A small percentage of the nutrients absorbed by plant roots may be obtained byroot interception as the roots grow through the part of the soil where they were located. However, most of the nutrients are transported to the root by the soil water by a combination of mass flow and diffusion. Mass flow carries nutrient ions along with it as the water is absorbed by plant roots. Plant growth requires a flow of such water to replenish the water lost by transpiration. Some nutrients, however, are absorbed so actively by the roots that their concentration in the water near the roots is depleted. This establishes a concentration gradient that causes diffusion to generate a net movement of their ions toward the roots. Both mass flow and diffusion utilize water as a transport medium.

Transport of nutrients in the plant occurs by bulk flow of water through the xylem from the root to the leaf. This bulk (or mass) flow results from evaporative water loss (transpiration) from the leaf. Removal of water at one end of the “pipe” by transpiration, complemented by the forces of adhesion and cohesion at work in the xylem vessels, results in bulk water flow and transport of nutrients through the plant. This is water movement in response to the negative pressure generated by evaporative loss. Similar return flow through the phloem carries soluble carbohydrates from the leaves to the roots, thus allowing the upper part of the plant to nourish its roots.

In Detail: Bulk Water Flow in Plants
Transpiration of water from a leaf dries the surrounding tissue and makes its water potential more negative. This produces a tension that pulls water through a continuous water column (held together by the cohesion of water molecules) through the petiole, the branch, the stem, and the roots of the plant. This produces a water potential gradient from the top to the bottom of the plant and between the plant roots and the soil. Consequently, the roots absorb water from the soil and pass it through the xylem tissue up to the leaves that need it to replenish the water they have lost through transpiration. Water can also move by diffusion under the influence of a concentration gradient, the water moving in one direction while the solutes move in the other direction. Any liquid or gas tends to seek homogeneity within a system and so it flows from a region of higher concentration to a region of lower concentration. This movement can be characterized mathematically by Fick’s Law which states that the rate of diffusive flux () is directly proportional to the concentration gradient ( $LaTeX: \bigtriangledown C$ ) and inversely proportional to the path length () of diffusion. Each diffusing substance has a characteristic rate of diffusion (diffusion constant, ) in the particular medium in which it is moving that is defined by the size of its molecule and other properties. Fick’s Law: $LaTeX: F=D\left(\frac{\bigtriangledown C}{L}\right)$ Differences in solute concentration are the principle factor that determines the differences in water concentration between soil particles and plant roots or between xylem and living cells. Cells are contained by a differentially semi permeable membrane, the plasmalemma, which is permeable to water but not to solutes. Water, therefore, moves from a region or cell of lower solute concentration (higher water concentration) to a region or cell of higher solute concentration (lower water concentration). Flux of water into a cell (osmosis) increases pressure within the cell and, if the cell wall is plastic, forces the cell to expand. This is how plants grow. Cells newly divided have high osmotic concentrations and so attract water to them. As water flows into the cell, the cell expands because of the increased turgor pressure until it meets an equal pressure force or until the cell wall becomes rigid through secondary thickening by laying-down of additional cellulose. This diffusive pressure of water does work that produces growth.