A general theory of vertical solute movement in a soil is presented, which takes into account uptake of water and solute by roots, irrigation or rainfall, and solute application and adsorption by the soil. Irrigation, rainfall, and the surface application of fertilizers are arbitrary functions of time. The main limitation of the theory is the neglect of the variability of soil-water conductivity with position. The theory is illustrated by comparing predictions and experimental observations of solute leaching losses measured in a lysimeter.