Assertion : In humid tropical climatic areas undissolved aluminium, iron and manganese accumulate and make soil unfertile Reason (R) : High bacterial activity destroys humus.

The assertion is true as humid tropical climates facilitate laterization, where intense leaching removes silica and soluble bases, leaving behind insoluble oxides of iron and aluminium [c1, t3]. These soils, often called laterites, are generally acidic and nutrient-deficient, making them unfertile for many crops [c1, t8]. The reason is also true; in humid tropical and equatorial climates, high temperatures and moisture lead to intense bacterial activity that rapidly oxidizes dead vegetation, resulting in very low humus content [c2]. However, while low humus contributes to poor soil quality, the primary cause of the specific accumulation of undissolved iron and aluminium mentioned in the assertion is the process of intense leaching (eluviation) of other minerals, not the bacterial destruction of humus itself [c1, t3]. Therefore, both statements are true, but the reason does not provide the correct causal explanation for the chemical accumulation described.

Sources

1. [1] NCERT. (2022). Contemporary India II: Textbook in Geography for Class X (Revised ed.). NCERT. > Chapter 1: The Rise of Nationalism in Europe > Laterite Soil > p. 11
2. [2] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Biological Activity > p. 45
3. [3] https://bhu.ac.in/Images/files/SSC-604(1).pdf