After reading this article you will learn about the beneficial and harmful roles of soil microorganism.

Beneficial Role of Soil Microorganism:

The beneficial role of soil microorganism are:

(a) Change and decomposition in organic matter

(b) Fixation of atmospheric nitrogen

(c) Formation and development of soil.

(a) Change and Decomposition in Organic Matter:

The organic materials when incorporated in the soil are attacked by the soil microorganisms. The microorganisms break up the various constituents of the organic materials and convert them into new substances.

(i) Ammonification:

The change of organic nitrogenous compounds into ammonia is called ammonification. The ammonification occurs as a result of action of enzymes produced by microorganisms in the presence of air.

Protein → Polypeptides → Amino acids → Ammonia

(ii) Nitrification:

The conversion of ammonia to nitrate (NO₃) is known as nitrification. Autotrophic bacteria performed this transformation. It is an aerobic process.

De-nitrification:

The conversion of soil nitrate into gaseous nitrogen or nitrous oxide is known as de-nitrification. It is an anaerobic process performed by bacteria belonging to genera Pseudomonas, Micrococcus and Bacillus.

Decomposition of Simple Products:

The more common simple products resulting from the activity of the soil microorganisms are as follows:

Carbon: CO₂, CO₃-,H CO₃-, elemental carbon

Sulphur: S,H₂S, SO₃-, SO₄– ,CS₂

Phosphorus: H₂ PO₄-,HPO₄–

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

Mineralization of Organic Sulphur:

Autotrophic bacteria (sulphur bacteria) oxidised sulphur into sulphate form.

Mineralization of Organic Phosphorus:

Organic phosphorus compounds are mineralized by the action of microorganism into inorganic phosphorus.

(b) Fixation of Atmospheric Nitrogen:

The conversion of elemental nitrogen to readily available form by nitrogen-fixing microorganisms is called biological nitrogen- fixation. The nitrogen fixed by microorganisms in the soil is known as ‘bio- fertilizers’.

The nitrogen-fixing microorganisms are:

(I) Bacteria

(II) Algae and

(III) Mycorrhizae (fungus root).

(I) Bacteria:

There are two main groups of bacteria which fix atmospheric nitrogen; symbiotic and non-symbiotic. Addition of nitrogen in the soil by bacteria is called ‘bacterial-fertilizers’.

(i) Symbiotic Nitrogen-fixing Bacteria:

This group of bacteria (Rhizobium) fix nitrogen in association with leguminous plants, called symbiotic bacteria. They are root nodule bacteria. The Rhizobium bacteria living in the soil enter the root hairs of leguminous plants, develop into colonies and form small nodules on the roots. They take their food (carbohydrate) from the leguminous plants and absorb nitrogen from the atmosphere.

They produce nitrogenous compound (available form of nitrogen) and supply to the leguminous host plants. Both the legume (pulse) crop and bacteria are benefitted by the association, known as ‘symbiosis’. The legume roots excrete available nitrogenous compound to the soil and enrich it.

The nodules of the legumes decay and become part of the soil. This process also adds nitrogen to the soil. There are different strains of legume—Rhizobium bacteria and specific strain works best with specific leguminous crop.

The strains of Rhizobium and their association with leguminous crops are given below:

When a legume is introduced in a new locality, it is necessary to inoculate the seed with proper Rhizobium culture otherwise crop may not produce nodules and not thrive. In the absence of proper Rhizobium culture, the seed can be mixed with the soil obtained from an area where the particular group of legumes grow well. The amount of nitrogen added to the soil by Rhizobium bacteria varies from 50-150 kg per hectare.

(ii) Non-symbiotic Nitrogen-Fixing Bacteria:

Azotobacter and Clostridium are the most important non-symbiotic nitrogen-fixing bacteria. These bacteria work independently of any host crop. Azotobacter is an aerobic nitrogen-fixing bacteria and thrives well in neutral soil. It is susceptible to a deficiency of phosphate. Azotobacter has been found to fix a considerable amount of nitrogen.

Clostridium is anaerobic which can adapt even in acid soil and fix nitrogen. This microorganism is fixing less amount of nitrogen than Azotobacter because of anaerobic fermentation releases only a small amount of energy. Whereas aerobic change produces large amount of energy which helps to fix more nitrogen.

