Small amounts of ammonium (NH $ _4^+ $ ) can increase growth of maize ( Zea mays )

Nitrate (NO $ _3^ - $ ) and ammonium (NH $ _4^+ $ ) are the predominant forms of nitrogen (N) available to plants in agricultural soils. Nitrate concentrations are generally ten times higher than those of NH $ _4^+ $ and this ratio is consistent across a wide range of soil types. The possible contribution of these small concentrations of NH $ _4^+ $ 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 NH $ _4^+ $ . 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 NH $ _4^+ $ 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 NH $ _4^+ $ supplied in addition to NO $ _3^ - $ can increase growth of maize.