Nitrate (NO3) and ammonium (NHþ4) are the predominant forms of nitrogen (N) available to plants in agricultural soils. Nitrate concentrations are generally ten times higher than those of NHþ4 and this ratio is consistent across a wide range of soil types. The possible contribution of these small concentrations of NHþ4 to the overall N budget of crop plants is often overlooked. In this study the importance of this for the growth and nitrogen budget of maize (Zea mays L.) was investigated, using agriculturally relevant concentrations of NHþ4. Maize inbred line B73 was grown hydroponically for 30 d at low (0.5 mM) and sufficient (2.5 mM) levels of NO 3 . Ammonium was added at 0.05 mM and 0.25 mM to both levels of NO 3 . At low NO 3 levels, addition of NHþ4 was found to improve the growth of maize plants. This increased plant growth was accompanied by an increase in total N uptake, as well as total phosphorus, sulphur and other micronutrients in the shoot. Ammonium influx was higher than NO 3 influx for all the plants and decreased as the total N in the nutrient medium increased. This study shows that agriculturally relevant proportions of NHþ4 supplied in addition to NO3 can increase growth of maize.