Question

Followings are the statements related to soil colloids.
(A). Soil colloids are inorganic in nature.
(B). Soil colloids are of extremely small size, so that they can not be seen with an ordinary light microscope.
(C). Soil colloids exhibit a large specific surface area.
(D). Soil colloids are always positively charged.
Choose the correct answer from the options given below:

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

Hint:

To solve complex true/false combinations in competitive exams, use the elimination method:
Since soil colloids are predominantly negatively charged (ruling out statement D), any option containing (D) can be safely crossed out, leaving the correct option instantly.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Soil colloids represent the most active portion of the soil, consisting of organic and inorganic particles with extremely small diameters.
These particles govern the physical, chemical, and biological properties of soils due to their tiny dimensions and surface charge.

Step 2: Detailed Explanation:
Let us evaluate each statement individually to determine its scientific accuracy:
Statement (A) asserts that soil colloids are inorganic in nature.
This is incorrect because soil colloids consist of both inorganic (clay minerals, crystalline and non-crystalline hydrous oxides of iron and aluminum) and organic (humus) fractions.
Statement (B) states that soil colloids are of extremely small size, so that they cannot be seen with an ordinary light microscope.
This is highly accurate as colloidal particles typically have a diameter of less than $1\,\mu\text{m}$ (or $2\,\mu\text{m}$ in some classification systems), which is below the resolving power of a standard light microscope.
Statement (C) claims that soil colloids exhibit a large specific surface area.
This is also true; because of their minute size, their total external and internal surface area per unit mass is extraordinarily high, ranging from $10$ to over $800\,\text{m}^2/\text{g}$.
Statement (D) claims that soil colloids are always positively charged.
This is false because the vast majority of soil colloids (such as silicate clays and humus) carry a net negative charge under normal soil pH conditions, though some hydrous oxides can carry positive charges in highly acidic soils.
Since Statement (D) is false, any option containing (D) is automatically incorrect.
This eliminates options (A), (B), and (C), leaving option (D) as the only logically sound choice.

Step 3: Final Answer:
By systematically eliminating options containing false statements, the correct option is (D).