Soil Organisms: Description, Condition and Effects | Soil Science

In this article we will discuss about:- 1. Introduction to Soil Organisms 2. Description of Soil Organisms 3. Role 4. Conditions Influencing the Activities 5. Effect of Cultural Practices.

Contents:

1. Introduction to Soil Organisms
2. Description of Soil Organisms
3. Role of Soil Organisms
4. Conditions Influencing the Activities of Soil Organism
5. Effect of Cultural Practices on Soil Organism

1. Introduction to Soil Organisms:

Soils are composed of minerals, organic matter, air, water and living organism. Living Organism both plants and animals, constitutes an important component of soil. The green or chlorophyll bearing plants and certain other organism (e.g. Euglena, Chrysamoeba etc.) having chlorophyll can synthesis food materials in presence of sunlight by taking carbon dioxide from air and nutrients from the soil.

So the plants are the Primary producer. The human beings and herbivorous animals feed on plants and carnivorous feed on animals. They are, therefore, consumers. Ultimately all plants and animal residues are returned to the soil and subjected to decomposition by micro-organisms. As a result of these, microbiological processes, the elements which were originally consumed by plants for organic synthesis are returned for reutilization.

The organisms that are lived in the soil are called ‘Soil organism’.

Important group of soil organisms are as follows:

Soil Flora – Algae, bacteria, actinomycetes, fungi etc.

Soil Fauna – Protozoa, nematodes, rotifer, earthworm, moles, ants etc.

Soil organisms are grouped into three groups on the basis of temperature requirement as follows –

Psychrophiles (< 10°C), mesophiles (20-40°C) and thermophiles (> 40°C).

Microorganisms are divided into two groups as follows:

(i) Aerobic – The organism which cannot exist without oxygen is called ‘aerobic organism’. Aerobic ones includes all fungi, majority of actinomycetes and mainly bacteria.

(ii) Anaerobic – The group of anaerobic microbes develop without any access of atmospheric oxygen. They take the oxygen for their respiration from oxygen containing compounds.

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2. Description of Soil Organisms:

Soil Flora:

1. Macro Flora:

(i) Roots or Higher Plants:

The space around the root surface of higher plant is called ‘Rhizosphere’. Rhizosphere provides a special environment for rapid proliferation of microorganism due to exudation of energy rich compounds. The root system of higher plants is associated with a vast community of metabolically active micro-organism. Plant roots grow and die in the soil and provide food to the soil organism.

2. Micro Flora:

(i) Algae:

Algae are chlorophyll bearing organism. They are autotrophic and capable of performing the photosynthesis, a process of synthesis of food material in presence of sunlight
by taking carbon dioxide from air and nutrients from the soil. Algae are universely distributed in soils but are relatively few in number as compared to the bacteria, actinomycetes and fungi. Number ranges from as few as one to forty thousand per gram of soil.

They are dominantly photoautotrophic nutrition and are dependent on sunlight and CO₂ (carbon dioxide), although the latter rarely becomes limiting. Algae are also of considerable economic importance in paddy growing areas in the Far East. Blue green algae is capable of fixing atmospheric nitrogen and they are used as ‘biofertilizers’.

Soil algae are divided in four classes depending on their colour such as:

a. Cyanophyta (blue green algae),

b. Chlorophyta (grass green algae),

c. Xanthophyta (Yellow green algae) and

d. Bacillariophyta (golden brown algae or diatome).

Algae are of considerable importance in the paddy growing areas in the Far East. Here their main role is the provision of a low level nitrogen supply through nitrogen fixation and by supplying oxygen to the submerged roots of the plants. Algae are capable of providing 30-40 kg of biologically fixed nitrogen per hectare in a paddy season. In general, the free living blue green algae fixes atmospheric nitrogen non-symbiotically. But there are certain blue green algae which possess the character of symbiotic nitrogen fixation in association with other organism such as aquatic fern (Azolla), mosses, fungi etc.

A blue green algae (Anabeana azolla) living in the epidermal cavity of the lower side of the leaf of azolla in symbiotic association fixes atmospheric nitrogen. This symbiotic association of Azolla pinnata and Anabeana azolla is termed as ‘Azolla anabeana complex’.

