PO 9. Describe how the processes of mass flow, diffusion, and root interception affect nutrient uptake.

Mass flow is the movement of dissolved nutrients into a plant as the plant absorbs water for transpiration.  The process is responsible for most transport of nitrate, sulfate, calcium and magnesium.

Diffusion is the movement of nutrients to the root surface in response to a concentration gradient.  When nutrients are found in higher concentrations in one area than another, there is a net movement to the low-concentration area so that equilibrium is reached.  Thus, a high concentration in the soil solution and a low concentration at the root cause the nutrients to move to the root surface, where they can be taken up.  This is important for the transport of phosphorus and potassium.

Root interception occurs when growth of a root causes contact with soil colloids which contain nutrients.  The root then absorbs the nutrients.  It is an important mode of transport for calcium and magnesium, but in general is a minor pathway for nutrient transfer.

The actual pathway of nutrients into the root itself may be passive (no energy required; the nutrient enters with water) or active (energy required; the nutrient is moved into the root by a "carrier" molecule or ion).