The experimental use of the relative addition rate as the driving variable for plant nutrition and growth is reviewed with special attention to the theoretical background and the technical and methodological problems. In this technique the culture solution is not a “nutrient solution” in the classical sense, i.e. a solution with a specified initial concentration of nutrient salts. Instead the solution is a carrier of repeatedly added nutrients to the roots. The nutrients are added quantitatively for a specified growth rate and period of time, to permit a steady state relative uptake rate. High accuracy in the control of nutrition and growth has been obtained in solution culture experiments in which the relative addition rate was applied as the treatment variable, when all necessary nutrients were accounted for, when the additions were made very often, and when the culture solution was in darkness and in good physical contact with the roots.