Nitrogen is the mineral element that most often limits plant productivity in natural and managed ecosystems. Plants acquire most of their nitrogen through root absorption of ammonium and nitrate from the soil solution. Competition among plants and soil microorganisms for this mineral nitrogen is often intense, and to compete successfully, a plant root must be in the right place, at the right time, and with the right equipment. This entails that root growth and development respond directly to ammonium and nitrate. We have examined these responses with ion-selective microelectrodes, a root extensiometer, and metabolomic analysis of root sections. The rhizosphere near the elongation zone of the root was more acidic than other regions, independent of nitrogen source. Lower pH promoted root elasticity, but did not influence the rate of root extension. Root growth accelerated in response to increasing rhizosphere ammonium and nitrate levels. This derived in part from the accumulation of nitrate as an osmoticum and from the improved nitrogen status of the tissue. As nitrogen levels in the rhizosphere increased to surfeit, root growth abated perhaps because of ensuing toxicity or osmotic stress. Rapid adjustments in root growth and development as a function of ammonium and nitrate availability enhanced root performance in the heterogeneous environment of the rhizosphere.