Which statement about diffusion is true?

• A. Nutrients move with water
• B. Nutrients move from low concentration to high concentration
• C. Nutrients move from high concentration to low concentration
• D. Nutrients move only with mechanical mixing

Answer: (C)

Diffusion is the spread of dissolved nutrients due to the random, jostling motion of molecules, moving from areas where they are more concentrated to areas where they are less concentrated until the concentrations even out. In soil water, nutrients diffuse down their concentration gradient, so they migrate from places with lots of nutrient to places with less, even if there’s no bulk water flow. This process happens on its own without mechanical mixing, though mixing can speed things up. That’s why moving from high concentration to low concentration is the true description of diffusion, rather than moving with water, moving from low to high concentration, or requiring mechanical mixing.