All organisms living within the soil are collectively termed soil life or soil biota. A soil organism is any organism inhabiting the soil during part or all of its life.
Soil organisms range in size from microscopic cells that digest decaying organic material to small mammals that live primarily on other soil organisms. They play a critical role in maintaining soil fertility, structure, drainage, and aeration.
They also break down plant and animal tissues, releasing stored nutrients and converting them into forms usable by plants.
Soil as a Complex Medium for Agricultural Processes
The soil is a highly complex medium where numerous chemical, biological, biochemical, geochemical, biogeochemical, and physical processes occur. It serves as the primary medium from which plants obtain most of their nutrients.
The soil hosts a vast population of living organisms, including micro- and macroflora, micro- and macrofauna, insects, and more. The activities of some of these organisms are detrimental to plants, particularly disease-causing organisms.
However, the activities of most soil organisms are beneficial to crops, particularly in terms of soil aggregation, nutrient cycling, biological nitrogen fixation, nutrient uptake, disease control/prevention, and production of growth hormones.
These organisms interact with one another in the soil, giving rise to diverse relationships such as symbiosis, parasitism, commensalism, protocooperation, neutralism, and competition.
Soil microorganisms constitute the highest populations of soil organisms and, due to their enzymatic capabilities, are more significant in soil processes than other soil organisms.
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Classification of Soil Microorganisms in Agriculture
Soil microorganisms can be classified based on physiology or nutrition, mode of respiration, and origin.
A. Physiological and Nutritional Classification of Soil Microorganisms
Microorganisms require food as sources of energy to enable them to carry out their activities, for growth and multiplication.
Microorganisms differ in their nutritional requirements, whereas some organisms can use the same source of food as carbon and energy, others require different sources. On this basis, microorganisms are divided into:
1. Autotrophs/Lithotrophs: These organisms use CO2 as the sole source of carbon. Based on the source of energy, they are further classified as:
2. Photoautotrophs/Photolithotrophs: These organisms derive their energy from the sun through the process of photosynthesis. Such organisms contain a pigment known as chlorophyll, which enables them to convert CO2 to carbohydrates in the presence of solar energy, e.g., algae.
3. Chemoautotrophs/Chemolithotrophs: These organisms derive their energy from biochemical oxidations. They release energy from reactions, e.g., oxidation of NH4+ to NO2- and NO2- to NO3-, as seen in Nitrosomonas sp. and Nitrobacter sp.
4. Heterotrophs: These organisms use organic compounds as their carbon and energy source. They derive both carbon and energy from the same source.
Most microorganisms belong to this class, and in addition to carbon, other nutrients like N, K, P, Na, Mg, Ca, Fe, etc., which they need, are obtained from organic matter.
B. Classification Based on Respiration of Soil Microorganisms
Based on the mode of respiration, soil microorganisms can be classified as aerobes, anaerobes, and facultative anaerobes.
1. Aerobes: These organisms require free oxygen for respiration and cannot survive in its absence. Most bacteria, all fungi, and actinomycetes fall into this class.
2. Anaerobes: These organisms grow optimally only in the absence of molecular oxygen. This group includes many bacteria, e.g., Clostridium.
3. Facultative Anaerobes: These organisms can survive either in the presence or absence of oxygen. Although they need oxygen, they do not necessarily require access to molecular oxygen.
They can survive by extracting the required oxygen from oxygen-rich compounds such as nitrates or sulfates, which are reduced, thus altering their availability to plants.
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C. Classification Based on Origin of Soil Microorganisms
Based on their origin, bacteria are classified as:
1. Autochthonous or Indigenous Microorganisms: These are the original residents in the soil, with constant numbers. They do not usually respond to additions of organic matter and grow very slowly. They may have developmental stages used to endure in soil for long periods without being active metabolically.
2. Allochthonous or Zymogenous or Invader Microorganisms: These microorganisms develop under the influence of specific soil treatments, such as the addition of organic matter, fertilization, or aeration. They do not contribute significantly to soil processes.
3. Transient Microorganisms: These microorganisms are introduced into the soil intentionally, e.g., Rhizobium sp., mycorrhizal fungi, or unintentionally, e.g., through diseased plants. They die rapidly or may survive in the soil for a period in the presence of a host plant or animal.
A soil organism is any organism inhabiting the soil during part or all of its life. They range in size from microscopic cells that digest decaying organic material to small mammals that live primarily on other soil organisms. They play a vital role in maintaining soil fertility, structure, drainage, and aeration.
Soil microorganisms can be classified based on physiology or nutrition, mode of respiration, and origin, all of which contribute to their critical functions in agricultural ecosystems.
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