Which statement best defines diffusion in soils?

• A. Nutrients moving from high concentration to low concentration
• B. Nutrients moving with water
• C. Nutrients moving against gravity
• D. Nutrients moving only during wind transport

Answer: (A)

Diffusion is the spreading of a nutrient due to the random motion of molecules, moving from areas of higher concentration to lower concentration. In soils, this process occurs through the pore water and does not require bulk water movement, so nutrients gradually balance themselves along concentration gradients even when there isn’t a strong flow of water. This gradient-driven motion is what makes diffusion distinct from transport that follows water flow (advection) or from physical displacement by wind or gravity-driven processes. So, the statement that nutrients move from high concentration to low concentration best defines diffusion.