Considering the importance of spice plants and their shelf life, as affected by various factors, the current study considered Summer savory plants (Satureja hortensis cv. Saturn) for evaluation under the application of different concentrations of ammonium sulfate (0, 40, 60, 80, and 100 kg/ha) as primary treatments. Based on the plant response, the control group and 100 kg/ha ammonium sulfate were selected as suitable treatments for storage experiments (i.e., storage at ambient, refrigerator, and freezer temperatures for 8 months). Based on the results, the highest percentage and yield of S. hortensis essential oil and biomass occurred in response to 100 kg ammonium sulfate, whereas the lowest amounts were observed in the control group (i.e., in the absence of ammonium sulfate). During the storage period, the essential oil content decreased, but the carvacrol content of the essential oil increased. During the different durations and conditions of storage, the stability of secondary metabolites varied. Essential oil, rosmarinic acid, and carvacrol contents maintained greater stability in plants treated with ammonium sulfate (100 kg/ha), compared with the control group during the storage period. It can be concluded that the preharvest application of ammonium sulfate on S. hortensis improved plant growth and quality indices at preharvest time, while also maintaining the stability of its active ingredients at the postharvest stage and storage time. It also led results to recommend storing Summer savory in the freezer to better preserve its secondary metabolites.