Which nutrient movement process depends on concentration differences?

• A. Mass flow
• B. Capillary action
• C. Transpiration pull
• D. Diffusion

Answer: (D)

Diffusion is movement of solutes from higher concentration to lower concentration, driven by random molecular motion. This concentration difference provides the driving force that causes nutrients to spread through soil water toward areas of lower concentration, such as near roots where uptake reduces the local nutrient level. In the rhizosphere, diffusion helps supply nutrients to roots when bulk water flow is limited or when nutrients are not being carried quickly by water movement.

Other processes involve movement of water rather than just solutes. Mass flow carries dissolved nutrients with the bulk movement of water, often tied to transpiration demand and water uptake. Capillary action draws liquid up through narrow pores due to cohesion and adhesion, not primarily because of concentration differences. Transpiration pull generates tension in the xylem from leaf evaporation, pulling water upward and indirectly moving nutrients with that water. These rely on water movement and pressure or potential differences, whereas diffusion specifically relies on concentration gradients.