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Abstract
Fertilizer inputs to overcome nutritional constraints only give maximum efficiency when nutrient losses are avoided and all other constraints are overcome so that maximum yields are grown. Examples are given of fertilizer efficiency being increased in experiments by control of diseases and pests which damage leaves and roots, and by the provision of extra water. Examples are given of three‐factor tests where biological and physical constraints interacted with crop nutrition. Progress to high fertilizer efficiency must depend on the identification of all constraints in a production system. Inputs to overcome these constraints must then be tested in multidisciplinary experiments which achieve the maximum potential yields. The results of these experiments, applied to models of the production system, will provide technological advice to farmers and its application will also depend on social and economic factors.