Why does nitrate move easily through soil after heavy rainfall?

• A. Nitrate is negatively charged and moves with water
• B. Nitrate volatilizes
• C. Nitrate is positively charged and attracted to cations
• D. Nitrate binds strongly to clay minerals

Answer: (A)

Nitrate moves easily through soil after heavy rainfall because it is an anion (negatively charged) and does not bind strongly to the negatively charged surfaces of soil minerals and organic matter. When rain is heavy, water infiltrates and moves through the soil profile, carrying dissolved nitrate with it. Since it isn’t held by the soil particles, it leaches downward with the percolating water, leading to rapid downward movement.

Volatilization isn’t the fate of nitrate—nitrate itself isn’t a gas, and ammonia volatility involves different forms and conditions. Nitrate also doesn’t bind strongly to clay minerals or other soil components despite the presence of cations; the negative charge of nitrate causes little attraction to the common negative soil surfaces. Hence the rapid leaching observed after heavy rain.