The uptake of nitrogen (N) in nitrate or ammonium (NH4+) form affects physiological and metabolic processes and toxicity may develop in plants receiving high concentrations of NH4+. The objective of the present study was to delineate the response of bell pepper plants to varying proportions of NH4+ combined with increasing concentrations of potassium (K) in the nutrient solution. Bell pepper plants were tolerant to moderate proportions of NH4+ (25% or less or 50% or less); however, higher proportions resulted in growth reduction. The application of higher K concentrations in the nutrient solution did not ameliorate the growth on vegetative plant parts; however, when K was increased to 9 mm, the yield was sustained even when 50% of total N was in the NH4+ form. Decreased shoot:root ratio and harvest index indicated that biomass accumulation was affected more in the shoot than in the root and in the fruit than in the shoot, respectively. There was a lower concentration of NH4+ in the roots compared with leaves, suggesting that the higher K concentration that resulted from the increased K in the nutrient solution was associated with NH4+ translocation through the xylem. A decrease in calcium and magnesium detected in leaves suggests an antagonistic relationship with NH4+ and K in the nutrient solution, which was correlated with the acidification of the growing medium. Higher yields when K was at 9 mm may be the result of the high photosynthetic rate and stomatal conductance (gS) detected in plants fertigated with 25% of total N as NH4+ and the higher leaf water potential when the proportion of NH4+ was 50%. The biochemical composition of fruits was affected because both high NH4+ and increased K resulted in higher ethylene production, lipid peroxidation, superoxide dismutase activity, and carotenoids.