Soil Microflora : Diversity, Function & Examples

  The soil flora is composed of a large number of microorganisms, root system of higher plants and animals such as rodents, insects and warm.

  • A spoonful of fertile soil is said to contain more microorganisms than there are people in this world.
  • Most soil organisms are found in the surface layer. Their number decreases with depth.
  • A well aerated soil contains more organisms than one lacking abundance of oxygen. The number and diversity of organisms found in soil depends on the following factors.
  1. Nature of soil
  2. Depth
  3. Season of the year
  4. State of cultivation
  5. Amount of organic matter
  6. Degree of aeration
  7. Temperature
  8. pH of soil
  9. Moisture
  • The existence of roots and their extensiveness also influence the soil flora. The climatic conditions also have effects on various organisms in soil.

Functions of Soil Microflora : 

One of the most important functions of soil organisms is to decompose various kinds of organic matter of plant and animal origin. This produces four distinct effects upon soil processes and upon plant growth.

  1. They provide inorganic nutrients to plants especially Nitrogen and Phosphorus.
  2. They affect the physical conditions of soil especially the moisture holding and buffering capacities.
  3. They provide certain specific elements that may be limiting factors for plant growth.
  4. They favour development of organisms that secrete antagonistic substances that control the growth of certain plant pathogenic species.

Diversity of Soil Microflora :

  Soil is an excellent culture medium for the growth of many kind of micro-organisms. The microscopic life of soil includes :
  (1) Bacteria
  (2) Fungi
  (3) Algae
  (4) Protozoa
  (5) Viruses.

(1) Bacteria :

  • Bacterial population of soil exceeds the population of all other groups of organisms.
  • Direct microscopic count of soil bacteria is as high as several billions per gram.
  • Viable plate count of soil shows only a fraction of this number (millions/gm). Forest soil contains about 4.8 x 10⁹ cells/cm³. 
  • There is a great variety of nutritional and physiological types of bacteria in soil, and no single laboratory environment can support growth of every viable cell type.
  • The soil bacteria may be autotrophs, heterotrophs, mesophiles, thermophiles, psychrophiles, aerobes, anaerobes, cellulose digesters, sulfur oxidizers, N2 fixers, protein digesters, etc.
  • The most abundant bacteria found in soil belong to the actinomycetes group. They produce geosmin a compound that gives a characteristic earthy odour to soil. 
  • They can degrade many complex organic substances present in soil and increase soil fertility. They produce antibiotics, which maintain equillibrium of soil microflora. Nocardia, Streptomyces and Micromonospora are the predominant genera of  actinomycetes found in soil. 
  • The other bacteria present in abundance include Arthrobacter, Pseudomonas, Clostridium, Bacillus, Micrococcus and Flavobacter. The less common are Chromobacterium, Sarcina and Mycobacterium.

(2) Fungi :

  • Many species of fungi inhabit the soil. 
  • They are usually found on the surface of soil. 
  • They exist both in spore and mycelial stage.
  • Both unicellular and multicellular fungi are present in soil. 
  • The unicellular fungi is known as yeast. 
  • The multicellular fungi is known as mold. 
  • The main function of fungi is to decompose plant dead organic material Including cellulose, lignin and pectin. Fungi improve the physical structure and water holding capacity of soil. 
  • Some fungal are predatory, they eat amoeba and nematodes, thereby maintain the microbial equilibrium. There are certain plant pathogenic fungi responsible for causing diseases in plants. 
  • The fungi form oxygen-impermeable structures called sclerotia and hyphal cords. Filamentous fungi which move nutrients and water over long distances are found within these structures.

(3) Algae :

  • Algal population in soil is smaller than that of fungi or bacteria. The major types found in soil include: Green algae, Diatoms and Blue-green algae (Cyanobacteria).
  • They are photosynthetic and some cyanobacteria can also fix atmospheric Nitrogen. They are found only on the surface of soil.

(4) Protozoa :

  • Most soil protozoa are flagellates or amoeba. Their number per gram of soil ranges from a few hundred to several hundred thousand in moist soils rich in organic matter. 
  • They eat bacteria and thereby maintain equilibrium of bacterial population.

(5) Viruses :

  • Viruses are obligate intracellular parasites. Bacterial, plant and animal viruses are often found in soil. 
  • Some viruses present in soil cause plant diseases, e.g., tobacco mosaic virus (TMV). 

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