SOIL MICROBIOLOGY EASY NOTES

SOIL MICROBIOLOGY

It is branch of science/microbiology which deals with study of soil microorganisms and their activities in the soil.

Soil:

It is the outer, loose material of earth’s surface which is distinctly different from the underlying bedrock and the region which support plant life. Agriculturally, soil is the region which supports the plant life by providing mechanical support and nutrients required for growth. From the microbiologist view point, soil is one of the most dynamic sites of biological interactions in the nature.  It is the region where most of the physical, biological and biochemical reactions related to decomposition of organic weathering of parent rock take place.

Rhizosphere: 

It is the zone/region of soil immediately surrounding the plant roots together with root surfaces, or it is the region where soil and plant roots make contact, or it is the soil region subjected to influence of plant roots and characterized by increased microbial.

Rhizoplane: 

Root surface along with the closely adhering soil particles is termed as rhizoplane.
Soil microorganisms: 

Soil is an excellent culture media for the growth and development of various microorganisms. Soil is not an inert static material but a medium pulsating with life. Soil is now believed to be dynamic or living system.

Soil Humus

Humus is the organic residue in the soil resulting from decomposition of plant and animal residues in soil, or it is the highly complex organic residual matter in soil which is not readily degraded by microorganism, or it is the soft brown/dark coloured amorphous substance composed of residual organic matter along with dead microorganisms. 

Composition of Humus:
In most soil, percentage of humus ranges from 2-10 percent, whereas it is up to 90 percent in peat bog. On average humus is composed of Carbon (58 %), Nitrogen (3-6 %, Av.5%), acids - humic acid, fulvic acid, humin, apocrenic acid, and C: N ratio 10:1 to 12:1. During the course of their activities, the microorganisms synthesize number of compounds which plays important role in humus formation. 

Functions/Role of Humus:

1.      It improves physical condition of soil

2.      Improve water holding capacity of soil

3.      Serve as store house for essential plant nutrients

4.      Plays important role in determining fertility level of soil

5.      It tend to make soils more granular with better aggregation of soil particles

6.      Prevent leaching losses of water soluble plant nutrients

7.      Improve microbial/biological activity in soil and encourage better development of plant-root system in soil

8.      Act as buffering agent i.e. prevent sudden change in soil PH/soil reaction

9.      Serve as source of energy and food for the development of soil organisms

10.  It supplies both basic and acidic nutrients for the growth and development of higher plants

11.  Improves aeration and drainage by making the soil more porous

Soil Organisms

A. Soil Flora
a) Microflora: 1. Bacteria 2. Fungi, Molds, Yeast, Mushroom 3. Actinomycetes, Stretomyces  

4. Algae eg. BGA, Yellow Green Algae, Golden Brown Algae.

b) Macroflora: Roots of higher plants

B. Soil Fauna

a) Microfauna: Protozoa, Nematodes

b) Macrofauna: Earthworms. moles, ants & others.

As soil inhabit several diverse groups of microorganisms, but the most important amongst them are: bacteria, actinomycetes, fungi, algae and protozoa. The characteristics and their functions / role in the soil are described in the next topics.

Bacteria

•      Most numerous in soil

•      Most diverse metabolism

•      Can be aerobic or anaerobic

•      Optimal growth at pH 6-8

•      Examples: Nitrosomonas and Nitrobacter in nitrification processes, N2 fixers, fire blight is caused by a bacterium

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

•      There are species of fungi that trap harmful plant-parasitic nematodes.

•      The mycorrhizae are fungi that live either on or in plant roots and act to extend the reach of root hairs into the soil.

•      Mycorrhizae increase the uptake of water and nutrients, especially phosphorus.

•      They are particularly important in degraded or less fertile soils. Roots colonized by mycorrhizae are less likely to be penetrated by root-feeding nematodes, since the pest cannot pierce the thick fungal network.

•      Mycorrhizae also produce hormones and antibiotics that enhance root growth and provide disease suppression.

•      The fungi benefit by taking nutrients and carbohydrates from the plant roots they live in.

Actinomycetes

•      Transitional group between bacteria and fungi

•      Active in degrading more resistant organic compounds

•      Optimal growth at alkaline pH

•      2 important products:
– produce antibodies (streptomycin is produced by an actino)
– produce geosmin

•      Negative impact - potato scab (Streptomyces scabies)

Algae

•      Many different species of algae live in the upper half-inch of the soil.

•      Unlike most other soil organisms, algae produce their own food through photosynthesis.

•      They appear as a greenish film on the soil surface following a saturating rain.

•      Algae improve soil structure by producing slimy substances that glue soil together into water-stable aggregates.

•      Some species of algae (the blue-greens) can fix their own nitrogen, some of which is later released to plant roots. Algae are classified as,

1.      Cyanophyta (Blue-green algae) 

2.      Chlorophyta (Green algae)

3.      Xanthophyta (Yellow-green algae)

4.      Bacillariophyta (diatoms or golden-brown algae)

•      Blue-green algae and grass-green algae are more abundant in soil.

•      The green-grass algae and diatoms are dominant in the soils of temperate region while blue-green algae predominate in tropical soils.

•      Green-algae prefer acid soils while blue green algae are commonly found in neutral and alkaline soils.

•      The most common genera of green algae found in soil are: Chlorella, Chlamydomonas

Protozoa

•      Protozoa are free-living microorganisms that crawl or swim in the water between soil particles.

•      Many soil protozoa are predatory, eating other microbes.

One of the most common is an amoeba that eats bacteria. By eating and digesting bacteria, protozoa speed up the cycling of nitrogen from the bacteria, making it more available to plants.