AbstractThe increase of rhizosphere pH in the course of nitrate nutrition results from H+consumption in the external medium during uptake of NO3−in a H+co‐transport and from internal OH−production during nitrate reduction. Synthesis of organic acids for NH4+assimilation as well as strong partial depolarization of membrane potential with NH4+uptake are the important reasons for rhizosphere acidification during ammonium nutrition. Despite differences in proton balance depending on N form, cytoplasmic pH changes are small due to physico‐chemical buffering, biochemical pH regulation, H+inclusion in vacuoles, and H+release into the rhizosphere.Because of the large capacity for proton excretion the plasmalemma H+ATPase of root cells plays an essential role during ammonium nutrition. An increase of the kinetic parameter Vmaxafter ammonium nutrition relative to nitrate nutrition suggests that the capacity of H+release may be adjusted to the particular requirements of ammonium nutrition. Moreover, H+ATPase is adjusted not only quantitatively but also qualitatively. The increase of the kinetic parameter kmas well as the capability of the plasmalemma vesiclesin vitroto establish a steeper pH gradient favours the supposition that H+ATPase isoforms are formed which allow H+release into the rhizosphere under conditions of low pH or poor H+buffering of the soil. In this respect species differences exist,e.g. between maize (efficient adaptation) and faba bean (poor adaptation).