Soil is formed over long periods of time from weathered rock and decayed organic matter. It consists of minerals like sand, silt, and clay as well as organic matter and hosts a diverse array of microorganisms. Soil microorganisms play important roles in nutrient cycling, decomposition, and supporting plant growth. The most abundant microbes are bacteria and fungi, which break down organic residues. Other microbes like actinomycetes and mycorrhizal fungi also contribute to soil fertility. Protists and nematodes regulate microbe populations as predators. Overall, the complex web of soil microorganisms drives key ecosystem functions.

2. What is Soil Microbiology?
Soil Microbiology is the branch of soil
science concerned with soil
inhabiting microorganisms, their
functions and activities

3. What is soil?
Soil is the outer, loose earth material
which is distinctly different from
the underlying bedrock and the
region which support plant life.

4. Soil is made up of many
things, such as weathered rock
and decayed plant and animal
matter.
Soil is formed over a long
period of time.
Soil is formed from the
weathering of rocks and
minerals
Weathering is the process of
the breaking down rocks. There
are three different types of
weathering. Physical, chemical
and biological weathering.

5. An example of biological weathering of a rock under the
influence of clams

6. Functions of Soil in the Global
Ecosystem
Soils perform five key functions
in the global ecosystem.
• medium for plant growth,
• regulator of water supplies,
• recycler of raw materials,
• habitat for soil organisms, and
• landscaping and engineering
medium.

7. The Components Of Soil
• Soils have five basic components:
1. Minerals
2. Organic Matter
3. Water
4. Air
5. Soil organisms

8. Gross Soil Composition

9. 1.Minerals
• The mineral part of soil
is composed of varying
amounts of sand, silt,
and clay. These particles
are inorganic. Their
characteristics greatly
influence soil behavior
and management needs.

10. Soil Particle Size
• Particle size is an important property that
allows us to make distinctions among the
different soil minerals.
• Soils contain particles that range from
very large boulders to minute particles
which are invisible to the naked eye.

11. To further distinguish particles based upon size,
particles are separated into the two categories: the
coarse fraction and the fine earth fraction.

12. Fine Earth Fraction
The Fine Earth Fraction
Size Texture Characteristics
Sand 2.0 mm -0.05 mm gritty Sand is visible to the naked eye, consists
of particles with low surface area, and
permits excessive drainage.
Silt 0.05 mm - 0.002 buttery Silt is not visible to the naked eye and
mm
increases the water holding capacity of
soil.
Clay < 0.002 mm sticky Clay has a high surface area, high water
holding capacity, many small pores, and
possesses charged surfaces to attract
and hold nutrients.

13. Sand
• Sand is mainly small
rock fragments and
hard minerals such as
quartz (silicon dioxide).
It contains few plant
nutrients
• Of the three types of
soil particles, sand is
the largest in size

14. • Silt consists of ground
Silt up sand (quartz) and
rock minerals. Silt
contains few nutrients
by itself, but it can have
nutrients clinging to its
surface.
• Silt is between sand and
clay in terms of size.

15. Clay
• Unlike sand and silt,
clays are aluminum-
silicate minerals that
also have varying
amounts of plant
nutrients such as
potassium, calcium,
magnesium, iron, etc.
• A good part of a soil's
native fertility can
come from its clay
portion

16. Relative size comparison between
sand, silt, and clay of the fine earth
fraction.

17. COARSE FRACTION
The coarse fraction of soil includes any
soil particles greater than 2mm.
The coarse fraction includes
boulders, stones, gravels, and coarse
sands.
These are rocky fragments and are
generally a combination of more than
one type of mineral.

19. • Organic Matter in the soil includes plant and
animal residues at various stages of
decomposition, cells and tissues of soil
organisms, and substances synthesized by
plant roots and soil microorganisms.
• Organic matter in the soil is frequently in the
form of humus, partially decomposed organic
matter that has become dark and crumbly and
continues decomposing at a slow rate

20. Humus benefits the soil in many ways:
 It can improve overall physical condition, especially in clay soils.
 It can help reduce soil erosion by wind and water because it acts as a
"glue" to bind soil particles together into "crumbs," called
aggregates, that improve water intake rates and lessen runoff.
 It stores and supplies nutrients, especially nitrogen, phosphorus, and
sulfur. These are slowly released for use by plant roots as organic
matter decomposes
 It increases the water-holding capacity of sandy soils.
 Its high negative charge helps prevent positively-charged nutrients from
leaching. In addition, negative charge improves a soil's buffering
capacity, or its ability to resist changes in pH.
 It can reduce the incidence of some soil-borne diseases and stimulate
growth of beneficial soil bacteria, fungi, and earthworms.

