2. Soil microbiology is the
study of organisms in soil,
their functions, and how
they affect soil properties
3. Normal, fertile soils teem with soil
microbes. In fact, there may be hundreds
of millions to billions of microbes in a
single gram . The most numerous
microbes in soil are the bacteria
(unicellular cells lacking a true nucleus)
followed in decreasing numerical order by
the actinomycetes, soil algae,
cyanobacteria and soil protozoa
(unicellular soil organisms that
decompose organic materials as well as
consume large numbers of bacteria).
4. Numbers of Microbes in Soil
Microbial Group No./Gram of Soil
Bacteria 100,000,000 - 1,000,000,000
Fungi 100,000 - 1,000,000
Algae and 1000 - 1,000,000
Cyanobacteria
Protozoa 1000 - 100,000
6. This is an excellent question because an
understanding of what it takes to
support the growth and activity of soil
microbes enables one to make decisions
about soil management. In general,
microbes need what all living things need
to prosper: air (oxygen), water, food and
a suitable habitat to live in
7. • Organic carbon
• Moisture
• Aeration
• pH
• Temperature
• Inorganic nutrients
• Grass clipping, crops residues,
organic wastes etc
• 50-6-% water holding capacity
• Balance of air and water filled
pores
• Near neutral
• 10-40 C
• Adequate N, P, K, S etc and trace
element
PRINCIPLE ENVIRONMENTAL FACTORS
AFFECTING SOIL MICROBES
9. SOIL MICROBES AND SOIL
STRUCTURE
• Soil structure dependent on the stable aggregates of
soil particles
• Soil organisms play an important role in soil
aggregation
• Constituents of soil are organic matter,
polysaccharides, gums, lignins synthesized by soil
microbes play an important role in cementing of soil
particle
• Cells and mycelial strands of fungi and actinomycetes
play an important role in soil aggregation
10. CONT’D
• Different soil microorganisms, having soil binding
properties are graded in the order as:
fungi > actinomycetes > gum producing bacteria >
yeasts
• EXAMPLES:
bacteria like Rhizobium, Bacillus, Azotobacter
fungi like Rhizopus, Mucor
11. SOIL MICROBES AND PLANT GROWTH
• Best medium for plant growth
• Convert complex organic nutrients in to simpler
inorganic forms which are readily absorbed by the
plant for growth
• Produce variety of substances like IAA, GIBBERLINS,
ANTIBIOTICS etc which directly or indirectly promote
the plant growth
14. The nitrogen cycle is the process by which nitrogen is
converted between its various chemical forms. This
transformation can be carried out through both
biological and physical processes.
15. Important processes in the nitrogen cycle include:
• fixation
• ammonification
• nitrification
• denitrification
PROCESS OF NITROGEN CYCLE
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20. NITROGEN FIXATION
• The nitrogen molecule (N2) is quite inert. To break it apart so that
its atoms can combine with other atoms requires the input of
substantial amounts of energy.
• Three processes are responsible for most of the nitrogen fixation
in the biosphere:
• atmospheric fixation by lightning
• biological fixation by certain microbes — alone or in a
symbiotic relationship with some plants and animals
• industrial fixation
21. NITRIFICATION
• Ammonia can be taken up directly by plants — usually through their
roots. However, most of the ammonia produced by decay is converted in
nitrates This is accomplished in two steps:
• Bacteria of the genus Nitrosomonas oxidize NH3 to nitrites (NO2
−).
• Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO3
−).
• These two groups of autotrophic bacteria are called nitrifying bacteria.
Through their activities (which supply them with all their energy needs),
nitrogen is made available to the roots of plants.
• Both soil and the ocean contain archaeal microbes, assigned to
the Crenarchaeota that convert ammonia to nitrites. They are more
abundant than the nitrifying bacteria and may turn out to play an
important role in the nitrogen cycle.
• Many legumes, in addition to fixing atmospheric nitrogen, also perform
nitrification — converting some of their organic nitrogen to nitrites and
nitrates. These reach the soil when they shed their leaves.
22. DENITRIFICATION
• The three processes above remove nitrogen from the atmosphere
and pass it through ecosystems.
• Denitrification reduces nitrates and nitrites to nitrogen gas, thus
replenishing the atmosphere. In the process several intermediates
are formed:
• Nitric oxide (NO)
• Nitrous oxide (N2O)(a greenhouse gas 300 times as potent as
CO2)
• Nitrous acid (HONO)
• Once again, bacteria are the agents. They live deep in soil and in
aquatic sediments where conditions are anaerobic. They use
nitrates as an alternative to oxygen for the final electron acceptor
in their respiration .
23. ANAMMOX
• Anammox is abbreviated as anaerobic ammonia oxidation
• Under anaerobic conditions in marine and freshwater sediments,
other species of bacteria are able to oxidize ammonia (with NO2−)
forming nitrogen gas.
• NH4+ + NO2− → N2 + 2H2O
• The anammox reaction may account for as much as 50% of the
denitrification occurring in the oceans.
• All of these processes participate in closing the nitrogen cycle.
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27. NITROGEN CYCLE IS VERY IMPORTANT IN
MAINTAINING OUR ENVIRONMENTAL
BALANCE
