Solution culture, greenhouse and field experiments were conducted to compare the effect of NH.‐N and NO3,‐N on growth and yield of barley (Hordeum vulgareL.). In a solution culture experiment, barley grew more with NO3, than with NH4, particularly at high N concentrations. The effect was not much altered by buffering the solution pH with CaCO3. In a greenhouse experiment, with 10 soils which received 30 μg N g‐1, barley yields were greater with NO3, than with NH4, when soil pH was less than 6.0. Yield from NH4.‐N, expressed as a % of yield from NO3‐N, correlated well with soil pH (r=0.88**). In another greenhouse experiment, urea or (NH4)2‐SO4, when placed in a band with or without thiourea, remained substantially as NH4, in the soil, and subsequently plant growth was reduced. The field experiments which received N fertilizers in the spring, did not show much difference in grain yield of barley between NH4and NO, sources. In 16 field experiments on acid soils (pH 4.5 to 5.8), the average grain yield was similar with NH4NO3and with (NH4)2SO4. In 10 other field experiments on soils with pH values ranging from 6.0 to 8.4, yields were not influenced by N source or addition of nitrification inhibitors to the NH4‐based fertilizers. Because of possibility for loss of NO3‐N through denitrification and leaching, the NH4form might be preferable in the field.