In soybean and common bean, enhanced topsoil foraging permitted by shallow root architectures is advantageous for phosphorus acquisition from stratified soils. The importance of this phenomenon in graminaceous crops, which have different root architecture and morphology from legumes, is unclear. In this study we evaluated the importance of shallow roots for phosphorus acquisition in maize (Zea mays L.). In a field study, maize genotypes with shallower roots had greater growth in low phosphorus soil than deep-rooted genotypes. For physiological analysis, four maize genotypes differing in root shallowness in the field were grown in solid media with stratified phosphorus availability in a controlled environment. Of the four genotypes, one shallow and one deep genotype were also inoculated with arbuscular mycorrhiza (AM). Shallower genotypes had significantly greater growth and phosphorus accumulation compared with deeper genotypes at low phosphorus availability. Mycorrhizal colonisation altered root shallowness under low phosphorus conditions, increasing shallowness substantially in a deep-rooted genotype but slightly decreasing shallowness in a shallow-rooted genotype. Mycorrhizal colonisation increased phosphorus acquisition under low phosphorus availability. Respiration costs of roots and shoots of phosphorus-efficient genotypes were significantly lower under low phosphorus conditions compared with inefficient genotypes. The physiological efficiency of phosphorus acquisition, expressed as root respiration per unit of phosphorus acquisition, was greater in shallow rooted genotypes. Our results demonstrate that genetic variation for root shallowness exists in maize, that phosphorus and AM can modulate root shallowness independently, and that a shallower root system is beneficial for plant performance in maize at low phosphorus availability. We propose that root architectural traits that enhance topsoil foraging are important traits for improved phosphorus acquisition efficiency of annual grain crops such as maize in addition to legumes.