High planting density is essential to increasing maize grain yield.  However, single plants suffer from insufficient light under high planting density.  Ammonium (NH4+) assimilation consumes less energy converted from radiation than nitrate (NO₃^–).  It is hypothesized that a mixed NO₃^–/NH₄⁺supply is more important to improving plant growth and population productivity under high vs. low planting density.  Maize plants were grown under hydroponic conditions at two planting densities (low density: 208 plants m^–2 and high density: 667 plants m–2) and three nitrogen forms (nitrate only, 75/25NO₃^–/NH₄⁺ and ammonium only).  A significant interaction effect was found between planting density and N form on plant biomass.  Compared to nitrate only, 75/25NO₃^–/NH₄⁺ increased per-plant biomass by 44% under low density, but by 81% under high density.  Treatment with 75/25NO₃^–/NH₄⁺ increased plant ATP, photosynthetic rate, and carbon amount per plant by 31, 7, and 44% under low density, respectively, but by 51, 23, and 95% under high density.  Accordingly, carbon level per plant under 75/25NO₃^–/NH₄⁺ was improved, which increased leaf area, specific leaf weight and total root length, especially for high planting density, increased by 57, 17 and 63%, respectively.  Furthermore, under low density, 75/25NO₃^–/NH₄⁺ increased nitrogen uptake rate, while under high density, 75/25NO₃^–/NH₄⁺ increased nitrogen, phosphorus, copper and iron uptake rates.  By increasing energy use efficiency, an optimum NO₃^–/NH₄⁺ ratio can improve plant growth and nutrient uptake efficiency, especially under high planting density.  In summary, an appropriate supply of NH₄⁺ in addition to nitrate can greatly improve plant growth and promote population productivity of maize under high planting density, and therefore a mixed N form is recommended for high-yielding maize management in the field.