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This article throws light upon five types of micro-organisms present in soil. The types are: 1. Nematodes 2. Protozoa 3. Rotifera 4. Mycorrhizae 5. Lichens.
1. Nematodes:
They are also called threadworms or eelworms. They are mainly microscopic in size and are the most abundant animals (maximum number is 50 per gm.). Nematodes live mainly in the water film that surrounds soil particles on or in plant roots.
They form cysts when conditions become unfavourable and are active when conditions become favourable. These organisms are round and spindle shaped. Their caudal end is pointed.
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They have been divided into three groups on the basis of their food requirements:
(i) Nematodes that live on decaying organic matter.
(ii) Predatory Nematodes feed on other nematodes, plant parasites protozoa, bacteria and the like.
(iii) Parasitic Nematodes that attack that roots of crops. These have a needle-like anterior end (stylet) with which they pierce the roots of the host, which develops galls or deformed roots.
2. Protozoa:
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They are much larger than bacteria although both are unicellular soil micro-organisms.
Phylum protozoa have been divided into five classes on the basis of their means of locomotion as shown below:
(a) Mastigophera:
Which moves by means of long whip-like appendages called flagella, e.g. Cerconodo in Fig. 8.1A.
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(b) Sarcodina:
They are also called rhizopods. Actually they consist of a naked mass of protoplasm which moves by pseudopodia e.g. Euglypus in Fig. 8.1B.
(c) Ciliate:
They bear hairs on their body which help them to move e.g. Colpoda in Fig. 8.1C. Protozoa encyst when the environment is unfavourable. The majority of them depend upon performed organic matter either as saprozoic feeders, obtaining their nutrition from soluble organic and inorganic substances, or by a holozoic nutrition characterized by direct feeding upon microbial cell Although they are the most varied and numerous because more than 250 species have been identified, their function in soil is still not fully known except that they serve to regulate the bacterial population.
3. Rotifera:
They occur in moist soils. Their anterior side is modified into a retractile disc that bears circles of hairs called cilia, which, while in motion, give the appearance of a moving wheel.
These hairs sweep floating food materials into the animal. The posterior end of a rotifer can attach itself to a convenient object (Fig.8.1D). Micro flora includes algae, fungi, actinomycetes and bacteria. Physiologically they may be heterotrophic or autotrophic in nutrition.
1. Algae:
Morphologically, algae may be unicellular or may occur in short filaments.
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They may be divided into the following four classes:
(i) Chlorophyceae:
They contain chlorophyll in chromatophores. So they are called green algae, e.g. chlorella.
(ii) Cyanophyceae or myxophyceae:
Chlorophyll is uniformly distributed in the cytoplasm of this class which also contains a blue pigment called phycocyanin in addition to chlorophyll. So they are called blue-green algae, e.g. Nostoc.
(iii) Xanthophyceae:
They contain a yellow-green pigment called xanthophyll. So they are called yellow-green algae, e.g. Bortrydium.
(iv) Bacillariaceae or Diatomaceae:
They are unicellular or colonial algae which are surrounded by a highly silicified layer. They are golden brown in colour. So they are called golden brown algae or diatoms e.g. Nevicula.
2. Fungi:
The vegetative body of fungi consists of elongate thread-like structures which are individually called hyphae. The entire vegetative body or groups of hyphae are called mycelium.
The hyphae of fungi are much broader than those of actionmyceters (about 0.5 to 1.2 micron in diameter).
Soil fungi belong to either of the three following classes:
(a) Class fungi imperfecti:
They possess non-septate myceliums which bear sexual spores called conidia, on specialized structures called conidiophores.
(b) Class phycomycetes:
They possess non-septate and filamentous hyphae and bear asexual spores called sporangiospores in Sparangia.
(c) Class ascomycetes:
They possess hyphae and form sexual spares called as cospores in a sac-like structure called an ascus. Fungi belonging to the classes Phycomycetes and Ascomycetes produce both asexual and sexual spores. Fungi usually occur in the soil @ 8000 to 100000 per gm. of dry soil. Fungi can decompose most complex organic molecules like staich, pectin, cellulose, fast and even lignins which cannot attacked by bacteria.
