Soil Microflora

Types of Soil Microorganisms

Bacteria

Among the soil microorganisms, bacteria are most dominant group of organisms. All kinds of bacteria are found in soil. This is because all kinds of organic refuse are disposed on the soil. Many of the soil bacteria perform useful functions like decomposition of organic matter, conversion of soil constituents into useful materials, production of antibiotics in the soil and biogeochemical cycling of elements like Carbon, Nitrogen, Phosphorus, Iron, Sulphur and Manganese. The bacterial population of the soil exceed the population of all other groups of microorganisms in both number and variety. 

More dominant group of microorganisms in the soil and equal to one half of the microbial biomass in soil. Population 100,000 to several hundred millions for gram of soil. (Common soil bacteria – Arthrobacter, Bacillus, Clostridium, Micrococcus).

Actinomycetes

The actinomycetes population is present as many as millions per gram of soil. Intermediate group between bacteria and fungi. Numerous and widely distributed in soil. Abundance is next to bacteria. 10 4 – 10 8/g soil. 70% of soil actinomycetes are Streptomyces. The most predominant genera present in the soil are Nocardia, Streptomyces and Micromonospora. Actinomycetes are capable of degrading many complex organic substances and therefore play an important role in building soil fertility. One of the most notable characteristics of the actinomycetes is their ability to produce antibiotics. Examples: Streptomycin, neomycin, erythromycin and tetracycline.

Fungi

Next to bacterial population in soil, fungi dominates in all kinds of soil. It possess filamentous mycelium composed of individual hyphae. All environmental factors which influence the distribution of bacteria and actinomycetes also influence the fungal flora of soil. The quality and quantity of organic matter present in the soil have a direct influence on the fungal numbers in soil. Fungi are dominant in acidic soils because acidic environment is not supportive for the existence of either bacteria or actinomycetes. Common genera in soil are Aspergillus, Mucor, Penicillium Trichoderma, Alternaria and Rhizopus. 

Soil Algae

Soil algae are ubiquitous in nature wherever moisture and sunlight are available. They are visible to the unaided eye in the form of green scum on the surface of soils. Numerically, they are not as many as Fungi, Bacteria or Actinomycetes. Some of the common algae in Indian soil are Chlorella, Chalmydomonas, Chlorochytrium, Chlorococcum and Oedogonium.

Blue green algae, or Cyanophyceae, are responsible for Nitrogen fixation. The amount of Nitrogen they fix depends more on physiological and environmental factors rather than the organism’s abilities. These factors include intensity of sunlight, concentration of inorganic and organic Nitrogen sources and ambient temperature and stability.

Soil Protozoa

Soil protozoa are unicellular. They are characterized by a cyst in their life cycle which can help the species to withstand adverse soil conditions. The protozoans prefer certain species of bacteria for their nutrition. Protozoa are abundant in the upper layer of the soil and their numbers are directly dependent on bacterial population.

Factors Influencing Microbial Population in Soil

The major factors that influence the microbial community in soil are

·        Moisture

·        pH

·        Temperature

·        Gases

·        Organic and inorganic fertilizer

·        Organic matter of soil

·        Types of vegetation and growth stages

·        Ploughing

·        Season

·        Depth of soil

Importance of Soil Microorganisms:

(Involved in nutrient transformation process)

1. Decomposition of resistant components of plant and animal tissue

2. Role in microbial antagonism

3. Participate in humus formation

4. Predator of nematodes

5. Surface blooming reduces erosion losses

6. Improve soil structure

7. Involved soil structure

8. Maintenance of biological equilibrium

(a) Rhizosphere:

The term ‘rhizosphere’ was introduced in 1904 by the German scientist Hiltner to denote that region of the soil which is modified as a result of the uptake and deposition of substances by a growing root or the root surface (rhizoplane) together with that region of the surrounding soil in which the microbial population is affected, qualitatively and/or quantitatively, by the presence of a root.

The rhizosphere may extend a few millimeters, or centimeters, from the rhizoplane.

(b) Rhizoplane:

oot surface along with the closely adhering soil particles is termed as rhizoplane.

(c) Antibiotics

A.  Important antibiotics:

Penicillin, Cephalosporin, Erythromycin, Carbomycin, Streptomycin, Neomycin, Oxytracycline, Polymixin G, Bulbiformin, aureofurgin Bacitracin etc.

B. Discovery of antibiotics:

(i) Alexander Fleming – discovered penicillin (first antibiotic) in 1929

(ii) S. A. Wakesman – streptomycin in 1943, used against Mycobacterium tuberculosis (the cause of tuberculosis)

(iii) Vasudeva I et al. – Bulbiformin in 1958

(iv) Thirumalachar et al, – Aureofungin in 1964, the antibiotic against fungi