Question

Arrange the following consequences of submergence in the soil in the correct sequence:
(A). Depletion of soil oxygen,
(B). Accumulation of toxic substances like hydrogen sulfide,
(C). Loss of beneficial soil organisms,
(D). Increased soil pH in acidic soil.

• A. (A), (C), (B), (D)
• B. (A), (B), (C), (D)
• C. (B), (A), (D), (C)
• D. (C), (B), (D), (A)

Hint:

Understand the biological and chemical changes occurring during waterlogging to answer sequence-based soil science questions.

Answer 1

The Correct Option is B

When soil is submerged, oxygen is depleted first, followed by accumulation of toxic substances like hydrogen sulfide. This harms beneficial organisms, and under prolonged submergence, pH may increase in acidic soils.

Answer 2

Reasoning from redox chemistry:
When a soil is flooded, free oxygen trapped in pore spaces is consumed by aerobic microbial respiration within hours, so oxygen depletion (A) is always the first event. As the soil becomes anaerobic, microbes switch to alternative electron acceptors (nitrate, manganese, iron, sulfate) in a defined redox sequence, and once sulfate reduction begins, hydrogen sulfide and other toxic reduced compounds (B) start accumulating. This increasingly toxic, oxygen-starved environment is unfavourable for strict aerobes such as many beneficial bacteria and fauna, causing loss of beneficial soil organisms (C). Only after prolonged reduction do the associated reactions drive the chemical changes that raise the pH of acid soils toward neutrality, so pH increase (D) occurs last. This gives the order A, B, C, D, confirming option 2.