Urea is 85% of the recycled nitrogen in a life support system. Urea is quickly converted to NH4+ but nitrification to NO3− is difficult. Supplying NH4+ directly to plants eliminates the need for a nitrifying bioreactor. Most plant physiology textbooks indicate that NH4+ is toxic to plants, but we now know that this may not be true if pH is rigorously controlled. However, the long-term effects of high NH4+/ NO3− uptake ratios are poorly understood. In four studies, two cultivars of wheat were grown to maturity with NH4+/ NO3− ratios from 0 to 0.85 in recirculating hydroponic solution. In the third and fourth studies, NH4+ was supplied as (NH4)2SO4, NH4CI, or both. Contrary to conventional wisdom, there was no beneficial effect of supplying 25% of the N as NH4+ compared to a nitrate control. The high NH4+ treatment (85% NH4+) reduced seed yield by 20% in the first two studies, but yield was not reduced in the third and fourth studies. Increasing calcium and potassium supply in the nutrient solution appears to be critical to ameliorating the detrimental effects of NH4+. Seed protein concentration was increased from 17 to 22% at the highest NH4+ level. These studies indicate that it may be possible to eliminate the need to recycle N as NO3− in regenerative life support systems.