The effect of various forms of nitrogen fertilizer and dates of application on Ocimum amercianum yield and essential oil (ton/feddan, 1 feddan = 4200 m2) was studied in two successive seasons. In both seasons the total yields of fresh and dry herb after the application of ammonium sulfate surpassed that of ammonium nitrate and urea. Dividing the full amount of fertilizer into two equal portions gave the highest yields in most cuts. Nitrogen fertilizers affected the percentage of essential oil in most cuts. The highest oil percentage was obtained after the application of ammonium nitrate and ammonium sulfate in both cuts in the first season, respectively, and urea in the second season. Early addition of the full amount of fertilizer was more effective in increasing oil percentage as compared to other two dates of application. Ammonium sulfate in the first season and urea in the second one produced the highest total oil yields either per plant or per feddan. One application of the full amount of ammonium nitrate at the early date produced the highest total percentage of monoterpenic compounds as compared to ammonium sulfate and urea. One application the full amount of nitrogen fertilizer at the early date was superior to other treatments. The opposite was true for sesquiterpenes. Oxygenated compounds showed the same trend of monoterpenic compounds. Thus, the application of ammonium sulfate at the rate of 60 kg N/fed/season at two equal portions in each cut could be recommended for maximizing herb, leaves and oil yields of Ocimum amercianum, L. var. pilosum plants.
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