AbstractAmmonia (NH3) is the third most abundant N species in the atmosphere and, due to various natural and anthropogenic sources, can reach high concentrations in some areas. While some plants show effects of toxicity, others are capable of using this N‐form and grow well without any utilization of soil‐N. Acquisition of atmospheric NH3will affect the acid‐base balance of the plants as absorption and dissolution causes an alkalinisation (production of OH−) and assimilation of NH3results in an acidification (generation of H+). As there is only a limited capacity for biochemical disposal of excess H+in shoots, pH regulation may involve H+/OH−extrusion into the media via roots and transport of (in)organic ions between roots and above‐ground parts of the plant.Our aim therefore was to assess NH3acquisition byLolium perenneand to study the effects of gas phase NH3on growth, acid‐base balance and mineral composition of the plants. The experiments therefore included application of a range of14NH3to the shoots and of15N as NO3−, NH4+or NH4NO3to the roots, from which the amount of gas phase NH3acquisition could be quantified. Analysis of the mineral composition provided data for calculation of acid‐base balance as well as for water use efficiencies of the plants.The results indicate that over the range of NH3supplied, plants from all treatments could utilize gas‐phase NH3as demonstrated by increases in growth and in N and C use efficiencies. Plants receiving NO3−via their roots had a higher capacity to use gaseous NH3than those growing with NH4+. NH3assimilation in shoots reduced both the acid load with NH4+nutrition and the alkaline load with NO3−supply to the roots. The results of the experiments are discussed in relation to possible acid‐base regulation mech