Question

Assertion (A): Waterlogged soil typically displays a grayish color due to reduced oxygen levels.
Reason (R): Upland soil usually maintains a reddish or brownish hue due to its well-aerated nature.

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

Soil color is an indicator of its aeration and redox status—gray indicates reduced (anaerobic), red indicates oxidized (aerobic).

Answer 1

The Correct Option is A

Waterlogged soils become anaerobic, which reduces iron and manganese compounds, leading to a grayish soil color. Upland soils, which are well-aerated, promote oxidation of iron, giving them a reddish or brown hue. Thus, both statements are correct and the reason explains the assertion well.

Answer 2

Redox potential approach:
Soil colour is governed by the oxidation state of iron and manganese, which in turn depends on the soil's redox potential (Eh). In waterlogged soils, oxygen is excluded and microbes use \( \text{Fe}^{3+} \) and \( \text{Mn}^{4+} \) as alternate electron acceptors, reducing them to \( \text{Fe}^{2+} \) and \( \text{Mn}^{2+} \); these reduced, soluble forms impart the characteristic grey/blue-grey (gleyed) colour. In upland, well-drained soils the high Eh keeps iron oxidised as \( \text{Fe}^{3+} \) oxides/hydroxides, producing red and brown hues. Since this Eh-driven mechanism directly accounts for both colours, Reason (R) is a correct explanation of Assertion (A).