A novel approach to the sustainable management of potassium ( K) resources in agro-ecosystems is through better exploitation of genetic differences in the K efficiency of crop plants. Potassium efficiency is a measure of genotypic tolerance to soils with low potassium availability and can be quantified as the K efficiency ratio ( the ratio of growth at deficient and adequate K supply). This study investigated the magnitude of variation in K efficiency among wheat ( Triticum aestivum L.) genotypes grown in a glasshouse and in the field. Genotypes differed significantly in response to low soil K availability in terms of shoot biomass during the vegetative growth phase and grain yield at maturity under glasshouse ( 144 genotypes) and field ( 89 genotypes) conditions. K-efficient and K-inefficient genotypes were identified. The main factor determining K efficiency for grain yield was the capacity of genotypes to maintain a high harvest index ( grain yield/total shoot weight) at deficient K supply. Genotypes that had reduced harvest index under deficient K supply were K-inefficient. Capacity to tolerate low concentrations of K in shoot tissue where K supply was deficient was also important in determining K efficiency for grain yield. Potassium-efficient genotypes have the potential to enhance the productivity and sustainability of cereal cropping systems.