Soil Microbiology

Soil microbiology and types of microbes present in soil.

Soil form an excellent culture media for the growth of many kind of organisms . The microscopic life of soil include bacteria ,algae ,fungi protozoans and diatoms .There are many factors which decide the number and kind of microflora in the soil

• Depth of soil

Most of the soil microbes are found in the surface layer, where the number decreases with depth .Some of the microbes have been found below 3 feet but none at 5 feet. Unless they are accompanying plant roots. The number and species of microbes in soil vary.In general, spoon full of fertile soil is said to contain more microbes than there are people in the world and the majority of microbes are found in the upper 6 to 12 inches of soil.

• Nature of soil

The texture of soil ,its physical condition and amount of organic matter and minerals present affected the population of microbes in the soil. Soil having more of salt concentration supposed for the growth of halophiles Examples: Staphylococcus

• STATE OF CULTIVATION

The soil microflora depends upon whether the crop grown on that soil is in seedling stage or fully matured or has been recently harvested. Similarly, the kind of the root system shown by the plants ,also affect soil microbes . Example :An agricultural soil after harvesting leguminous crop will support more of Rhizobium ,Such soil show more of soluble substances like proteins, carbohydrates and lipids. These substances undergo hydrolysis which results in increased acidity of soil than support many types of heterotrophes and facultative autotrophs .

• pH OF SOIL:

The hydrogen ion concentration of a good topsoil ranging from 6 to 8 at which most kind of soil microbes grows best ,however pH range of 1 to 10 have been reported from many soils .Soil bacteria generally prefer neutral pH i.e 7 ,Fungi prefer acidic pH i.e 3 to 4.

• SOIL TEMPERATURE

The temperature of soil varies from day to day controlled by climate. Soil, as long as, it is not frozen can support the growth of microbes .Most soil microbes grows best at temperature between 15 to 25 degree Celsius. psychrophiles will prefer temperature between 0 to 15 degree Celsius and thermophile at 45 degree Celsius.

• MOISTURE:

Some microbes are capable of living in highly humid soil and many other in soil water (aquatic microbes).On a volume basis, a well drained soil may contains 40% water which is available in soil microbes for respiration and growth. Some soil microbes prefer dry conditions. Example Actinomycete- A class of bacteria, multiplies asexually like fungi (used to synthesise antibiotics ) .Many bacteria and fungi are able to adjust themselves to different moisture condition

• NUTRIENTS IN SOIL:

Application of fertilizers to the soil improove the microbial activity because of the availability of more radily obtainable nutrients.Similarly application of green manure stimulate some bacteria like Actinomycetes.

• ECOLOGICAL FACTORS:

Rain ,height of the place, thunder and storm also affect the microflora

TYPES OF MICROFLORA:

In a given soil the population of the microbes are ever changing but the microbes are in the dynamic equilibrium.

The important groups of microbes constituting in the soil are as follows

1 BACTERIA

In the most dominant group of microbes in soil population ranges from 10 thousand to several hundred millions per gram of soil . Bacteria live in soils in the form of bacilli , coccii and spiral forms and some of these are grown in unfavourable conditions, by the formation of endospores. Majority of soil bacteria are heterotrophs among them some are nitrogen fixers like Acetobacter, Rhizobium , Azospirillum (as azo means nitrogen )

Both autotrophs and heterotrophs bacteria are recognised in soil. Autotrophs are capable of synthesizing their food from simple inorganic materials for nutrients while heterotrophs depend upon reformed food for nutrition, all autotrophs use carbon dioxide from atmosphere as carbon source while photo autotrophs derive energy from sunlight chemoautotrophs drive energy from oxidation of simple substances some chemoautotrophs present in the soil are nitrosomonas and nitrobacter.

