Which pair correctly matches an organism with its role in soil?
Answer: C
Only the first pairing is right. Rhizobium fixes nitrogen rather than oxidising nitrite, which is Nitrobacter's work; Thiobacillus oxidises sulphur rather than fixing nitrogen; and sulphate reduction belongs to Desulfovibrio, not to the nitrogen-fixing Azotobacter.
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Which soil organism group is most sensitive to soil compaction?
Answer: D
Compaction collapses the larger pores that hold air, so oxygen diffusion falls and aerobes are the first to suffer. Anaerobes actually benefit, which is why compacted patches become sites of denitrification. Light never penetrates below the surface in any case.
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A soil with a very low organic matter content will generally support
Answer: A
Nearly all soil organisms are heterotrophs and depend on organic carbon for both energy and cell material, so where organic matter is scarce the population is small however favourable the other conditions. This is why building organic matter is the surest way to raise biological activity.
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Which is the correct sequence of the nitrogen cycle steps acting on dead organic matter?
Answer: B
Organic nitrogen is first mineralised to ammonium by ammonification, that ammonium is then oxidised through nitrite to nitrate by nitrification, and only nitrate can be reduced to gas by denitrification. The order follows the oxidation state, from most reduced to most oxidised and back to gas.
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Actinomycetes are most abundant in soils that are
Answer: C
Actinomycetes tolerate dryness better than most bacteria but are among the least acid-tolerant groups, so they thrive in warm neutral to alkaline soils and dwindle as pH falls. Their filamentous growth also lets them bridge dry gaps between soil particles.
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The nitrogen in a legume crop residue becomes available to a following cereal chiefly after
Answer: D
Plants take up nitrogen only as ammonium or nitrate, so the organic nitrogen in the residue must first be released by decomposers. That is why the benefit to the following crop is spread over weeks rather than being available the moment the residue is incorporated.
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Which factor most strongly determines how quickly a pesticide is degraded in soil?
Answer: A
Microbial degradation is the main route of loss for most pesticides, so persistence depends chiefly on whether an organism present can metabolise the molecule. Compounds with unusual synthetic structures are persistent precisely because no such enzyme is common.
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Repeated application of the same pesticide to a soil sometimes leads to
Answer: B
Once a population capable of using the compound establishes itself, each subsequent application is degraded faster than the last, which can shorten a product's useful life in the field. This adaptation is a recognised practical problem rather than a laboratory curiosity.
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Which measurement would tell you most directly how biologically active a soil is at the moment of sampling?
Answer: C
Respiration integrates the activity of the whole living community as it stands, including organisms that cannot be cultured. Total nitrogen and clay content are properties of the soil that change over years, and colour is only an indirect clue to organic matter.
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Nitrogen fixation and nitrification differ in that fixation
Answer: D
Fixation converts atmospheric dinitrogen into a combined form, adding to the soil's total nitrogen, whereas nitrification merely oxidises ammonium already present into nitrate. Nitrification is the aerobic process of the two, and neither is carried out by plants.
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