Question

The SI unit for cation exchange capacity of soil is

• A. \(\text{dSm}^{-1}\)
• B. \(\text{Joules } ^\circ\text{C}^{-1}\)
• C. \(\text{Mg kg}^{-1}\)
• D. \(\text{Cmol kg}^{-1}\)

Hint:

CEC SI unit: $\text{cmol}(p^+)\text{/kg}$ (or $\text{cmol kg}^{-1}$). $1\text{ meq/100g} = 1\text{ cmol/kg}$.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Cation Exchange Capacity (CEC) quantifies the total capacity of a soil to hold exchangeable cations on its negatively charged colloidal surfaces.
Key Formula or Approach:
\[ 1\text{ meq/100g} = 1\text{ cmol}(p^+)\text{/kg} = 1\text{ cmol kg}^{-1} \]

Step 2: Detailed Explanation:
In classical soil science, CEC was expressed in milliequivalents per 100 grams of soil ($\text{meq/100g}$).
Under the modern International System of Units (SI standard):
CEC is expressed as centimoles of positive charge per kilogram of dry soil ($\text{cmol}(p^+)\text{ kg}^{-1}$ or simply \(\text{Cmol kg}^{-1}\)).
Numerically, \(1\text{ meq/100g} = 1\text{ cmol kg}^{-1}\).

Step 3: Final Answer:
Therefore, the SI unit for cation exchange capacity of soil is \(\text{Cmol kg}^{-1}\), matching option (D).