Why does nitrate not bind strongly to soil?

• A. Both nitrate and soil particles are negatively charged.
• B. Nitrate is negatively charged while soil is neutral.
• C. Soil particles are positively charged.
• D. Nitrate binds to clay minerals.

Answer: (A)

Nitrate doesn’t bind strongly to soil because both the nitrate ion and most soil surfaces carry negative charges. Nitrate is an anion (NO3−), and the common soil surfaces—clay minerals and soil organic matter—are negatively charged. That like-charge situation leads to electrostatic repulsion, so nitrate is not held by the solid phase and stays in the soil water. This makes nitrate relatively mobile and prone to leaching with percolating water, especially in soils with low cation exchange capacity where positively charged ions are not strongly retained. While soils do have sites that hold cations (the cation exchange capacity), nitrate is not one of them, so strong binding doesn’t occur.