NPK fertilizer and yields of peppermint, Mentha x piperita

Field experiments conducted in the experimental gardens of the Higher Institute of Agriculture, Plovdiv, Bulgaria, studied the effect of nitrogen, phosphorus, and potassium on leaf (from the first swath) and essential oil (from the second swath) of three Bulgarian cultivars (Tundja, Zefir, and Clone 1) of peppermint (Mentha X piperita L.). The fertilization treatment constituted of four levels of nitrogen, two levels of phosphorus, and 3 levels of potassium. Fertilization was based on preliminary analyses on available soil nutrients, and nitrogen, phosphorus, and potassium uptake by peppermint plants. An attempt was made to achieve calculated levels dry leaf and essential oil yields, with special reference to sustainable peppermint production. Plant height and weight of all three cultivars increased with increased levels of fertilization, while the ratio of leaves to stems within plants was not altered. Yields of fresh material from the first swath were increased by 18 to 79 percent as compared with the nonfertilized control. The most responsive cultivar was Clone 1. Yields of essential oil from the second swath were increased by 23 to 86 percent as compared with the control. Of the three cultivars, highest essential oil yields were obtained from cv. Zefir (due to the higher essential oil content), while the lowest oil yields were obtained from Clone 1. Fertilizer application increased the menthol content in the essential oil in cv. Zefir and cv. Tundja. The content of the other constituents was not significantly altered by the tested fertilizer rates.