What happens to the mobility of cations as cation exchange capacity increases?

The correct answer is that the mobility of cations decreases as cation exchange capacity (CEC) increases. Cation exchange capacity refers to the total capacity of soil to hold cations, which are positively charged ions like calcium, magnesium, potassium, and sodium.

When CEC is high, the soil has a greater number of negatively charged sites available for cation adhesion. This allows the soil to hold onto cations more effectively, thereby reducing their mobility within the soil solution. Instead of readily moving through the soil, these cations are held tightly at exchange sites, which can limit their immediate availability for plant uptake and reduce overall mobility.

In contrast, with lower CEC, fewer exchange sites are available, leading to higher cation mobility as they can move more freely in the soil solution without being held tightly to these sites. Therefore, as CEC increases, the ability of the soil to retain cations strengthens, leading to a decrease in their mobility. This understanding is crucial for managing soils and ensuring that plants have access to the necessary nutrients.