(II) Algae:

Blue-green algae is capable of fixing atmospheric nitrogen to the soil. The main genus of algae which fix nitrogen are Anabaena, Nostoc and Cylindrospermum. The blue-green algae utilise a variety of nitrogen sources, besides the free nitrogen from atmosphere. They prefer usually neutral or slightly alkaline’ soil.

In water-logged rice field, algae grow well and fix about 20-30 kg nitrogen per hectare. Nitrogen-fixation has been found to be greater in the presence of the crop than m the absence. Cylindrospermum is also found in maize and and sugarcane fields. Algae culture may be inoculated in a field to increase the soil fertility. The algal material after decomposition add organic matter to the soil and improves the physical condition of the soils.

(III) Mycorrhizae (Myco = fungus ; rhiza = root):

Mutually beneficial association between certain fungi and roots of higher plants is called mycorrhizae (fungs roots). By this symbiotic association, fungi get sugars and organic exudates from the roots of higher plant. In return, the fungi provide several essential nutrients, including phosphorus, calcium, magnesium, zinc, copper, manganese and iron to the roots of plant.

Mycroohizae are divided into two types: ectomycorrhiza and endomycorrhiza. The ectomy corrhiza group includes Amanita, Boletus etc., primarily associated with trees, such as oak, fir, pine etc. This group develops externally forming a mantle outside the roots. The hyphae of the fungi penetrate the roots and develop around the cells of the cortex but do not penetrate these cell walls.

The endomycorrhiza group are called vesicular-arbuscular mycorrhizae (VAM), penetrates the root cell walls, enters the root cells and forms hyphal masses within the cell. Almost 90 per cent of plants, including the most important agricultural field and fruit crops (wheat, maize, cotton, sugarcane, potato, dry land rice, beans, alfalfa, apple, grapes, citrus, coffee, rubber etc.) have VAM association. They increase the uptake of phosphorus, enhance resistance against drought and certain root- infecting fungus. The growth and biomass production of plants are also increased in the association of VAM fungus.

Microscopic examination of mycorrhizae shows the presence of aseptate, branched and hyaline mycelia. The fungus also produces spherical double walled chlamydospore. The root cortical cell walls of host plants are penetrated by the hyphae of VA mycoorhizae.

Inside the plant cell highly branched, small structures known as arbuscules are formed by the fungi. These structures are considered to be the site of transfer of mineral nutrients from the fungi to the host plants. Other structures, called vesicles, serve as storage organs for the plant nutrients and other products.

(c) Formation and Development of Soil:

(i) Decomposition of Rock and Minerals:

The soil microorganisms help in weathering of rocks and formation of soil.

(ii) Decomposition of Organic Matter:

They help in decomposition of organic matter into simpler and available forms.

(iii) Soil Structure Improvement:

Bacteria, fungi and algae aid in the development of desirable soil structure by their secretions of gummy substances. Addition of organic matter in the soil also helps in the development of good soil structure.

(iv) Movement of Soil Material:

Organic acids and carbon dioxide that are released by decomposition make insoluble phosphates and other unavailable compounds more available to plants.

Harmful Role of Soil Microorganism:

There are following harmful effects of soil microorganism:

(a) De-nitrification

(b) Development of plant disease

(c) Formation of toxic compounds aid

(d) Competition for nutrients.

(a) De-Nitrification:

The biological reduction of nitrate to gaseous nitrogen is called de-nitrification. Denitrifying bacteria that live in the soil under anaerobic condition which convert available nitrogen to unavailable form (molecular nitrogen or oxides of nitrogen) and hence, undesirable in agriculture. Water-logging in rice fields will increase nitrogen losses due to de-nitrification. Nitrate fertilizers, therefore, should not be applied in rice fields.

(b) Development of Plant Disease:

Plants of great economic importance to man are destroyed by soil microorganisms. The blight disease of rice, apple and pear is caused by bacteria. Fungi cause more serious damage to crop plants. For example, smuts and rusts of cereal crops and late blight of potatoes.

(c) Formation of Toxic Compounds:

Under anaerobic conditions toxic substances such as methane, hydrogen sulphide are formed due to improper decomposition of organic matter.

(d) Competition for Nutrients:

Soil microorganisms utilise carbon, hydrogen, oxygen, nitrogen, phosphorus for their growth and development. Competition for plant nutrients between soil microorganisms and crop plants is quite high. At the time of decomposition, microorganisms uptake more quantity of nutrients; as a result, plants get insufficient nutrients for their growth.