24 Soil – I (Formation, Structure and Chemical Composition)

Prof. Ajay Taneja

epgp books

 

Contents:

  1. Soil Definition
  2. Formation of Soil
  3. Weathering
  4. Soil Forming factors
  5. Soil Composition
  6. Soil Structure: Formation and Description
  7. Soil Grade
  8. Soil Profile
  9. Inorganic Components in soil
  10. Soil Humus
  11. References

 

Soil Definition

The word soil is derived from word‘solum’(Latin word) which means earthly material in which plants grow. The study of soils as naturally occurring phenomena is called pedology (from the Greek word pedon, meaning soil or earth). Soil possess complexity and heterogeneity. It documentation is as early as 5000 BC. Vedas and Upanishads regarded soil as ‘Mother of all’ that supports and nourishes all life on Earth. By definition, Soil is a dynamic natural body developed as a result of pedogenic processes during and after weathering of rocks, consisting of mineral and organic constituents, possessing definite chemical, physical, mineralogical and biological properties, having a variable depth over the surface of the earth and providing a medium for plant growth for land plants.

 

Formation of Soil

Soil is formed as a result of long term process of complex interactions disintegration and decomposition of rocks due to weathering leading to the production of mineral matrix in close association with interstitial organic matter. Two types of rocks that are recognized are (1) Soft rock that is formed fromdecomposition of rocks and soil development proceed simultaneously and (2) in hard rockdecomposition of rocks and rock minerals occurs prior to development of soil.

 

Weathering

The disintegration and decomposition of rocks and rock minerals by physical, chemical and biological agents in single or in combination is termed as weathering. Two types of weatheringare recognized:

 

(1) Geochemical weathering occurs before soil is developed and continues beneath soil cover. It is also termed as “geogenesis” and (2) Pedochemical weathering that occurs as result of disintegration and transformation of minerals that take place within the soil. It is also termed as “pedogenesis”. Weathering on the basis of agents caused can be classified into following three types. The Physical/ Mechanical weathering takes place when rock is split or broken into smaller pieces of the same material without changing its composition. Agents include temperature, water, ice, gravity, wind, mutual rubbing of rock fragments and air blast of sand grains. It is more predominant in the desert and arctic regions. Biological weathering occurs due to weakening and subsequent disintegration of rock by plants, animal and microbes. Their agents include micro-organisms such as bacteria, fungi, protozoa, lichens and mosses and also burrowing animals like earthworms. These micro- organisms decompose a part of the organic matter and add inorganic nutrients to soil. Chemical weathering includes processes whereby rock material are decomposed by alteration of present material. Their agents include atmospheric gases, especially oxygen, water and acids. It takes place through six different chemical reactions:

 

(a) Solution- Weathering of water soluble minerals (limestone, gypsum) by solvent action. It increases in presence of carbon dioxide and organic acids formed as a result of decay of organic plant and animal remains.

(b) Oxidation- Oxides dissolve in water weakening rock and causing weathering. It occurs best in well aerated and well drained soils.

(c) Hydrolysis- Here, weathering by the chemical reaction occurs between hydrogen ions and hydroxyl ions of water and a mineral’s ions.

(d) Reduction- It occurs in poorly aerated deep zones of earth’s crust

The red ferric oxide reduces to grey ferrous oxide

(e) Carbonation- Combination of carbon dioxide and water to form carbonic acid. Carbonic acid combines with hydroxides of Ca, Mg to form carbonates and bicarbonates.

(f) Hydration- Attack of water molecule to the rock material resulting in increment in volume and softening of hydrated mineral with its subsequent weathering.

 

Soil Forming Factors

Factors that contribute to soil formation can be classified into three types: Active factors include rainfall, temperature, wind, humidity, evaporation. Passive factors includes parent material, topography (drainage, runoff, soil erosion) while micro-organisms eg.worms, centipedes etc. come under biosphere factors.

