1. Igneous Rocks:

Igneous rocks are those rocks which at one time were in fluid molten state before their solidification. These rocks are formed by solidification of molten lava.  They are classified into three - Organic Rocks, Plutonic Rocks and Intrusive Rocks:

2. Sedimentary Rocks:

Sedimentary rocks are derived from igneous rocks and are formed from the deposition and re-cementation of weathering products of igneous rocks. These rocks develop due to gradual accumulation of weathering products by water or wind on the surface of the earth. Such rocks are characterized by the presence of distinct sediment or layers in them.  Sedimentary rocks are of different types such as Mechanical Sediment,  Organogenic Sediments and Chemical Sediment

3. Metamorphic Rocks:

Metamorphic rocks are those rocks which have undergone some sort of metamorphism or change. Igneous and sedimentary masses subjected to tremendous pressure and high temperature have succumbed to metamorphism. The common examples of metamorphic rocks are gneisses, derived from granite, quartzite from quartz and marble from limestone.

Rocks are also classified on the basis of their silica content as follows:

Acid rocks – This rock contains 65-75 per cent silica, e.g. granite, sandstone, gneiss etc.

Basic rocks – This rock contains 40-55 per cent silica, e.g. basalt, lime stone, gabbro, diabase etc. Basic rocks are also rich in calcium, iron, magnesium and sodium.

Intermediate rocks – This rock contains 55-65 per cent silica, e.g. andesite, diorite, syenite etc.

4. Relief or Topography:

The topography or relief of the land is the difference of elevation  and it influences soil formation through its effect on drainage, run-off, soil erosion and exposure of land surface to the sun and wind. The character of soil depends on the topography. A group of soil which has developed from same parent material in same climate but under different topographical condition is called soil catena.

5. Time:

The actual length of time that the materials are subjected to weathering plays an important role in soil formation.

 

 

Formation of Soil profile

The ultimate result of soil formation is the development of soil profile which is formed by interaction of various pedogenic factors under special sets of conditions.

·         Addition of mineral and organic matter to the soil.

·         Losses of mineral and organic matter from soil.

·         Translocation of mineral and organic matter from one point of soil profile to another.

·         Transformation of mineral and organic matter in the soil and formation of definite layers.

The processes that results in the development of a soil profile are

1.      Humification

2.      Eluviation

3.      Illuviation

Humiflcation

The top layer of soil (‘A’ horizon) contains dead remains of plants, animals and products of microbial metabolism. These undergo decomposition to form the humus.  The process of decomposition of organic matter and synthesis of new organic substances is called ‘humification’. The humus and organic compounds are mixed with fine particles of weathered rock. The water percolating through humus layer dissolves organic acids and influences the development of lower horizons.

Eluviation and Illuviation

Water percolating downward through A horizon or top soil removes several mineral and organic substances from the top soil. The process of washing of soil constituent by percolation from upper layers to lower layers is termed as eluviation.  The surface layer from which components are lost is called eluvial layer or A horizon.

The eluviated substances move downward and are deposited in the lower zone or B horizon, which is termed as illuvial layer or B horizon. The process of accumulation of eluviated material is called illuviation.

The humification, Eluviation and Illuviation are influenced by the following processes

Gleization -  is the process of reduction of ferric compounds like ferric phosphate and ferric sulphide in water logged soil in presence of organic matter.

Podzolization - a type of eluviation in which humus and sesquioxides becomes mobile, leach out from upper horizon and becomes deposited in the lower horizons under acid condition (pH 4-5)

Laterization - is the process of desilication, the removal of silica and accumulation of sesquioxides. Hydroxides of iron and aluminium are precipitated in the form ‘laterite’.  Laterite are also formed from clayey sandy rocks when large amounts of iron are accumulated in them. Laterization is favoured under climate with high temperature and sufficient moisture.

Calcification - In sub-humid and dry regions, soil accumulates considerable amount of soluble materials due to lack of excessive moisture in the soil and carbonates of calcium and magnesium are deposited in the B. horizon.

Salinization -  the process of accumulation of soluble salts in soils. Soluble neutral salts of calcium and magnesium are deposited on the surface of the soil when water evaporates or it may take place through capillary rise of saline ground water or by inundation with sea water. Desalinization is the process of leaching soluble salts from soil by rain water or irrigation water.

Alkalization (Solonization) - Alkalization is the process by which soils with high exchangeable sodium are formed. Dealkalization (solodization) is the process by which sodium ion is replaced from the clay and humic micelle by hydrogen ion.

Hydromorphic Soils (Gleysoils) - regarded as intrazonal soil and this soil evolves when the soil is over-moistened either from the surface or groundwater. The anaerobic condition promotes the reduction reaction for the formation of gley horizon or a gley stratum in swamp, bog, marsh, muck and peat soils.