The algae fixes atmospheric nitrogen for azolla and in exchange the plants provides home and food to the algae. As this complex fixes atmospheric nitrogen, it has a great potentiality for use in agricultural field as bio-fertilizer and can be used as an alternative to nitrogenous fertilizers.

(ii) Bacteria:

Bacteria are single cell organism devoid of chlorophyll. It is the most abundant as well as the most diversified group of microorganism. In the soil, bacteria exists as mats and filaments, called ‘colonies’ on and around the soil particles wherever food and other conditions are favourable.

Bacteria are divided into three groups on the basis of their morphology as follows:

(i) Cocci or round spherical cells.

(ii) Bacilli or rod like cells.

(iii) Sprilla or spiral cells with long wavy chains.

Soil bacteria are commonly classified into two types on the basis of their sources of energy and carbon as follows:

(i) Autotrophic Bacteria:

The autotrophic bacteria obtain their energy from the oxidation of mineral constituents such as ammonia, sulphur and iron and most of their carbon from carbon dioxide. This bacteria are of different types such as Nitrifying bacteria, Denitrifier, Sulphur oxidizer, Iron oxidizer, Hydrogen oxidizer etc.

(ii) Heterotrophic Bacteria:

The heterotrophic bacteria depend on the oxidation of organic compounds for their energy and are responsible for the decomposition of cellulose, hemicelluloses, starches, sugars, protein, fats, waxes and other plant and animal remains that find their way to soil. The heterotrophic bacteria can be divided most conveniently on the basis of their nitrogen requirements in nitrogen fixing organism and those that require nitrogen in a combined organic or inorganic form.

The nitrogen fixing organisms are relatively few in numbers, but the three representative groups namely, the symbiotic nitrogen fixing organisms and the aerobic and anaerobic non-symbiotic fixer, play an important part in the soil economy. The most abundant species of soil bacteria are Azotobacter, Aerobacter, Arthobacter, Agrobacterium, Bacillus etc. The important species of bacteria from plant nutrition point of view are Beijerinekia, Clostridium, Nitrobacter, Nitrosomonas and Rhizobium.

Bacteria are involved in several processes and transformation of nutrients. Nitrogen fixation is an important process carried out by bacteria and this has considerable influence on agriculture. Several steps in mineralisation of organic matter are carried out by bacteria which releases several nutrients like nitrogen (N), phosphorus (P), potassium (K), sulphur (S) and iron (Fe). Bacteria also involved in decomposition of cellulose and oxidising sulphur (S) and manganese (Mn). The number of bacteria in soil is variable depending on the type of soil, climatic conditions and other environmental factors.

Conditions Affecting the Growth and Activity of Soil Bacteria:

There are some conditions that affect the growth and activity of soil bacteria as follows:

(i) Nutrients – The activity and number of soil bacteria depends on the availability of required nutrients. They need all the nutrients required by higher plants. Azotobacter is susceptible to a deficiency of phosphorus. The bacteria obtain mineral nutrients from soil or decomposing organic materials.

(ii) Oxygen – In normal arable soil, aerobic bacteria predominate as they use oxygen gas. On the other hand, the anaerobic bacteria dominate in poorly aerated soil as they use mostly combined oxygen.

(iii) Water – Bacteria do their function well in a soil having optimum moisture level for higher plants. Periodic variation in soil water brings about parallel variation in bacterial population.

(iv) Temperature – Soil temperature influences the activity of soil bacteria. The optimum range of soil temperature is between 25°C and 35°C. But some bacteria thrive well below or above this range.

(v) Organic matter – The heterotrophic bacteria draw carbon and energy from organic matter, decaying roots and plant residues. This bacteria decompose organic wastes and dead bodies of plants and animals and they bring about mineralisation of organic matter.

(vi) Soil pH – Soil pH influences the activity of soil bacteria. The bacterial population is highest in neutral (pH 7.0) or near neutral soil (pH 6.5-7.5). High acidity and high salinity are unsuitable for bacteria. Soil pH 6.0-8.0 is generally best for most of bacteria.

3. Soil Fungi:

Fungi are heterotrophic plants having no chlorophyll. They cannot manufacture their food and so they depend on the organic matter of the soil for their energy and carbon.

Fungi are of two types as follows:

(i) Parasites – Parasites obtain their food from living organism.