22. Who is at home in the
soil?

23. Soil organisms are creatures that spend
all or part of their lives in the soil
Soil organisms can be grouped on the
basis of:
Size: how big they are
Species: ecological functions
Function: how they make their living

24. Size of Soil Organisms
Macro or large Meso or mid-size Micro or small
(>2 mm) (2–0.2 mm) (<0.2mm)
Earthworm Springtail Yeast
Alfalfa root Bacteria
Mite

25. Based on the ecological function:
(1) herbivores that subsist on living plants,
(2) detritivores that subsist on dead plant debris,
(3) predators that consume animals,
(4) fungivores that eat fungi,
(5) bacterivores that eat bacteria,
(6) parasites that live off, but do not consume, other
organisms.
 Another classification of soil organisms groups
them into:
(1) heterotrophs that rely on organic compounds for
energy
(2) autotrophs that obtain their energy from
photosynthesis

26. Commensalist
Parasitic
Dietrich Werner, Marburg, Germany
Symbiotic

27. • Earthworms
– Mix fresh organic materials into
the soil
– Brings organic matter into contact
with soil microorganisms
• Soil insects and other arthropods
– Shred fresh organic material into
much smaller particles
– Allows soil microbes to access all
parts of the organic residue

28. Soil Microorganisms
• Bacteria
Most numerous in
soil
Most diverse
metabolism
Can be aerobic or
anaerobic
Optimal growth at
pH 6-8

29. Nitrogen-fixing bacteria form symbiotic associations
with the roots of legumes. The plant supplies simple
carbon compounds to the bacteria, and the bacteria
convert nitrogen (N2) from air into a form the plant
host can use
Nitrifying bacteria change ammonium (NH4+) to
nitrite (NO2-) then to nitrate (NO3-) – a preferred form
of nitrogen for grasses and most row crops.
Denitrifying bacteria convert nitrate to nitrogen (N2)
or nitrous oxide (N2O) gas; anaerobic

30. Actinomycetes
• a large group of
bacteria that grow as
hyphae like fungi
• Transitional group
between bacteria and
fungi
• Active in degrading
more resistant organic
compounds
• Optimal growth at
alkaline pH

31. • 2 important products:
– produce antibodies
(streptomycin is produced by
an actinomycetes)
– produce geosmin;
responsible for the
characteristically “earthy”
smell of freshly turned,
healthy soil
• Negative impact - potato scab
(Streptomyces scabies)

32. Soil Microorganisms
• Fungi
Dominate the soil biomass
Obligate aerobes
Can survive desiccation
Dominate in acid soils
Negative impacts:
– Apple replant disease
(Rhizoctonia, Pythium, Fusarium,
andPhytophtora)
– Powdery mildew is caused by a
fungus
Beneficials:– Penicillium

33. What are mycorrhizal fungi?
• Mycorrhizas are associations between fungi and plant
roots that can be beneficial to both the plant and the
fungi
• The fungi link the plant with soil by acting as agents of
nutrient exchange
• In general, mycorrhizas play an important role in
sustainable plant productivity and maintenance of
soil structure.
• Mycorrhizal associations occur on almost all
terrestrial plants and are not as plant-specific as other
plant-microbe associations that formed between
some plants (e.g. legumes) and bacteria (e.g.
rhizobia).

34. Mycorrhizal fungi
Soil structure benefit
Mycorrhizal fungi present Mycorrhizal fungi absent
• Soil structure stabilized and • Soil structure is weak
strengthened • Structure is not maintained when
• Structure is maintained when immersed in water
immersed in water

35. Protists and nematodes- the predators
–Feed on the primary Rotifer
decomposers
(bacteria, fungi,
actinomycetes) Amoeba
–Release nutrients
(nitrogen) contained
in the bodies of the Predatory nematode
primary
decomposers

36. Abundance of soil organisms
Number Biomass1
Organism per gram soil (lbs per
(~1 tsp) acre 6”)
Earthworms – 100 – 1,500
Mites 1-10 5 – 150
Nematodes 10 – 100 10 – 150
Protozoa up to 100 thousand 20 – 200
Algae up to 100 thousand 10 – 500
Fungi up to 1 million 1,000 – 15,000
Actinomycetes up to 100 million 400 – 5,000
Bacteria up to 1 billion 400 – 5,000
1 Biomass is the weight of living organisms

37. Why are soil organisms important?
Soil animals perform several functions in soil
that make them a vital part of all
ecosystems, including agriculture. Soil animals
are involved in:
• degradation of organic matter and
mineralisation of nutrients,
• controlling populations of pathogens,
• improving and maintaining soil structure and
• mixing organic matter through the soil.