 

Soil Composition

Figure 1: Constituents of soil (by percentage) in a typical top soil

 

(a) Mineral matter- geochemically classified on basis of their bonding characteristics into five maingroups:

Table 1: Components of Mineral matter of soil

 

Element Characteristics Examples
Lithophile Readily ionized forming oxyanions Li, Na, K, Ca, Mg, Fe, Si, B, Bi, Ni, Co, S, Cu,Zn, halogens, rare earths.
Chalcophile Forms covalent  bonds with sulphides. Fe, Ni, Co, Cu, Zn, Pb, Ag, Sb, As, Bi, S, Se, Te, As.
Siderophile Capable of forming metallic bonds Fe, Co, Pd, Os, Au.
Atmosphile Tend to remain in atmospheric gases. He, Ne, Ar, N, O
Biophile Remain associated with living organisms. C, H, O, N, P, S, Ca, Mg

 

(b) Organic matter -It varies from 3.5 – 5% by weight in top soil. It contains partly decayed plant and animal residues that are microbially synthesized contributing in maintenance and improvement of soil properties. It acts as storehouse of plant nutrients. Five major classes of organic matter exist in soil namely humus, saccharides, fats, resins and waxes, nitrogen organics and phosphorus containing organics:

(i) Humus- The formation by microbial degradation of plants and animals is called humification. The composition of humic substances follows order: C: 45-55%,O :30-45%, H :3-6%, N :1-5%, S :1%. It possesses aromatic character and are instrumental in decomposing soil minerals

(ii) Saccharides- It assist in stabilization of soil aggregates from major food sources.e.g.sugar, cellulose, starches, gums.

(iii) Fats, resins and waxes: They are lipid extractable. The enzyme lipase degrades fats into glycerol and fatty acids which further breaks down into carbon dioxide and water.Being phototoxic affects soil properties by repelling water.

(iv) Nitrogen organics: In this, nitrogen is attached to humus, amino acids and amino sugars

(v) Phosphorus organics- It is the source of plant phosphate and occurs as phosphate esters, phospholipids

Soil Water

It provides medium to plants for obtaining necessary nutrients. Itaids in maintenance of texture, arrangement and compactness of soil particles. The total amount of water present in the soil is called holard. Water that can be absorbed by plants ischesard while unabsorbed is called echard.

 

Soil Air

Its composition nearly similar to atmospheric air except CO2 level being high. Water clogging between air spaces leads to anaerobic condition in which gases are unable to diffuse inwards. It is essential for respiration of soil microorganisms and plants and enhances permeability of roots to soil.

 

Soil Structure

The texture and structure of soil influences the amount of water held and hence behavior of any soil. Soil bulk density with soil texture influences water infiltration rate.Clay soil has slow water drainage than sand and volcanic soils. A loam soil with humus containing 34% of air and 66% of water is considered best for majority of crops. Secondary particles called aggregates or peds formed by the arrangement or grouping of primary particles (sand, clay, slit and organic matter) is defined as soil structure.

 

Formation of soil structure

The process of soil formation occurs through physio-chemical process characterized by aggregate formation, flocculation and swelling, shrinking of clay masses and biological process where alleviation of aggregates by burrowing animals, binding action of plant roots and production of organic glues by micro-organisms occurs.

Soil is known to contain air spaces and generally possess a loose texture

 

Figure 2: Fine structure of soil showing solid, water, air phases

 

Describing Soil Structure

Soil possess varying shapes or types. Soils having no true structure are termed as structure less. Sandy soil possess negligible attraction between sand grains. Deposits of sand in a sand dune are called single

grain. Clay soil have large cohesive masses on textural basis and are thus massive in structure. Structural units are called Peds. Peds can take several definite and repeatable shapes such as granular, blocky, prism-like and platy.

