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Physicochemical properties of soil include soil texture, water, air, inorganic chemicals, and organic matter. The biological factors of soil are soil flora and fauna.
Mechanical composition (texture) of soil is determined on the basis of size of soil particles i.e. sand, silt and clay particles (Table 30.1). However, the ratio of soil particles governs the porosity (pore size of soil), soil water (present in pores), air temperature, pH, inorganic and organic matters and microorganisms and their community- size. The amount of each component is changed with soil types.
The sources of inorganic material in soil are the parent rocks which get changed by the physical and chemical processes of weathering. Therefore, soil consists of different sized mineral particles (Table 30.1), the ratio of which determines the characteristics of soils.
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On the basis of particle size soil minerals are divided into:
(a) Sand particles (about 50 µm diameter) which are the fragments of rock materials,
(b) Silt particles (2- 50 µm diameter) which contain primary minerals (quartz), and
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(c) Clay particles (less than 2 µm diameter) composed of secondary minerals (e.g. kaolinite, montmorillonite, ellite, etc.).
Clay particles are negatively charged particles which are the important components of soil environment influencing the physicochemical and biological properties of soil.
Chemical nature of soil minerals differ, and it has been divided into silicates and non-silicates. The non-silicate group includes oxides, hydrides, sulphates, chlorides, carbonates and phosphates. The silicates are very complex structures but vary widely in its stability and resistance to decomposition.
Among these, the most influential soil particles, as far as microbial activity is concerned, are the colloidal size clays and humic materials. It plays significant role in determining the availability of nutrients and in the interaction of extracellular enzymes and antibiotic substances produced by the microorganisms.
Organic Matter:
The surface layer of soil consists of a relatively unchanged mass of plant/ animal remains called litter. After microbial decomposition organic matter is converted into unidentifiable amorphous material which is known as humus.
Soil-Water and Air:
Soil-water and air play a significant role as it influence the metabolic activities of macro and micro biota. Soil water and air are directly related to the soil texture because the portion of pore space devoid of water is filled with gas, and water film in pores, gives a mean for the movement, germination and growth of spores therein. The detailed account of microbial activity affected by various edaphic factors associated with water regime has been discussed by Griffin (1972).
Is ‘Habitat’ a Better Term for Microorganism?
It is rather better to use the term ‘niche’ than ‘habitat’, as the habitat is the place where an organism lives, whereas niche speaks the habitat as well as the role of the organism in that habitat with respect to other organisms and the environment. Odum (1971) has used the two analogies, ‘address’ for habitat and “profession’ for niche of the organisms.
Among the active microbial species in soil, food specialization makes possible the existence of a large number of ecological niches within a given habitat. Even in such specialized and restricted habitat, as the rhizoplane (see microbial interactions) of an individual plant, it is possible that most of the microbial species that remain present therein are not competitive.
Dwivedi and Saravanamuthu (1985) have discussed this restriction as the inherent genetic potential of a species which determines the range of tolerance to the varied physicochemical and environmental parameters and called as fundamental niche.
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Moreover, if an ecological niche is a ‘substrate’ the key for understanding seems to be the realization that the scale of time and distance applicable to microorganisms are extremely small, and the soil is extremely heterogeneous on these scales.
Thus, soil is the best heterogeneous ecological niche harbouring a variety of microorganisms which play many fold roles (both beneficial and harmful) with the subsequent effect on soil properties.
Microbial Balance:
Soil is a complex ecosystem in a state of dynamic equilibrium, bounded by physicochemical parameters. The relative stability of this system depends upon the relative stability of its biological composition and regulating parameters.
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However, long association between organisms in the same environment brings about a kind of permanence or balance among them which is commonly known as equilibrium between needs of organisms and their numbers.
The equilibrium is possible only when, during coexistence action and interaction between different microorganisms of varied potential go on. Thus, at a given time population of one species increases, while that of the others perhaps decreases due to microbial interaction in an ecological niche.
In natural soil if the greater number of interacting factors, results in the more stationary microbial balance. In rhizosphere the balance would be less stable due to continuous release of energy sources (host tissues/exudates) for microorganisms.
Baker and Cook (1974) pointed out that the presence of microorganisms at a given place and time is determined by:
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(a) its having or being introduced there,
(b) the existence of physicochemical environment favourable to its development,
(c) the presence of associated organisms (symbionts, hosts) favourable to its development, or organisms (host or parasite) required for its survival,
(d) the inhibition or absence of organism (disease organisms, pests, antagonists) so detrimental to it as to cause its extinction.
An organism will increase until the limitations imposed by the biotic and abiotic environment just counter balance the rate of increase. Thus, in biological sense limitations check the chaos and epidemics of microorganisms.
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Owing to the presence of complex substrates the ecological niches become very complex. However, physicochemical and genetical diversity of microorganisms even in nutritionally deficient types, allow them to range over many environments with varying degree of success.
For example, Macrophomina phaseolina is a root parasite pathogenic to many plants under high soil temperature (30-35°C) and dry conditions, but it was found to parasitize quickly the roots of weeds growing in cool environment where the fungus was not pathogenic.
Man is the supreme in living organisms in disrupting the balance of nature. This act has been exploited in the case of microorganisms what we call as biological control of soil inhibiting pathogens.
In contrast, when man started cultivation innumerable microbial balances were broken resulting in an increase in parasites harmful to cultivated plants. Ordish (1967) stated “we are civilized only because we have upset the balance of nature for our advantage; unless we continue to do so we shall perish”.