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Current Category » Introduction to Soil Science

Decomposition of Soil Organic Matter

The organic materials (plant and animal residues) incorporated in the soil are attacked by a variety of microbes, worms and insects in the soil if the soil is moist. Some of the constituents are decomposed very rapidly, some less readily, and others very slowly. The list of constituents in terms of ease of decomposition:
1. Sugars, starches and simple proteins Rapid Decomposition
2. Crude proteins
3. Hemicelluloses
4. Cellulose
5. Fats, waxes, resins
6. Lignins Very slow Decomposition

The organic matter is also classified on the basis of their rate of decomposition

Rapidly decomposed: Sugars, starches, proteins etc
Less rapidly decomposed: Hemicelluloses, celluloses etc
Very slowly decomposed: Fats, waxes, resins, lignins etc

Simple decomposition products

Aerobic – CO2, H2O, NO3, SO4

When organic material is added to soil, three general reactions take place

a. The bulk of the material undergoes enzymatic oxidation with CO2, water, energy and heat as the major products.
b. The essential elements such as N, P and S are released and / or immobilized by a series of reactions.

Compounds very resistant to microbial action is formed either through modification of compounds or by microbial synthesis.

A. Decomposition of soluble substances: When glucose is decomposed under aerobic conditions the reaction is as under:
Sugar + Oxygen --------------> CO2 + H2O

Under partially oxidized conditions,
Sugar + Oxygen --------------> Aliphatic acids
                                                (Acetic, formic etc.) or
                                                Hydroxy acids
                                                (Citric, lactic etc.) or
                                                Alcohols (ethyl alcohol etc.)

Some of the reactions invoiced may be represented as under:
C6H12O6 + 2O2 -------------> 2 CH3. COOH + 2CO2 + 2H2O
2C6H12O6 + 3O2 ------------> 2 C6H8O7 + 4 H2O
C6H12O6 + 2O2 -------------> 2C2H5OH + 2 CO2

i) Ammonification: The transformation of organic nitrogenous compounds (amino acids, amides, ammonium compounds, nitrates etc.) into ammonia is called ammonification. This process occurs as a result of hydrolytic and oxidative enzymatic reaction under aerobic conditions by heterotrophic microbes.

ii)Nitrification: The process of conversion of ammonia to nitrites (NO2) and then to nitrate (NO-3) is known as nitrification. It is an aerobic process by autotrophic bacteria.
            Nitrosomonas            Nitrobacter
NH3 --------------------------> NO2 -------------------> NO-3
Ammonia                                  Nitrite                             Nitrate

The net reactions are as follows:
NH4 + O2 -------------------> NO2 + 2H+ + H2O + energy
NO2 + O2 ------------------- >NO-3 + energy

iii) Denitrification: The process, which involves conversion of soil nitrate into gaseous nitrogen or nitrous oxide, is called Denitrification. Water logging and high pH will increase N loss by Denitrification.

             Pseudomonas / Bacillus
Nitrate --------------------------------> Nitrogen Gas

B. Decomposition of Insoluble Substances

i) Breakdown of Protein: During the course of decomposition of plant materials, the proteins are first hydrolyzed to a number of intermediate products and may be represented as under:

              Hydrolysis         Proteases Aas
Proteins --------------> Peptones --------> Amides ------> Ammonia Peptides
           Aminization                                         Ammonification
Aminization: The process of conversion of proteins to aminoacids.
Ammonification: The process of conversion of aminoacids and amides to ammonia.

ii) Breakdown of cellulose: The decomposition of the most abundant carbohydrates is as follows:

                 hydrolysis                     hydrolysis
Cellulose ---------------> Cellobiose ----------------> Glucose
                 (cellulase)                        (cellobiase)
---------------> Organic acids -------------> CO2 + H2O
This reaction proceeds more slowly in acid soils than in neutral and alkaline soils. It is quite rapid in well aerated soils and comparatively slow in poorly aerated soils.

iii) Breakdown of Hemicellulose: Decompose faster than cellulose and are first hydrolyzed to their components sugars and uronic acids. Sugars are attacked by microbes and are converted to organic acids, alcohols, carbon dioxide and water. The uronic acids are broken down to pentose and CO2. The newly synthesized hemicelluloses thus form a part of the humus.

iv) Breakdown of Starch: It is chemically a glucose polymer and is first hydrolyzed to maltose by the action of amylases. Maltose is next converted to glucose by maltase. The process is represented as under:

(C6H10O5)n +nH2O -------------> (C6H12O6)

C. Decomposition of ether soluble substances:

Fats------------------------> glycerol + fatty acids
Glycerol -------------------> CO2 + water

D. Decomposition of lignin:
Lignin decomposes slowly, much slower than cellulose. Complete oxidation gives rise to CO2 and H2O.

Current Category » Introduction to Soil Science