3. Actinomycetes:
Actinomycetes are intermediate between fungi and bacteria. They are unicellular micro-organisms that consist of slender branched mycelia which are narrower than the fungal mycelium (0.5 to 1.2 microns in contrast to 5 microns in the case of fungi).
If they break up into spores they resemble bacteria. So they are called thread bacteria. In addition to proliferation by vegetative means, certain actinomycetes produce asexual spores called conidia. They develop in moist, well aerated soil at a pH of 6.0 to 7.5.
Very few of them occur below pH 5. Actinomycetes decompose organic matter and attack even the resistant compounds. Usually they occur @ 100000 to 600000 per gm. of dry soil. They are especially numerous in humus rich soils where they may be as much as 0.25 tonnes per hectare.
4. Bacteria:
They are unicellular micro-organisms. Larger ones seldom exceed 4 to 5 microns, and smaller bacteria are the size of clay particles i.e. less than 2 microns. A rod shaped bacterium is called Bacillus (Plural Bacilli) a spherical shaped bacterium is called a coccus (Plural, Cocci) and a spiral shaped bacterium is called spirillum (Plural Spirilla).
Bacteria occur in soil @ 0.1 to 20 million per gm. of soil. Some produce spores which allow them to survive during unfavourable conditions. They occur in the greatest number near the surface of the land where favourable conditions occur most of the time.
Most of the soil bacteria are heterotrophic i.e. they obtain their carbon and energy from organic matter. A few of them may get their energy from sunlight when they are called photoheterotrophic bacteria.
Some bacteria are autotrophic, and obtain their carbon from carbon dioxide and energy sunlight when they are called photoautotrophic bacteria or from oxidation of inorganic compounds when they are called chemoautotrophic bacteria.
Bacteria are the most important soil micro-organisms. They are responsible for basic enzymatic transformation like nitrification, nitrogen fixation and sulphur oxidation. Aerobic bacteria can use only oxygen gas. Anaerobic bacteria can only use oxygen in the combined form. They grow best at 50 to 75 per cent of the maximum water holding capacity of soils, between 20 to 40’C and at a pH of 6.0 to 8.0. Most of them require abundant calcium.
4. Mycorrhizae:
The symbiotic association between the roots of some plants and some fungi is called Mycorrhiza. The fungus is highly habitat limited and is usually found in the immediate vicinity of or within the roots.
The fungal hyphae on the exterior of roots usually serve as an extension of roots and store large amounts of carbohydrates. Roots supply carbohydrates to the fungi which absorb nutrients from the soil and supply them to the crop.
The mycorrhizae are of two kinds:
(1) Ectomycorrhizae and
(2) Endo-mycorrhizae.
In the former case, the fungus forms a mantle around the exterior of the roots a network composed of a mass of hyphae entering the intercellular spaces while in the latter case it penetrates the root cells.
Endo-trophic mycorrhizae are quite common among Ericaceae and Orchidaceae as well as in fruit trees especially citrus, coffee, rubber and various legumes. Coils of hyphae or branched hyphal structures formed within the root cells are called arbuscles.
The fungus benefits the host in the following way:
1. The effective root surface is increased, so absorption of nutrients, especially phosphorus, by the roots, increases.
2. The rootlets function for a longer time.
3. The capacity of the host plant to tolerate heat and drought increases.
4. Soil nutrients become more easily available.
5. The host plant becomes more disease resistant. This last benefit occurs only in the case of ectomycorrhizae.
Mycorrhizae develop well in soils low in phosphorus and nitrogen. They are most vigorous on roots which have a large reserve of available carbohydrates.
5. Lichens:
Symbiotic association between some algae and fungi are called Lichens. The algae benefits in part because the fungal hyphae envelop it and protect it from environmental stresses, while the fungus gains by absorption of carbohydrates chat have been photosynthesized by the algae.
If a blue green is part of the lichen, the fungus additionally gains nitrogen because the blue green alga fixes atmospheric nitrogen. Lichens release compounds that cause weathering of rocks.
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