(ii) Saprophyte – Saprophyte lives on dead tissues of organic substances and derives nutrients from their host.

Fungi perform two important functions as follows:

(i) Fungi act as scavengers and decompose the organic matter to the form available to the plants. Fungi are capable of decomposing lignin. Fungi play an active role in transforming nitrogen constituent of soil.

They are associated with the decomposition of different materials as follows:

Cellulose – Penicillium, Aspergillus, Fusarium, Trichoderma etc.

Lignin – Basidomycetes etc.

Protein – Aspergillus, Cytromyces, Fusarium, Mucor etc.

(ii) Certain fungi belonging to the groups, phytomycetes and Fungi imperfecti, feed on protozoa and nematodes in soil and thus maintain biological equilibrium of soil.

Fungi may be divided into three groups such as molds, yeasts and mushroom.

The chemical substances such as streptomycin, a widely used antibiotics and penicillin are obtained from streptomyces and Penicillium notatum respectively.

Unlike the bacteria and actinomycetes, the fungi have high acid tolerance and under conditions of extreme acidity these organisms may become predominant flora. They are exceptionally well equipped to undertake the rapid decomposition of virtually all of major plant constituents-cellulose, hemicelluloses, pectin, starch, and even the lignins. They also play an active role in transforming the nitrogen constituents of soil.

4. Micorrhizae:

(Myco = fungus, rhizae = roots or fungus roots) – Micorrhizae are found as a result of unique association of certain fungi with the roots of plant.

Micorrhizae are divided into two general classes based on the inter-relation of threadlike fungus hypae and the root cells as follows:

(i) Ectotrophic – In this case, the fungus hypae penetrate between cortex cell of the root but do not enter the cell. e.g. Amanita. Boletus, etc.

(ii) Endotropic – In this case, the fungus hypae, actually penetrate the epidermal and cortex cell of the root. e.g. Phoma, Rhizoctonia etc.

5. Soil Actinomycetes:

Actinomycetes are thread like organism and are intermediate between bacteria and fungi. So they are sometimes called ‘fungi like bacteria’. They are next to bacteria in abundance in soil. They are aerobic and heterotrophic and need plenty of organic material, optimum water and aeration for growth. They are of great importance in the decomposition of soil organic matter and the liberation of nutrients from them.

They can degrade all sorts of organic substances such as cellulose, fats, proteins, organic acids, polysaccharides etc. However the rate of decomposition is slow compared to bacteria and fungi. They are more abundant in dry soils than in wet soils, grassland and Pasteur soils than in cultivated soils, surface soil than in sub-soils and tropical soils than in the soils of temperate region. Streptomyces is the most common genera of actinomycetes.

Soil Fauna:

1. Macrofauna:

(i) Earthworm:

Earthworm is one of the most important macrofauna and it is found in moist soil containing organic matter. It feed on the dead organic tissues in soil and soil material along with it. The soil is finally ground in its gizzard and moved through its alimentary canal. The excreta which are casts on the surface soil are richer in bacteria and available mineral nutrient. They make burrows in the field and bring the sub soil to the surface. The burrows improve the aeration of the soil. It is generally called as “Farmer’s friend”. They are more common in fine textured soil (i.e. clay, loam etc.) than coarse sandy soils.

Factors Affecting the Activity of Earthworm:

There are some factors affecting the activity of earth worm as follows:

(i) Water – Earthworm prefers moist soil. So it is found abundantly in moist soil.

(ii) Air – Well aerated soil is a good habitat of earth worm.

(iii) Soil – Earthworms are found mostly in fine textured upland soil. Generally they are more common in fine textured soil (clay, loam etc.) than coarse sandy soils.

(iv) Organic matter – It is found in a soil having sufficient organic matter as a source of food. Consequently they thrive in a soil where farm manure or plant residues are added to the soil. It requires a high level of exchangeable calcium for optimum activity.

(v) Soil pH – Earthworm thrives best in a soil which is not too acid. A few species are reasonably tolerant to low soil pH.

(vi) Soil temperature – Soil temperature affects the number and distribution of earthworm. A temperature of 10°C is ideal for earthworm, and the number of earthworm declines at above or below this temperature.

