Microbes play an essential role in soil properties and plant growth. They are responsible for decomposing organic matter, fixing nitrogen, and managing soil stability through various biochemical processes. The four main types of microbes found in soil are bacteria, fungi, actinomycetes, and algae. Each group serves important functions like nutrient cycling, organic matter breakdown, and maintaining balances in the soil environment. Microbes also influence soil structure by producing compounds that bind soil particles together and form stable aggregates.
1. Role of Microbes in Soil
DR. S. ANU KIRUTHIKA
BENGALURU NORTH UNIVERSITY
BENGALURU
2. Soil Microbiology
• It is branch of science dealing with study of soil microorganisms
and their activities in the soil, their functions, and how they affect
soil properties form a very small fraction of soil mass (volume of
less than 1%)
• In the upper layer of soil (top soil up to 10-30 cm depth i.e.
Horizon A), the microbial population is very high which decreases
with depth of soil
3. TYPES OF SOIL
1. Sand
• It is most extensively used construction material.
• It consists of particles of rock and hard minerals, such as silicon dioxide.
• They are the largest type of soil particles, where each particle is visible to naked eye.
• The large, relatively stable sand-particle size increases soil aeration, improves drainage in tight soils and creates plant-growth
supporting qualities, or tilt.
• The particle size of course sand ranges from 2 – 4.75mm, Medium sand ranges from 0.425 – 2 mm and fine sand ranges from
0.075 – 0.425 mm.
• The bigger particle size of the sand gives wet or dry sandy soil a grainy texture when you rub it between your fingers, and it
makes the soil light and crumbly even when you try to stick it together in your hand.
• The particle shape is angular, sub angular, rounded, flat or elongated.
• The texture is rough, smooth, or polished.
4. 2. Silt
• Silt is a sediment material with an intermediate size between sand and clay.
• Carried by water during flood it forms a fertile deposit on valleys floor.
• The particle size of silt ranges from 0.002 and 0.06 mm.
• Silt is a non-plastic or low plasticity material due to its fineness.
• Due to its fineness, when wet it becomes a smooth mud that you can form
easily into balls or other shapes in your hand and when silt soil is very wet, it
blends seamlessly with water to form fine, runny puddles of mud.
5. 3. Clay
• Clay particles are the finest of all the soil particles, measuring fewer than 0.002 mm in size.
• It consists of microscopic and sub-microscopic particles derived from the chemical decomposition of rocks.
• Clay is a fine grained cohesive soil.
• They stick together readily and form a sticky or gluey texture when they are wet or dry.
• Clay is made of over 25 percent clay, and because of the spaces found between clay particles, clay soils hold a high
amount of water.
• Clay expand when in contact with water and shrink when getting dry.
• Compared to sand particles, which are generally round, clay particles are thin, flat and covered with tiny plates.
• Organic clay is highly compressible and its strength is very high when dry, which is why it is used in construction as mud
mortar.
6. 4. Loam
• Loam is a mixture of clay, sand and silt and benefits from the qualities of these 3
different textures, favouring water retention, air circulation, drainage and fertility.
• These soils are fertile, easy to work with and provide good drainage.
• Depending on their predominant composition they can be either sandy or clay loam.
• The way the other particles combine in the soil makes the loam.
• For instance, a soil that is 30 per cent clay, 50 per cent sand and 20 per cent silt is a
sandy clay loam, with the soil types before “loam” listed in the order their particles
are most dominant in the loam.
• The labels “clay loam,” “silt loam” and “sand loam” are used to refer to soils that are
composed predominantly of those ingredients.
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9. Importance of microorganisms
Affect the structure and fertility of different soils.
Contribute to nutrient availability in soil(Organic matter decomposition,
humus formation, N-fixation, seed germination).
Manage soil stability by different biochemical processes.
Degrade pesticides and chemicals in soil.
Contribute the growth and success of the plants and overall ecosystem of a
soil environment.
11. BACTERIA
• Bacteria smallest organisms in the soil.
• Prokaryotic(simple cell structure with no internal organelles).
• Most abundant microorganisms.
