Question

Which of the following is a key component of Cation Exchange Capacity of the soils?

• A. Nitrate ions
• B. Anions
• C. Exchangeable cations
• D. Soil water content

Hint:

• Cation Exchange Capacity (CEC) is the total capacity of a soil to hold exchangeable cations.
• Soil colloids (clay and organic matter) have negative charges on their surfaces.
• These negative charges attract and hold positively charged ions (cations) like Ca$^{2+}$, Mg$^{2+}$, K$^+$, Na$^+$, H$^+$, Al$^{3+}$. These are the exchangeable cations.
• CEC is a measure of soil fertility and nutrient retention.

Answer 1

The Correct Option is C

Cation Exchange Capacity (CEC) is a measure of the soil's ability to hold positively charged ions (cations). Soil particles, particularly clay minerals and organic matter, have negatively charged surfaces. These negative charges attract and hold onto cations from the soil solution. The key components related to CEC are:

• Negatively charged soil colloids: Clay particles and soil organic matter (humus) are the primary sources of negative charges in the soil.
• Exchangeable cations: These are the positively charged ions that are adsorbed onto the negatively charged sites of soil colloids. Common exchangeable cations include calcium (Ca$^{2+}$), magnesium (Mg$^{2+}$), potassium (K$^+$), sodium (Na$^+$), aluminum (Al$^{3+}$), and hydrogen (H$^+$). These cations can be "exchanged" with other cations present in the soil solution.

CEC itself is a measure of the total quantity of these exchangeable cations that a soil can hold, usually expressed in milliequivalents per 100 grams of soil (meq/100g) or centimoles of positive charge per kilogram of soil (cmol$_c$/kg). Option (a) Nitrate ions (NO$_3^-$) are anions (negatively charged) and are involved in anion exchange, not cation exchange. They are repelled by negatively charged soil surfaces. Option (b) Anions are negatively charged ions. CEC deals with cations. Option (c) Exchangeable cations are precisely what CEC refers to – the cations held on exchange sites that can be replaced by other cations. They are a key component *measured by* or *represented by* CEC. Option (d) Soil water content is important for soil processes, including nutrient availability and ion movement, but it's not a direct component defining CEC itself, although it influences the expression of CEC and ion exchange. Thus, "exchangeable cations" are intrinsically linked to the concept of CEC. \[ \boxed{\text{Exchangeable cations}} \]