Chemical composition of soil, minerals, soil organic matter

Chemical composition of soil

The stoichiometry of the soil is not forward and almost about ten elements are in the proportionately decreasing order as,

O> Si> Al > Fe > C > Ca > K > Na > Mg > Ti

The amount of oxygen present in the soil is about 490g/kg and silica is about 310 g/kg and together in combined form they make about 80% of the soil makeup. They do not exist in a single form, rather they combine to form the minerals, and the dominant structural unit in these is SiO bond which is much stronger than the metal- oxygen bond. Such processed formed rocks are Quartz, feldspar, mica, amphibole, pyroxene, and Olivine, which have their origin in the parent rock. The Secondary minerals are those that results from the weathering of the primary rocks. In the chemical composition of the soil, the climate has the crucial role in the composition of the soil.

Chemical composition of the common soil minerals

A. Sand and silt minerals

1. Quartz or Silica

2. Feldspar

a. Orthoclase

b. Plagioclase

c. Calcium feldspar

3. Micas

a Muscovite

b. Biotite

4. Pyroxene

5. Amphibole

6. Olivine and Serpentine

7. Calcite, Magnesite, and Dolomite

8. Iron oxides

a. Haematite

b. Magnetite

c. Limonite

B. Clay minerals

1. Kaolin

2. Montmorillonite

Minerals

The naturally occurring substances which are homogeneous and have a definite composition with the physical properties like color, shape, hardness, melting point etc. are commonly known as minerals. The types of minerals in the soil can be categorized as:

a. Primary minerals

The minerals that are present from the original rocks or are formed during the cooling of the magma are known as primary minerals. The minerals also contain silicon and the oxygen along with their metal cations and are also known as the primary minerals.

b. Secondary minerals

The minerals that are formed by the decomposition of the primary minerals or by the recrystallization or the precipitation of the soluble substances. These minerals include clay minerals and are found in the sedimentary rocks.

Minerals

Quartz

Chemical composition: SiO₂

Comments: Hard, weathered slowly, and is the major material of most of the sands.

Feldspar

Orthoclase

Chemical composition: (K, Na)AlsiO₈

Plagioclase: (Ca, Na) Al (Al, Si) Si₂O₈

Comments: Hard, weathers slowly or moderately and provide important nutrients and clay in the weathered products.

Micas

Muscovite

Chemical composition: KAl₃SI₃O₁₀(OH)₂

Biotite

Chemical composition KAl(Mg,Fe)₂Si₄O₁₀(OH)₂

Comments: Glitter in rocks or the wet sands. Used as the important source of potassium and the clay.

Dark minerals (augite, hornblende, biotite, mica and others)

Chemical composition: Ca₂(Al, Fe)₄ (Mg, Fe)₄ Si₆

Includes several minerals that weather moderately fast and good clay formers.

Apatite

Chemical composition: 3Ca₃ (PO₄)₂ CaF₂

Comments: The most common minerals that supply phosphorus.

Secondary minerals and their composition

Calcite

Chemical Composition: CaCO₃

Dolomite

Chemical Composition: (Ca, Mg). (CO₃)₂

Comments: These are slightly soluble minerals in limestone or dolomite rock common in arid region soils; calcium and magnesium as the source.

Gypsum

Chemical Composition: CaSO₄.2H₂O

Comments: A soft, moderately soluble mineral found in arid region soils.

Iron Oxides

Chemical composition: Fe₂O₃.xH₂O

Comments

A group of minerals with different amount of water which provides the soil to their yellow to red colour.

Quartz

Chemical composition: SiO₂

Reprecipitate forms such as opal, agate, and petrified wood.

The clays, Kaolinite, and montmorillonite also remain as the secondary source.

Soil organic matter

The complex mixture of the organic compounds that are formed from the microbial decomposition of the plant and animal materials where the micro-organisms derive energy from their feeding. The soil organic matter consists of the decomposition of the organic matter of any living or the non-living forms. It supplies energy and the building constituents for the most of the organic matter. The organic matter influences the physical and the chemical properties of the soil far out of the proportion of the small quantities present. It commonly accounts for at least of the half proportion of the small quantities present.

Sources of the organic matter

The natural source of the organic matter is the plant tissue. Under the natural conditions, the top of the root of the trees, shrubs, grasses and other natural plants usually supply large quantities of the organic residue. When the plants are removed out from the top soil then the 1/3^rd part of the residue remains in the soil. As these materials are decomposed and is digested by the microorganisms, they become the part of the underlying horizons by physical incorporation. This shows that the plant tissue is the primary source not only of the food for various soil organisms but also of the organic matter which is essential for the soil formation. The animals are the secondary source of the organic matter. As they attack the original plant tissue, they contribute waste products and leave their own bodies as their life cycle are consummated. The earthworm, centipedes, and ant play important role in the translocation of the plant residue.

Advantages of the Organic matter

• It increases the water-holding capacity of the soil.
• It increases the cation exchange capacity.
• It improves the condition of the microbial growth.
• It supplies polysaccharides which are vital for the improvement of the soil structure.
• It helps to reduce the alkalinity of the soil.

Decomposition of organic matter in the soil

The bulk of the material undergoes enzymatic oxidation with CO₂, water, energy and heat as the major product.

The essential elements like nitrogen, phosphorus and sulphur are released or immobilized by a series of specific reactions relatively unique for each element.

The compounds, resistant to the microbial reaction are formed either through modification of the compounds in the original plant tissue or by the microbial synthesis.

The composition is fast under the aerobic conditions through the oxidation.

(C, 4H) + O₂ → CO₂ + H₂O + energy- H-containing compounds.

Composition

Carbohydrates which are made up of C, H and O range are in the complexity from simple sugar to the cellulose. The fats and oil are the glycerides of the fatty acids such as the butyric acid, stoic and the oleic acid. These are associated with the resins of many kinds and are somewhat complex more than the carbohydrates. They are too made up of C, H, and O The lignins occur in older plant tissue and are complex and very resistant to the decomposition.

References:

Keller, E.A. Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company, Bell, and Howell Company, 1985.

Mahapatra, G.B. Textbook of Physical Geology. Shahadra,Delhi-110032: CBS Publishers and Distributers Pvt.Ltd., 1992.