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)

The main idea here is how soils store and supply nutrients. Cation exchange capacity (CEC) is the soil’s ability to hold positively charged ions on exchange sites that live on clay minerals and organic matter. Soils with higher CEC can trap and retain more nutrients, acting as a larger reservoir and buffering changes in soil chemistry.

Base saturation is about what portion of those exchange sites are occupied by the basic, plant-available cations—calcium, magnesium, and potassium. A higher base saturation means more of the exchange sites carry nutrients that plants can use, and it often corresponds to better soil structure and higher pH buffering.

These concepts matter for fertility management because they guide lime and gypsum use and fertilizer planning. Liming raises pH and increases base saturation by supplying calcium (and sometimes magnesium), which reduces aluminum toxicity in acidic soils and improves nutrient availability. Gypsum can supply calcium and sulfur without dramatically changing pH. Knowing the soil’s CEC helps estimate how much fertilizer the soil can hold and release, and base saturation helps indicate which nutrients are already present on exchange sites and which might be limiting. In short, CEC tells you how much can be held; base saturation tells you how much of that holding capacity is already filled with essential nutrients, guiding both pH adjustment and nutrient additions.