2. Microfauna:

(i) Protozoa:

Protozoas probably are the simplest form of animal life and they are microscopic unicellular organisms existing in all arable soils but numerically form only a small part of the soil population. They live in the surface soil. Amoeba, Paramecium, Euglena etc. are free living form. Soil protozoa feeds either on organic matter or on bacteria, thus help to maintain a favourable balance of microfauna of the soil.

(ii) Nematodes:

Nematodes are the abundant soil microfauna. They live in the soil and cause diseases in many crops plant. So they are harmful microfauna.

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3. Role of Soil Organisms:

A vast number of organism live in the soil.

The role or function of soil organism can be divided into beneficial function and harmful function as follows:

I. Beneficial Functions of Soil Organism:

The different beneficial functions of soil organism are as follows:

1. Decomposition of Organic Debris:

Soil organisms play a role in the decomposition of organic debris. Plant cannot utilize the raw organic matter directly as a food materials. A large number of bacteria and fungi bring about the decay of organic substances and their mineralization process releases a variety of nutrient to be made available to plants.

Compounds like sugar, starch and protein are decomposed first in the decomposition process and then cellulose, fatty substances and lastly lignin and woody substances are degraded. Protein when acted upon by microbes are converted into amino acid, ammonium salt, nitrites and nitrate. The change of nitrogen containing organic compound into ammoniacal nitrogen is called ammonification.

In the decomposition process, a number of complex mineral compounds are also converted into simpler and soluble compound. Organic acid and carbon dioxide that are released by decomposition of organic substances make insoluble phosphates and other unavailable compound more easily available to plants.

Humus, an intermediate product, of decomposition process, is formed by microorganism in optimum physical condition. Decomposition of dead organic matter primarily helps in the feeding and growth process of these micro-organism and secondly, increase the nutrient content of the soil. Bacteria and soil fungi are the main agents which bring about the process of decomposition of organic compound in the soil.

(ii) Nitrogen Fixation:

A great number of bacteria such as symbiotic (e.g. Rhizobium etc.), non-symbiotic (e.g. Azotobacter, Clostridium, Pasteurianum etc.) and blue green algae are known to fix free atmospheric nitrogen gas into nitrogenous compound such as nitrite and nitrates and thereby increases the fertility of the soil. De and Fritsch (1938) have found that certain blue green algae are able to fix 20 lb (9 kg) of atmospheric nitrogen per acre in a paddy field. Biological nitrogen fixation plays an important role in the economy of crop production.

(iii) Formation and Development of Soil:

Soil organisms play an important role in the weathering of rocks and formation of soil. Bacteria, fungi and algae helps in the development of desirable soil structure by their secretions of gummy substances. The excreta of soil animals is deposited on the surface of soil in the form of casts which increases the fertility of soil.

(iv) Improvement of Soil Aeration:

Soil organism improves aeration of soil. The large organism like earthworm, rodents lives in the soil. They improve the aeration of the soil due to their burrowing habit. Roots of higher plants take active part in the disintegration of rocks mass and also make the compact soil loose.

(v) Production of Growth Stimulating Substances:

Many soil organisms including soil fungi and bacteria produce growth stimulating substances such as 3-Indol acetic acid, gibberellins and gibberellic acid in the soil. Fusarium species have been found to secrete gibberellin and gibberellic acid.

II. Harmful Role of Soil Organism:

Soil organisms have some harmful role as follows:

(i) Production of Some Toxic Compounds:

Under anaerobic condition, some soil microbes secrete chemicals such as aldehyde, organic acids etc. which may show toxic effects to many plants. Algae, bacteria and fungi secretes toxic substances. Fusarium lini, a causal organism of wilt of Flax (Linum usitatissimum Linn) secretes hydrocyanic acid (HCN), a deadly poisonous substance and Fusarium udum, a causal organism of wilt of Pigeon pea/arhar (Cajanus cajan) secretes fusaric acid in the root of host plant. These toxic substances secreted by the fungi may be responsible for causing wilt in the flax and arhar respectively.

(ii) Denitrification:

The biochemical reduction of nitrate nitrogen to gaseous nitrogen is called ‘denitrification’. It is not desirable in agriculture as in this process the available nitrogen is converted to unavailable form. Water logging in paddy fields will increase nitrogen losses due to denitrification. Nitrate fertilizers should not be applied in paddy field as they are subjected to be lost by denitrification.

