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� CS 566: ADVANCED��SOIL MICROBIOLOGY�

Module 1

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Soil Organisms

Most obvious property of soil – physical constituents.

Biological element most considered – Macro fauna e.g. earthworm.

Why overlook the diversity, complexity and continual activity of the living organisms in the soil?

-Due to microscopic nature of this group of soil inhabitants.

Microscopic nature compensated for by large numbers (10⁷– 10⁹) per gram soil.

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Without the vital process of the biological activity of soil microorganisms, the soil would become a repository of dead plant remains.

There would be no facility for the recycling of C, N, and P for plant growth.

A naturally fertile soil – A soil in which soil organisms are releasing inorganic nutrients from organic reserves at a rate sufficient to sustain rapid plant growth.

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Soil Fauna

Compared to microbial studies, the soil Fauna until recently received little attention.

Relative neglect has been due partly to:

Difficulty of isolating the animals from the soil
Problem of systematic classification
Little was known about the actual food of many of the members

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Advances in Fauna Studies

Fixing and preparing casts and sections of soil without disturbing its structure.

Identification of food sources by examination of gut contents, using microscopical and immunological methods.

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Enumeration of Soil Fauna

Soil Monolith Procedure

Tullgren method

Floating in salt solution

Watering soil with emulsion of o – dichlorobenzene.

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Soil Inhabiting Mammals

Various small mammals – mice, rabbits, etc are present in appreciable numbers in some undisturbed soils.

Though their total weight per ha is usually small, they can cause loosening of the surface layers of the soil by their often extensive excavations.

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Functions of Soil Fauna

Decomposition of organic matter and cycling of nutrients – numerous enzymes in gut including protease, lipase, amylase, cellulose and chitinase.

Provide well comminuted medium for microbial activity which greatly influence the total soil metabolism.

Burrowing activities have some importance in relation to soil drainage and aeration.

Act as predators of soil organisms.

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Bacteria

These are unicellular organisms without organelles or nucleus they are one of the simplest forms of life.

Most numerous organisms in soil, with viable populations estimated at up to 200 million individual cells per gram of soil.

The size ranges from 1-5 microns. The shape vary from cocci (round shaped), to bacilli (rod shaped) and to spiral

Number of bacteria in the soil is not evenly distributed but usually correlated to the amount of organic matter.

Numbers of bacteria immediately adjacent to roots are much higher than in the surrounding regions – Rhizosphere effect.

The Soil Microflora

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Optimum pH condition for bacteria growth is slightly acidic to neutral, however some groups survive under highly acidic conditions and they are termed acidophilic bacteria.

The optimum temperature range for most bacteria is 25 to 35 oC and these are termed mesophiles, however, some are able to tolerate extreme temperatures these are Psychrophiles (0 to 20 oC) and

Thermophiles (40 to 65 oC).

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Importance of bacteria

Bacteria are very important in the general decomposition of organic matter in soil.

They carry out specific functions important in nutrient cycling such as nitrification.

A group of bacteria are important in in nitrogen fixation- conversion of atmospheric nitrogen to plant available forms.

Some soil bacteria cause diseases.

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Fungi

They have well developed organelles including nuclei, mitochondria, they are more developed than bacteria.

The most important characteristic of fungi is the possession of a filamentous body consisting of strands of hyphae.

Form the second major group of soil microorganisms.

Depending on local conditions, particularly pHand ambient moisture content, fungi or bacteria may predominate.

Despite the fact that bacterial are more numerous, many times fungi constitute the largest proportion of microbial protoplasm in cultivated soils.

The numbers of fungi are greatest on the soil surface and decrease with increase in depth.

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Importance of FUNGI

Fungi are important in decomposition of organic residues in soil

They are especially important in decomposing woody material which many bacteria cannot decompose.

They are important in processes leading to humus formation.

They play important roles in the formation of stable aggregates.

Some soil fungi cause plant and animal diseases.

Some fungi form symbiotic association with roots of higher plant.

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Actinomycetes

Are typically aerobic organisms and are uncommon in waterlogged soils.

They are largely intolerant to acidity.

Structurally, these organisms lie between bacteria and fungi, they bear similarity to bacteria in terms of cell size and structure characteristic and they are filamentous organisms like fungi.

But in many aspects of their morphology and growth patterns resemble those of fungi.

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Importance of ACTINOMYCETES

They are important in decomposition of organic matter, especially cellulose, chitin and phospholipids.

Some actinomycetes produce antibiotics eg Streptomyces sp.

Some actinimycetes cause plant diseases eg potato scab disease.

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Algae

The algae are photosynthetic micro-organisms which contain chlorophyll and in some cases other pigments.

In many soils, the algae make no significant impact.

Their importance is found in certain situations where they play unique roles:

Where drastic erosion has occurred
Where severe burning has taken place.

In these situations, the photosynthetic ability of algae permits their growth as primary colonizers.

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Cont’d

Retaining inorganic nutrients that might otherwise be leached away.

Bringing them to the matrix the initial impact of organic material necessary for the transformation into soil to begin.

In the most severe conditions where algae might not be able to survive; association with fungi – lichen symbiosis allows initial colonization.

They form symbiotic associations with fresh water fern (azolla) which are very important in fertility management of rice paddies.