Q.73 Which one of the following represents the abundance of the organisms found in
soil?
(A) Fungi > Aerobic bacteria > Anaerobic bacteria
(B) Aerobic bacteria > Fungi > Anaerobic bacteria
(C) Aerobic bacteria > Anaerobic bacteria > Fungi
(D) Anaerobic bacteria > Aerobic bacteria > Fungi
Aerobic bacteria represent the highest abundance of organisms in soil, followed by fungi and then anaerobic bacteria. This order reflects typical well-aerated soil conditions where oxygen supports dominant aerobic populations.
Option Analysis
Each option proposes a different hierarchy for the abundance of organisms in soil among aerobic bacteria, anaerobic bacteria, and fungi.
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(A) Fungi > Aerobic bacteria > Anaerobic bacteria: Incorrect, as fungi rank second in abundance after bacteria overall, not first; bacteria (especially aerobic) outnumber fungi by billions per gram.
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(B) Aerobic bacteria > Fungi > Anaerobic bacteria: Correct, matching relative percentages where aerobic bacteria comprise about 70%, fungi around 3%, and anaerobic bacteria 13%.
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(C) Aerobic bacteria > Anaerobic bacteria > Fungi: Incorrect, since fungi exceed anaerobic bacteria in most aerated soils; anaerobic forms thrive only in low-oxygen microsites.
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(D) Anaerobic bacteria > Aerobic bacteria > Fungi: Incorrect, as anaerobic bacteria are minority (less than 1/3 of total bacteria ideally) in healthy, oxygenated soils.
Soil Microbial Abundance
Soil hosts billions of microbes per gram, with bacteria dominating at 10^8-10^9 cells, far exceeding fungi (10^5-10^6 propagules) and actinomycetes. Aerobic bacteria thrive in oxygen-rich topsoil, driving nutrient cycling, while fungi form networks for decomposition. Anaerobic bacteria persist in compacted or waterlogged zones but remain lowest in abundance.
Factors Influencing Order
Oxygen availability dictates aerobic dominance in most soils; compaction boosts anaerobes. Organic matter and pH favor bacteria over fungi in fertile topsoil. Rhizosphere zones amplify aerobic bacteria near roots. For CSIR NET, recognize this as standard for aerated agricultural soils.