(iii) Development of Plant Diseases:

Not all the soil organisms are beneficial in their properties and behaviour. Some microbes becomes parasites of higher plant and causes considerable damage. Soil bacteria, fungi, nematodes etc. causes many diseases of crops such as seedling blight, damping off, root rot etc.

(iv) Competition for Nutrients:

Soil organism utilizes the nutrients such as carbon, hydrogen, oxygen, nitrogen, phosphorus etc. for their growth and development. As a result, plant do not get sufficient nutrients for their growth and development.

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4. Conditions Influencing the Activities of Soil Organism:

There are some factors that are affecting the activities of soil organism are as follows:

(i) Water:

Water is essential for all life process. Soil organisms do function well in a soil having adequate water. Soil moisture is the most important factor in deciding the type of microorganism present in the soil. When soil moisture is near or below field capacity, aerobic bacteria flourish. Anaerobic bacteria exists in a soil which is saturated or submerged. Actinomycetes predominates in dry-region.

(ii) Air:

Aerobic organism requires oxygen for their growth and activity. Aeration is very important for nitrification as these bacteria are aerobic.

(iii) Temperature:

Soil organisms are most active and multiply fast at the optimum temperature which ranges between 30°C. to 37°C. The optimum temperature for nitrification is 30°C to 35°C. Most microflora of the soil is mesophiles and grows well at a temperature of 20°C to 40°C with optimum being 37°C. Direct sunlight is injurious to most microflora except algae.

(iv) Soil pH:

The soil with pH 7.0 (i.e. neutral condition) is most suitable for soil organism. The optimum soil pH differs with the type of microflora. Bacteria grows well in a soil having pH range of 6.5-8.0. Fungi and actinomycetes prefer acidic condition (pH 4.5-6.5) and slightly alkaline condition respectively. Very acidic and alkaline condition are not favourable for soil micro-organism.

(v) Nutrients:

Soil organism needs nutrients such as nitrogen, phosphorus, sulphur etc. and carbohydrates for building up of their body cells.

(vi) Organic Matter:

Bacteria require organic matter in the soil and they are very active when fresh organic matter is added to soil.

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5. Effect of Cultural Practices on Soil Organism:

(i) Tillage:

Tillage is tilling of land for bringing about conditions that are favourable for the cultivation of crops. Tillage operation facilitates aeration in the soil and expose soil surface to sunshine and thereby increases biological activity of soil organism, particularly of bacteria.

(ii) Crop Rotation:

Crop rotation is the planned sequence of cropping. Crop rotation with legumes increases the number of nitrifying bacteria in the soil.

(iii) Application of Manures and Fertilizers:

Manures and fertilizers are the good source of soil nutrients for the crops as well as the soil organism. Organic manures provide a readily available source of carbon for the heterotrophs. Phosphatic fertilizer has a special action on leguminous crops. It induces nodule formation of this crop and rhizobial activity. Thus it helps in fixing more of atmospheric nitrogen in root nodule of leguminous crops.

Application of some oilcakes viz., neem, Karanja and groundnut encourages the growth of nematode trapping fungi. The decomposition products of these cakes are also toxic to nematode. On application of nitrogenous fertilizers, it was found that the bacterial count was relatively lower, but that of fungi and actinomycetes was relatively higher.

(iv) Irrigation:

Soil organisms require optimum soil moisture for their growth and activity. Irrigation should be given to provide optimum moisture in the soil as and when needed. Irrigation of soil, particularly in arid regions, brings about significant-proliferation of soil microbes.

(v) Application of Soil Amendment:

Generally lime and gypsum are applied in soil for the reclamation of acid and alkali soil respectively. Liming of acid soil increases the activity of bacteria and actinomycetes and the population of the fungus. On the other hand, application of gypsum to alkali soil is favourable for the activity of bacteria.

(vi) Application of Plant Protection Chemicals:

In intensive agriculture, plant protection chemicals need to be applied to control insect pests, nematodes and weeds. Application of plant protection chemicals, particularly when applied in soil, reduces the number of soil fauna as well as total population number of the various soil organism. The bacteria, fungi and nematodes seem to be most affected by plant protection chemicals, particularly pesticides.