2 ACTINOMYCETES

Actinomycetes are usually regarded as an intermediate group between bacteria and fungi. They differ from fungi in their composition of cell wall. ( they do not have chitin and cellulose) these are known as filamentous bacteria and are widely distributed in soil .There number increases in the presence of the composite organic matter and the most favourable pH for them is between 6. 5 to 8 and temperature between 25 degree celsius to 30 degree celsius .Although thermophiles species growing in 55 degree celsius to 65 degree celsius are common in composed heaps. Example Actinomycetes is present in soil are – Streptomyces,Nocardia and Micromonospora .They have an odoury water and are responsible for the foul smell of a freshly ploughed field. Species of Actinomyces, Actinoplains are also encountered in soil. Sometimes are called as Ray fungi due to radial arrangement of branching.

3 FUNGI:

The number of fungi in soil varies from a few thousand to few million per gram. They are most abundant near the surface where oxygen is readily available .Some of the most common species of fungi in soil are Penicillium,Mucor,Aspergillus, Trichoderma, Cladosporium Trichogerma, Rhizopus. Most of these belong to deuteromycotina. Fungi are heterotrophic in nutrition and play an important role in the degradation of cellulose ,hemicellulose ,starch, pectin and Lignin ,Moreover the accumulation of mold mycelia improve the physical structure of soil by increasing crumble structure. These refers to the binding fine soil particles to form larger water stable aggregates. Yeast are most likely to occur in to the soil of wine yards and orchards. They are abundant on the leaves ,stems , roots grow into the soil.

4 ALGAE:

Algae are abundant in soils with enough moisture and sunlight .They may form scum on the surface of the soil visible to the naked eye. these are unicellular filamentous Or Colonial. for example Chlamydomonas, Green algae and blue green algae are photoautotrophic and use carbon dioxide from the atmosphere and give out oxygen .Among the species of given green algae ,those numerous in soil are Chlorella, Chlamydomonas Oedogonium , Volvox ( blue green Algae )are prokaryotic. They prefer neutral to alkaline environment of PH Those generally present are Oscillatoria, Nostoc ,Anabaena. Some of the blue green algae possess specialised cells known as known as heterocysts which are thought to be involved in nitrogen fixation .Blue green Algae are Cyanobacteria prefer neutral to alkaline environment. Important forms generally found in soils are Oscillatoria ,Anabaena, Scytonema, Tolypothrix. The water logged dry soil provide an ideal environment for the growth of the nitrogen fixation by Cyanobacteria. In some algae ,have been found to perform important beneficial changes example barren and eroded lands ,they may initiate the accumulation of organic matter because of their photosynthetic activity

5 PROTOZOA

Soil Protozoa are unicellular with relatively simple structure .These are abundant in upper layer of the soil. The applications of organic manure is increasing the number of soil Protozoa with corresponding increase in bacterial flora. Soil Protozoa are characterized by a cyst stage in their life cycle which have to withstand Adverse soil conditions. When the water supply is too low the Protozoa encyst in soil and remain as such until the environment become favourable. For growth most of the soil Protozoa fall under the group flagellates,Amoebae and ciliates Important examples are Bodo cercomonas, spiromonas cercobodo, Amoeba trinema ,Vorticella. Protozoa derive their nutrition by predating soil bacteria i.e Enterobacter, Agrobacterium ,Bacillus, Micrococcus and Pseudomonas are the genera of bacteria usually attached by Protozoa. when Protozoa grow in soil the bacterial Colony disappear.

6 VIRUSES

Soil viruses are obligately parasitic on other soil microbes like bacteria, actinomyces ,fungi and algae. The plant and animal viruses also rich in soil .Though viruses can be seen under only electron microscope thus their lysogenic activity on host can be demonstrated in the form of plaques on Agar plates .Similarly Actinophages, Cyanophages and bacteriophages has been studied in detail. Some viruses in soil cause diseases of plants ,these viruses do not multiply in soil but possessed there for a year or more .Similarly some animal and human viruses remain in soil for periods ranging up to several months .However the role of viruses in soil has not been clearly understood. The soil microbes exhibit the phenomena of syntrophism this is that ecological relationship in which the organism provide nourishment for each other