N-fixation ability limited to few bacteria, either as free-living organisms or in symbiosis with higher

1. Institute of Agricultural science RGSC BHU
SSC-411-(0+10)10
Topic:- Soil Organisms, functions and their role in
Soil fertility
Submitted to:
Dr. Ashish Latare
(Assistant professor)
Submitted by:
Anup Kumar
B.sc.(Ag) 4th year
ID No.:- 16311
Ex Roll No:-16230AGC008

2. CONTENT
1. Soil Organisms
2. Classification of soil organisms
3. Classification of Bacteria
4. Soil flora
5. Nitrogen fixing systems
6. Soil fauna
7. Interaction among soil organisms
8. Role of organism in soil fertility
9. Soil fertility evaluation by biological tests-
10. Harmful Activities of Soil Organisms

3. Soil Organisms
1) The organisms that live in the soil are called soil organism.
2) Micro organisms are numerous in number with monopolized
biological activity in soils (60-80% of total biological
activity).The study of living organisms in soil is called Soil
biology.
3) The organisms in the soil, not only help in development of
soils but carryout a number of transformations facilitating
the availability of nutrients to the plants. In the absence of
the activities of these organisms, in soil, life on earth would
have come to a halt, as all available nutrients would have
ended up locked in the organic, disrupting the nutrient
cycles.

4. Continued…..
4) The soil is teeming with millions of living organisms which
make it a living and a dynamic system. Under microscope it
reveals a complex arrangement of soil particles and pore
spaces filled with air and water. It is in these pore spaces
that plant roots and millions of organisms develop, ranging
from microscopic to macroscopic in size.
5) The soil organisms are classified into two broad groups ,viz.
soil flora and soil fauna,important groups as follows

5. Classification of soil organisms:
A. Soil flora (Plant kingdom)
1) Macro-flora - Plant roots, Macro -algae
2) Micro-flora - Bacteria, Actinomycetes
Fungi, Algae
B. Soil fauna (Animal kingdom)
1) Micro fauna - protozoa, nematodes
2) Macro-fauna - Earthworms,Termites,
Ants,Grubs

6. Classification of Bacteria
A. Heterotrophic
1) nitrogen fixing-
a) Symbiotic(Rhizobium)
b) Non-Symbiotic- Aerobic(Azatobactor)
Anaerobic(Clostridium)
c) Associative symbiotic -Azospirillum
1) Non-nitrogen fixing-
a) Aerobic-Ammonifiers
b) Anaerobic- Denitrifiers
A. Autotrophic-a) Chemoautotrophic(Thiobascillus )
b) Photoautotrophic (Chlorobium)

7. SOIL FLORA
1) SOIL MACROFLORA- Eg. Roots of higher plants
Functions in soil-
1) The dead plant
contribute to the
formation of the soil
organic matter which ,
provides food, energy
and nutrients to
microorganisms and
also higher plants – a
process of cycling of
plant nutrients .

8. Continued…..
2) Decay of plant roots add O.M. to soil , thereby changing
soil properties viz. soil aggregation ,CEC,water and
nutrient retention capacity, etc. of soil.
3) While proliferating , the roots exert tremendous
pressure on surrounding particles. When roots decay ,
the vacant space makes room for water and air to move
it as well as more food to the microbes, influencing
therby microbial activity in soil.

9. 2) SOIL MICROFLORA-
a) Bacteria :
1) Bacteria are primitive ,very
minute, unicelluar
organisms.
2) They are known for rapid
proliferation. Size varies
from 0.5 to 1 micron in
diameter and 1 to 10 micron
(length).
3) They have flagella and are
motile.
4) They are most abundant in
soil. 300 to 3000 kg of live
weight of bacteria/ha., 0.01
to 0.4 % of total soil mass

10. Continued…..
5) Shape may be round (cocci), rod like (bacilli) or spiral
(spirilla). In the soil, the rod shaped ones seem to
predominate.
6) Bacterial population vary from few billion to three trillion
in each kilogram of soil.
7) Types on the basis of temp.
- <10 0C
Psychrophiles
Mesophiles -
Thermophiles -
20 0C-40 0C
>40 0C
5) The genera –Pseudomonas, Azotobacter, Bacillus,
Agrobacterium, Arthrobacter- are most abundant .The
genus Bacillus has largest species followed by
Pseudomonas.

11. Functions of Bacteria –
1) Biological fixation of N.
2) Symbiotic and non-symbiotic N-fixation.
3) Decomposition of carbohydrates and lignins.
4) Decomposition of proteins with the liberation of
ammonia or ammonification, nitrification ,and
denitrification.
5) transformation of C,N,P,S,Fe,Mn.

12. Fungi
1) They are longer than
bacteria , aerobic and
heterotrophic in nature
(Saprophytic in nature).
2) Fungi are broadly
grouped in to yeasts,
molds and mushrooms,
out of which molds and
mushrooms are
important in soils.
3) Fungi dominates in acid
soils.

13. Continued…..
4) Important molds in soils are Penicillium, Mucor,
Fusarium and Aspergillus.
5) Prefer acid medium (4.5-6.5). Some fungi can
tolerate even pH 9.0.
6) Mushroom fungi are found in forests and
grasslands. Mushroom fungi are not widely
distributed like molds, but are of much significance
especially in breaking down of woody tissue.

14. Functions of fungi-
1) A mutually beneficial (symbiotic) association
between numerous fungi and the roots of higher
plants is called ‘Mycorrhizae” (fungus root), is
useful in adsorption of nutrients.
Eg. VAM-fungi
2) Fungi decomposes almost anything in organic that
cannot tackle by bacteria.They decompose
carbohydrates,celluloses,sugars,proteins,fats and
lignins.

