Certified Crop Advisor Practice Exam

Cations are held on the cation exchange capacity (CEC) by what type of charges?

1. Positive charges

2. Neutral charges

3. Negative charges

4. Variable charges

The correct answer is: Negative charges

Cations are positively charged ions that are held on the cation exchange sites in soil due to the negative charges present on soil particles, particularly in clay and organic matter. This negative charge is crucial because it attracts and retains cations, allowing them to be available for plant uptake. The cation exchange capacity (CEC) is a measurement of how well soil can hold cations. Soils with a higher CEC typically have more negative charge sites where cations can bind. This variety of negatively charged surfaces allows for the retention of cations even when there are fluctuations in moisture or nutrient supply in the soil solution. Neutral charges do not attract or hold cations since they lack the ability to exert an electrostatic pull on the positively charged ions. Variable charges can occur in some soil minerals, but they mainly refer to changes in surface charge under different pH conditions and are not the primary mechanism for cation retention. Thus, the consistency of the negative charges provides a stable and reliable means for cations to be held in the soil, which is fundamental for plant nutrition and soil fertility.