Which statement correctly describes cation exchange capacity (CEC) and base saturation, and why they matter in fertility management?

Which statement correctly describes cation exchange capacity (CEC) and base saturation, and why they matter in fertility management?

• A. CEC is the soil's ability to hold positively charged ions; base saturation is the proportion of exchange sites occupied by basic cations (Ca2+, Mg2+, K+). They matter for nutrient holding and lime or gypsum adjustments.
• B. CEC measures soil pH; base saturation measures moisture; they determine irrigation scheduling.
• C. CEC and base saturation are only important for sandy soils; they have no effect on fertilizer rates.
• D. CEC is the soil's ability to bind organic matter; base saturation is the ratio of acids to bases; they determine composting rates.

Answer: (A)

At the heart of the idea is how soil holds and supplies nutrients. Cation exchange capacity (CEC) is the soil’s ability to hold positively charged nutrients on negatively charged sites on clay minerals and organic matter. Base saturation is the share of those exchange sites that are occupied by basic cations—primarily calcium, magnesium, and potassium (and sometimes sodium). This matters because it determines how much nutrient the soil can hold and how readily nutrients are released to plant roots, as well as how the soil will respond to liming or gypsum.

If a soil has a higher CEC, it can store more nutrients and buffer against short-term nutrient losses, but the actual availability depends on base saturation. When base saturation is low, more exchange sites are taken by acidic ions like hydrogen and aluminum, which can reduce nutrient availability and, in acidic soils, raise the risk of aluminum toxicity. Liming raises soil pH and increases base saturation by displacing H+ and Al3+ with Ca2+ and Mg2+, while gypsum can supply Ca (and sulfate) to improve base saturation without necessarily raising pH.

So the statement that CEC is the soil’s capacity to hold positively charged ions and base saturation is the proportion of exchange sites occupied by basic cations, with practical relevance to nutrient holding and lime or gypsum adjustments, best captures how these properties influence fertility management. Other options misstate what CEC and base saturation measure (pH, moisture, or binding of organic matter) or imply limits (e.g., only for sandy soils), which aren’t accurate.