• In the soil serve many important purposes, one of those being
nitrogen fixation among other biochemical processes
13. Soil Bacteria – Role in Soil
• Degrade organic matter.
• Fix N2 – and other steps of N cycle.
• Rhizobium-nitrogen fixation.
• Nitrification-Nitrosomonas ,Nitrobacter.
• Organisms involved with vital functions may be in lower abundance.
• Organisms present will depend on many factors- Nutrients, O2, moisture, pH.
14. Actinomycetes
• No. in soil second to bacteria.
• Prokaryotic in nature,classified with bacteria.
• Form mycelium and asexual spores like fungi.
• Cell wall composition same as bacteria i.e. peptidoglycan.
15. Most Abundandant Actinomycetes in Soil
• Actinomycetes are slow growing compared to bacteria and fungi.
• Streptomyces-may be upto 65 % of soil population.
• Nocardia and Micromonospora-upto 35 % of population.
• Other common genera - Streptosporangium, Actinoplanes,
Thermoactinomyces, Thermomonospora.
16. Role of Actinomycetes in soil
1. Organic matter decomposition - Starch, cellulose,
hemicellulose, lignin, humus.
2. Antibiotic production - Streptomycin, tetracycline,
chloramphenicol.
3. Maintain Microbial equillibrium in soil - Control pathogenic
organisms. Produce proteases-lyse cells of pathogens.
17. FUNGI
• Grow as long threads (hyphae).
• Push through soil particles, roots, rocks.
• Often group into masses called mycelium (look like roots).
• Higher fungi have basidium : club-shaped structure , bearing fruiting
body.
19. Role of Fungi in Soil
• Organic matter degradation: Degrade starch, hemicellulose, cellulose,
lignin.
• Ammonification; Degrade proteins, nucleic acids and release NH3.
• Control of other organisms; Some fungi kill nematodes.
• Pathogenicity; Many fungi associated with plant disease- pythium,
Sclerotinia, Puccinia.
• Formation of mycorrhizae; symbiotic relationship-Glomus
20. ALGAE
• Filamentous, colonial, unicellular.
• Photosynthetic
– Most in blue-green group, but also yellow-green, diatoms, green
algae
– Form carbonic acid (weathering)
– Add organic matter to soil; bind particles
– Aeration
– Some fix nitrogen
21. Algal groups in soil
• Chlorophyaceae- Green algae
• Cyanophyaceae- Bluegreen algae
• Bacillariophyaceae- Diatoms
• Xanthophyaceae- Yellow green algae
22. Importance of Algae in Soil
• Provide organic matter.
• Many algae produce polysaccharides ,improve soil structure.
• Produce oxygen, provide oxygen to rice plants.
• Fix nitrogen and provide to rice plants after decomposition.
23. PROTOZOA
• Eukaryotic organisms.
• Sexual reproduction.
• Biological control agent.
• Maintain equilibrium in soil microbes.
• Protozoa can be split up into three categories: flagellates, amoebae,
and ciliates
24. Types of flagellates
smallest members of the protozoa group, and can be divided further based on whether.
Non chlorophyll-containing flagellates found mostly in soil and flagellates that contain
chlorophyll typically occur in aquatic conditions.
distinguished by their flagella.
Amoeba
larger than flagellates and move in a different way slug-like properties and pseudopodia
does not have permanent appendages
Ciliates
largest of the protozoa group move by means of short, numerous cilia.
25. Soil microbes and soil structure
• Soil structure dependent on stable aggregates of soil particles.
• Soil organisms play important role in soil aggregation.
• Constituents of soil are organic matter, polysaccharides, lignins and gums
synthesized by soil microbes plays important role in cementing of soil
particles.
• Cells and mycelial strands of fungi and actinomycetes play important role in
soil aggregation.
26. • Different soil microorganisms, having soil binding properties are graded in
the order as:
fungi > actinomycetes > gum producing bacteria > yeasts
• Examples : Fungi like Rhizopus, Mucor, Chaetomium, Fusarium,
Cladasporium, Rhizoctonia, Aspergillus, Trichoderma
• Bacteria like Azotobacter, Rhizobium Bacillus and Xanthomonas.
