Agriculture for Engineers 4 (3+1)

Lesson 10. Soil Organic Matter

10.2 COMPOSITION OF PLANT RESIDUES

Composition of organic residues has un-decomposed soil organic matter (mainly plant residues together with animal remains. i.e. animal excreta etc). The moisture content of plant residues varies from 60 to 90 % (average 78 %) and 25 % dry matter (solid). Plant tissues (organic residues) may be divided into 1) organic and (2) inorganic (elemental) composition. The compounds constituting the plant residues or undecomposed soil organic matter is shown in the following diagram.

Fig. 10.1 Compounds constituting the plant residues or un-decomposed soil organic matter

10.3 DECOMPOSITION OF ORGANIC MATTER

The organic materials incorporated in the soil do not remain as such very long. They are at once attacked by a great variety of microorganisms, worms and insects present in the soil especially if the soil is moist. The microorganism for obtaining their food, break up the various constituents of which the organic residues are composed, and convert them into new substances, some of which are very simple in composition and others highly complex. The whole of the organic residues is not decomposed all at once or as a whole. Some of the constituents are decomposed very rapidly, some less readily, and others very slowly.

A. Decomposition of soluble substances:

Sugar and water-soluble nitrogenous compounds are the first to be decomposed as they offer a very readily available source of carbon, nitrogen and energy for the microorganisms. Thus, when glucose is decomposed under aerobic conditions the reaction is as under:

     1. Ammonification:

The transformation of organic nitrogenous compounds into ammonia is called ammonification. During the course of action under aerobic conditions by heterotrophic organisms, oxygen is taken up and carbon dioxide is released. Ammonification process involves a gradual simplification of complex compounds. The ammonification occurs as a result of action of enzymes produced by microorganisms. Their action is chiefly hydrolytic and oxidative (in presence of air).

Organic nitrogen ---> NH3 or protein ---> polypeptides ----> amino acids ----> ammonia or salts

       2. Nitrification:

The process of conversion of ammonia to nitrite (NO₂) and then to nitrate (NO₃) is known as nitrification. The production of nitrate is more rapid than that of nitrite, while the formation of ammonia is the slowest process. That is why soil usually contains more nitrate nitrogen than nitrite at any time. Nitrification is an aerobic process involving the production of nitrates from ammonium salts. It is the work of autotrophic bacteria.

The process which involves conversion of soil nitrate into gaseous nitrogen or nitrous oxide is called denitrification.

 

 Water-logging (e.g., rice field) and high pH will increase nitrogen loss   by denitrification.

 B. Decomposition of insoluble substances

1. Breakdown of protein

Proteins are complex organic substances containing nitrogen, sulphur, and sometimes phosphorus, in addition to carbon, hydrogen and oxygen. During the course of decomposition of plant materials, the proteins are first hydrolyzed to a number of intermediate products. e.g. proteoses, peptides, etc., collectively known as polypeptides.

The process of conversion of proteins to amino acids is known as aminization.

2. Breakdown of cellulose

Cellulose is the most abundant carbohydrate present in plant residues. The microorganisms break up cellulose into cellobiose and glucose. Glucose is further attacked by organisms and converted into organic acids.

The decomposition of cellulose in acid soils proceeds more slowly than in neutral and alkaline soils. It is quite rapid and well-aerated soil and comparatively slow in those poorly-aerated.

3. Breakdown of hemi-cellulose

When subjected to microbial decomposition, hemicelluloses are first hydrolyzed to their component sugars and uronic acids. The sugars are further attacked by microorganisms. They are converted to organic acids, alcohols, carbon dioxide and water.

The uronic acids are broken down to pentose and carbon dioxide. The newly synthesized hemicelluloses thus form a part of the humus. Hemicelluloses decompose faster than cellulose

4. Breakdown of starch

Chemically it is glucose polymer. It is first hydrolyzed to maltose by the action of enzymes (amylases). Maltose is next converted to glucose by another enzyme (maltase). Glucose being soluble in water is utilized for growth and other metabolic activities.

 C. Decomposition of ether-soluble substances

Fats are first broken down by microorganisms through the agency of enzyme lipase into glycerol and fatty acids. Glycerol is next oxidized to organic acids which along with the other fatty acids are finally oxidized to carbon dioxide and water.

D. Decomposition of lignin

Lignin is deposited on the cell wall to impart strength to the skeleton framework of plant. Lignin decomposes slowly, much slower than cellulose. Complete oxidation of lignin gives rise to carbon dioxide and water.

E. Simple decomposition products

As the enzymic changes of the soil organic matter proceed, simple products begin to manifest themselves. Some of these, especially carbon dioxide and water, appear immediately. Others such as nitrate-nitrogen accumulate only after the peak of the vigorous decomposition is over. The more common simple products resulting from the activity of the soil microorganisms are as follows:

 Carbon : CO_2, CO₃^-  HCO₃^-  , elemental carbon

Nitrogen: NH₄⁺, NO₂^- , NO₃^-   , gaseous nitrogen

Sulphur: S, H₂S, SO₃^-- , SO₄^--   , CS₂

       Phosphorus: H₂PO₄^- , HPO₄^-

      Others H₂O, O₂, H₂, H⁺, OH^- , K⁺, Ca⁺⁺ , Mg⁺⁺ etc.

10.4 Mineralization of organic sulphur:

Many organic compounds especially those of a nitrogenous nature, carry sulphur, Heterotrophic bacteria simplify the complex organic compounds, and then autotrophic bacteria (sulphur bacteria) oxidize it into sulphate form.

10.5 Mineralization of organic phosphorus:

A large proportion of the soil phosphorus is carried in organic combinations. Upon attack by microorganisms, the organic phosphorus compounds are mineralized; that is, they are changed to inorganic combinations. It depends upon soil pH. As the pH goes up from 5.5 to 7.5 the available phosphorus changes from H₂PO₄^-  to HPO₄^-2 ,both of these are available to higher plants.