Plants rely on their roots to acquire the water and mineral elements necessary for their survival in nature, and their yield and nutritional quality in agriculture. White et al. examine how the roots of land plants evolved, describe how the ecology of roots and their rhizospheres affects the utilization of soil resources, and discuss the influence of plant roots on biogeochemical cycles. They then describe the roles of roots in overcoming the constraints to crop production imposed by hostile or infertile soils, illustrate root phenotypes that improve the acquisition of soil resources, and discuss high-throughput methods to screen for these traits in the laboratory, glasshouse and field. Finally, they consider how adaptations to root systems might enable sustainable agriculture in the future.