This document discusses soil microorganisms and their classification. It notes that soil microorganisms play important roles in soil nitrogen utilization, organic matter decomposition, nutrient transformations, and soil formation. The microorganisms are classified as bacteria, actinomycetes, fungi, and algae. Bacteria are the most abundant and play key roles such as organic matter decomposition and enzymatic reactions. Actinomycetes are filamentous bacteria that also decompose organic matter. Fungi possess filamentous mycelium and are important for organic matter decomposition. Algae are photoautotrophs that fix carbon and are present in soils.

1. SOIL MICROORGANISMS
Dr. K. VANANGAMUDI
Formerly Dean (Agriculture), AC & RI, Coimbatore
Dean, Adhiparashakthi Agricultural College, Kalavai
Professor and Head - Seed Science and Technology,
Tamil Nadu Agricultural University,
Coimbatore 641 04, Tamil Nadu, India.
1. Role of soil microorganisms
 Soil N utilization by plants.
 Dead organic matters decomposed.
 Nutrient transformation.
 Physical and chemical properties of soil.
 CO2 content.
 Soil formation.
2. Classification
 Soil organisms are classified broadly into soil flora and soil fauna.
 Subdivided into micro and macro flora.
 Micro flora again classified into bacteria, actinomycetes, fungi and algae.
1. Bacteria
 Single celled.
 Rod shaped and spherical.
 About 1 µm wide and up to 3 µm long and about 2 µm in diameter.
 Most abundant microorganism in the soil.
3.Classification of bacteria
a). Based on O2 requirement:
 Aerobic facteria: Bacillus subtilis, Azotobacter, Arthrobacter, Mycobacterium.
 Anaerobic bacteria: Actinomycetes, Clostridium, Propionibacterium, Bacterioides,
Fusobacterium, Bifidobacterium.

2. b) Based on temperature:
 Facultative: Optimal. An organism that can use only O2 but has anaerobic method of
energy production.
 Facultative aerobes – Chlorobium limicola.
 Facultative anaerobes – Escherichia coli, Bacillus anthracisis, Pseudomonas,
Klebsiella, Staphylococcus spp, Streptococcus spp, Salmonella, Listeria.
 Psychrophiles: Grows best at moderate temperature close to freezing.
 Arthrobacter, Psychrobacter, Chryseobacterium greenlandensis.
 Mesophiles: Grows best at moderate temperature.
 Listeria, Thiobacillus novellus, E.coli, Clostridium
 Theromophile: Grows best at high temperature.
 Chloroflexus aurantiacus, Thermus thermophilus, pyrolobus fumari
c) Based on their food preparation:
 Autotroph: Able form nutritional organic substances form inorganic substances such
as carbon dioxide.
 Cyanobacteria, Green and purple sulfur bacteria, Nitrosomonas.
 Heterotroph: Derives nutritional requirements from complex organic substances.
 Clostridium, Mycobacterium, Rhizobium.
 Chemoautotroph: Obtains energy by the oxidation of reduced compounds.
 Nitrosomonas, Thiobacillus thiooxidans.
 Obligate chemoautotrophs: Prefer specific substrates,
 Nitrobacter – Nitrite as substrate,
 Nitrosomonas – Ammonia as substrate,
 Thiobacillus - Converts sulphur compounds to SO4.
 Ferrobacillus – Converts ferrous to ferric,
d)Based on symbiotic relationship
Symbiotic N fixers:
 Associated with a host plant.
 Both the host and the bacteria get the benefits.
 Fix atmospheric N.
 Rhizobium (Legume association)
 Bradyrhizobium (Soybean association)
 Azorhizobium (Sesbania rostrata association)

3.  Anabaend azollae (Azolla – Anabaena)
 Frankia (Casuarina association)
Non symbiotic N fixers:
 Bacteria present without the association of a plant.
 But fix atmospheric N.
 Symbiotic, non symbiotic and cellulose decomposers come under Heterotrophs.
 Nitrifiers, denitrifiers nitrate formers and sulphur oxidizers are autotrophs.
 Azotobactor, Beijerinckia (named after scientist Beijerinck), Clostridium, Chlorobium,
Rhodopseudomonas, Desulphovibro, Thiobacillus, Nostoc.
2. Role of bacteria
 Bacteria carryout the decomposition of organic matter and synthesis of humus .
 Enzymatic transformations are carried out by bacteria
 Bacteria oxidizes or reduces many chemical reactions such as N fixation, sulphur
oxidation, nitrification etc.,
2. Actinomycetes
 Unicellular like bacteria.
 Have same size as bacteria.
 Filamentous and profusely branched.
 Mycelial threads are smaller than those of fungi.
 No nuclear membrane as in bacteria.
 Also called as filamentous.
 Sensitive to acid soils.
 Potato scab, a disease due to actinomycetes, can be controlled by lowering the soil pH
by applying sulphur.
 Heterotrophic - optimum temperature 25-30oC, pH 6.5 – 8.0.
 Actionomycetes are important for organic matter decomposition.
 Chitin and phospholipids are reduced to simple compounds.
 Aroma of freshly ploughed land at certain times of the year is probably due to
actinomycetes as well as certain molds.
 Actinomycete population in soil exceeds all other organisms except bacteria.
 Their proportion increases with soil depth. Their population and biomass are almost
equal to that of bacteria.
 Eg. Frankia family (Actinobacteria).

4.  Streptomyces flavovirens.
3. Fungi
 Soil fungi may be parasitic or saprophytic.
 Possess filamentous mycelium composed of individual hyphae which are 5-20 µm in
diameter and several centimeters in length.
 Most fungi are heterotrophic and hence, they depend on the organic matter content of
the soil.
 Dominant in acid soils and some can tolerate a pH upto 9.0.
 Strictly aerobic.
 Classified into phycomycetes, ascomycets, basidiomycetes and fungi imperfecti.
 Fungi may also may be classified as molds, yeast and mushrooms.
Molds: Filamentous microscopic molds develop vigorously in all types of soils.
 In acid forest soils, involves in decomposing of organic matter.
 Common genera - Mucor, Fusarium and Aspergillus.
 Average population is 10-200 billion / m2.
 In humus formation and aggregate stabilization, molds are more important than
bacteria.
 Continue to decompose complex organic substances, after bacteria and actinomycetes
have stopped their function.
Yeast: Group of fungi which exist as an unicellular organism.
 Reproduce by fission or budding.
 Used as food supplement and also for the production of alcoholic beverages.
 Yeast is not common in soils but produce several plant diseases.
Mushroom: Present in forests and grasslands where ample moisture and organic residues are
present.
 Some mushrooms are edible.
 Mushrooms are also not common in cultivated soils.
 Their fruiting body is above the ground.
3. Algae
 Filamentous, 10µm in diameter. Population in soil is around 1-10 billion / m2.
 Their mass in soil may be 50-600 kg/ha.
 Algae are photoautotrophs.
 Divided into 4 general groups - blue green, green, yellow green and diatoms.

5.  Blue green algae are numerous in rice soils.
 Blue green algae growing within the leaves of aquatic fern.
 Azolla can also fix atmospheric N.