 

  • Granular structure -It is spherical in shape. It is easily separable into aggregates.Improper locking of outer surface.Size- < 1 mm to 10 mm (in diameter). It is found in organic matter enriched surface horizon (A- horizon). Its occurrence supported by prevalence of grassland vegetation and earthworm activity.
  • Block-like structure- It is cubic in shape. Size- <5 mm to 50 mm (in diameter). It does not occur individually but derive their shape from surrounding peds. It is found in sub soil (B-horizon), It is characterized by angular blocky-shape précised edges & diverse rectangular peds.
  • Prism-like structure: The prismatic structure has flat/angular tops where strong association exists between swelling type of clays. It occurs in subsurface B- horizon of soil profile while Columnar structure is characterized by flat/ rounded tops. It occurs in soil horizon of arid and semi- arid region in the regions of high sodium content.
  • Platy- structure: It has horizontal oriented peds stacked over one another. Size- <1 mm to 10 mm (in diameter). It is found in surface or sub surface horizon. Its structure is known to be inherited from the soil’s parent material by action of water

                                                           Figure 3: Different structures of soil

 

Soil Grade

It describes the uniqueness of the structure that is further combined with the cohesion of soil within units compared to cohesion between individual units. Weak Grade is characterized by poor observation of aggregates where peds break into whole and broken units on disturbance.In the moderate type of grade, the structural units are easily identified. Peds break into mixture of entire, broken & little aggregates, on shaking. The ‘clean’ separation from other peds is noticeable. Strong grade is characterized by distinct structural units and identifiable shape. Peds retain their shape on shaking (clean separation).

 

Structural Class

It describes the size of units. The smallest dimension of any structural unit is referred by its size limit.

Table 2: Structural classes of soil

 

Soil Horizons- Profile

It refers to series of distinct horizontal layers (horizon) with varying chemical compositions and physical characteristics. The layered structure of soil results from the action of living organisms and climatic influences of region.Leachingis a process by which inorganic nutrients are transported from horizon A to B by flowing water.

 

                                          Figure 4: Representation of soil horizon/ profile

 

 

Soil Profile

  1. Horizon O – Consists of organic matter plant remains
  2. Horizon A (top soil)- Rich in inorganic nutrients and humus (dark in color)
  3. Horizon B (sub soil)– Light in color due to scarce organic matter
  4. Horizon C- Initial stages of fragmentation of rocks
  5. Horizon D- Existence of patent material & influence soil’s rate of water absorption & retention

 

Inorganic Components in Soil

They act as storehouse of water and plant nutrients. They assist in absorption of substances and their further detoxification and contributes in determining soil productivity. Elemental composition varies from O (46.6%), Si (27.7%), Al (8%), Fe (5%), Ca (3.6%), Na (2.8%), K (2.65), Mg (2%). Soil Minerals like quartz, silicates, iron oxides, clay occurs in soil.

 

Soil Humus

Humus comprises of two parts: Humic&Fulvic acids (base soluble)and humin (base insoluble). Humification refers to the process by which humus is formed.During process, N/C ratio increases as carbon is lost to CO2 evolved during biodegradation and nitrogen fixed by nitrogen fixing bacteria is incorporated into the humic residue.Humic substances exert a strong influential impact on the soil properties. They are known to bind metals strongly and also enhance water binding capacity of soil.They are also known to act as buffers in soil due to their strong acid-base character. They enhance the water holding capacity of soil. They also stabilize to increase the sorption of organic compounds by soil.

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References:

  • Bridges, E. M.,1978, World Soils, II ed. Cambridge: Cambridge University Press.
  • Biswas,T.D., Mukherjee,S.K.,1994, Text Book of Soil Science, II ed. Tata Mc Graw–Hill Publishing Company Limited, New Delhi
  • Bear, F. E.,(Ed.), 1976, Chemistry of the Soil II ed, Oxford & IBH Publishing, New Delhi
  • Tan, K. A., 1982, Principles of Soil Chemistry, Marcel Dekker, Inc, New York.
  • McBride,M.B., 1994, Environ. chemistry of Soils, Oxford University Press, New York.
  • Stehouwer, R., 2004 Soil Chemistry and the Quality of Humus, Biocycle, 45, 41-46.
  • Chesworth, W., (Ed.) 2008 Encyclopedia of Soil Science, Springer, Dordrech, The Netherlands.
  • Eash, N.S., 2008 Soil Science Simplified 5th ed., Blackwell Publishing, Ames, IA.
  • Malcolm E Sumner, 2000 handbook of Soil Science, CRC Press, New York.
  • www.isric.org, ISRIC-World Soil Information