15. Actinomycetes
1) Thread / Filamentous
bacteria. Next to
bacteria in abundance.
2) A common genus is
streptomyces.
3) Actinomycetes have
transitional
charecteristics between
bacteria and fungi,
sometimes called fungi-
like bacteria.
4) They are more abundant
in dry and tropical soil.

16. Functions of Actinomycetes
1) Actinomycetes can degrade all sorts of organic
substances such as celluloses, polysaccharides,
proteins,fats,etc.
2) But, they are slower activity than bacteria and
fungi.For this reason , organic residues first attacked
by bacteria and fungi, later on actinomycetes.
3) It produce very little mycelium ,so less contribute to
humus.

17. Algae
Charecteristics-
1) Chlorophyll containing
organism, Aerobic,
Photoautotrophic.
2) Blue green algae(BGA) ,
also called cynobacteria
are free living N-fixer
3) ‘Heterocyst’ cells found in
which N-fixation done.It
provide anaerobic
environment
Algae in rice field

18. Continued…..
4) Soil algae have been divided mainly into four classes,viz.
Ex:-a) Blue green algae/Cynophyta -(Anabaena, Nostoc,)-
dominates in tropical soil
b) Chlorophyta-grass green algae-dominates in
temperate regions.
c)Xanthophyta- yellow green algae
d)Bacillariophyta- golden-brown algae or diatoms
Use in agriculture-
Azolla-anabaena symbiosis can increase 10-15% rice yield

19. Nitrogen fixing systems
N- Fixing System Organism
Involved
Plant Involved Site
of fixation
Nitrogen
kg N/ha/year
Legumes
(Symbiotic)
Rhizobium
(bacteria )
Legumes Nodules 50-100
Non- legumes
(Symbiotic)
Frankia
(Actinomycetes)
BGA(Anabaena)
Alnus
Casuarinas
Azolla(Fern)
Nodules
Leaf
50-100
Associative
symbiotic
Associative
symbiotic
Non- legumes Rhizosphere 5-30
Non- Symbiotic BGA
Azatobacter,
Beijerinckia,
(aerobic)
Clostridium
(anaerobic)
Not Soil & Water 10-50
5-30
5-20

20. B) SOIL FAUNA
1)SOIL MACROFAUNA-
Earthworms-
1. These having slender
cylindrical bodies with varing
diameters.
2. These feed on dead organic
tissues and soil materials.
3. Mineral soil constituents are
digested and decomposed in
digestive system.
4. Their excretaare richer in
bacteria and available
mineral nutrients.
5. They are more common in
fine texture soil

21. Continued…..
Ants-
1. Ants are the most
widespread species of
insects in soil, and
consume plant
residues.
2. But are more active in
humifying insects than
plants.

22. Others macrofauna are- Termites,Grubs,
Slugs,Snails,Centipedes, Millipedes etc

23. 2) SOIL MICROFAUNA-
1) Soil protozoa-are
unicellular but larger than
bacteria, size varing from
few microns to few cm,
found in all arable soils.
Functions-
1. The role of protozoa in soil
is not definitely known.
2. As they feed on bacteria
and actinomycetes , they
probably help to maintain
a favourable balance of
the microflora in soil.

24. 2) Nematodes-
Also called eelworms, thread
worms or round worms.
There are various types of
nematode found as-
1. saprophytic(feed on decaying
O.M.),
2. predatory(feed on
earthworm) and
3. parasitic (feed on roots of
higher plants), this one
(parasitic) cause lossof vigour
of root system and make
plants growing in nematode
infested soil soil liable to
diseases.

25. 3) SOIL VIRUSES
1. These are ultramicroscopic
obligate parasite(have head-
tail structure , with diameter
0.05 to 0.1 micron) always
requiring a living host for their
multiplication.
2. They are much smaller than
bacteria , only visible through
electron microscope.
3. The viruses in soil is known as
bacteriophases.
4. When they parasitize and
destroy a bacterium of
agricultural importance like
Rhizobium, they attain
economic importance.

26. Interaction among soil organisms-
Soil organisms interact with each other, with the plant roots, and
with the environment.
Some interactions are as follows-
1. The association existing between one organism and another
whether of symbiotic or antagonistic influences the
population and activity of soil microbes.
2. The predatory habit of protozoa and some mycobacteria
which feed on bacteria may suppress or eliminate certain
bacteria.
3. Activities of some of the microorganisms are beneficial to
each other. For instance organic acids liberated by fungi,
increase in oxygen by the activity of algae, change in soil
reaction etc. favours the activity or bacteria and other
organisms in soil.

27. Role of organism in soil fertility
1. Soil microbes break down organic matter.
2. Soil microbes recycle nutrients.
3. Soil microbes create humus.
4. Soil microbes create soil structure.
5. Soil microbes fix nitrogen.
6. Soil organisms promote plant growth.
7. Soil microbes control pests and diseases.

28. Soil fertility evaluation by biological tests-
1. Neubauer seedling method
2. Standard and Demont technique
3. Microbiological methods
4. Sacket and Stewart technique
5. Melich Cunninghamella-plaque test
6. Mulder Aspergillus niger-test

29. Harmful Activities of Soil Organisms
1. Rodents, snails, slugs, termites, insect larvae and
nematodes cause great damage to plants.
2. Mainly fungi, but bacteria and actinomycetes ,also, are
responsible for many soil borne diseases of crop plants.
3. Offer lot of competition to higher plants.
4. Under conditions of poor drainage, depletes the available
oxygen and hampers the normal growth of plants.
5. Under anaerobic conditions, bacteria transform some of
the nutrients to unavailable form (k & Zn etc.) and some
nutrients to be available in toxic levels ( Fe & Mn etc.)

30. “There can be no life without soil and no soil
without life; they have evolved together.”
-Charles E. Kellogg