Mass flow involves large amounts of water flowing to roots as the plant transpires water. Diffusion is the dominant transport mechanism for phosphorus and potassium. For a nutrient to cross a cell membrane into the cell, each nutrient ion must be attached to some carrier . The carrier-nutrient complex can pass through the membrane or in some other unknown manner move the ion into the cell. Electrical balance also seems to be involved in ion absorption and accumulation.
Plants also absorb nutrients through small openings in leaves, the stomata.
Direct absorption through the leaves seldom exceeds a few pounds per acre per application. The small concentrations of micronutrients needed can be satisfactorily added to a foliar spray. Chapter 9, pages 261-264.