EFFECTS OF ORGANIC FERTILIZERS ON GROWTH AND YIELD ...

spearmint oil. Marotti and co-authors (1994) used different levels of nitrogen and phosphorus in
peppermint trials. They reported that mineral fertilization seemed to increase the menthol content
of essential oil compared with untreated plants. Hornok (1974), in an experiment with
peppermint fertilization, stated that the most effect on yield was achieved first by the level of
nitrogen and second by the level of calcium. Phosphorus fertilization had no effect on yield.
However, a high quality of peppermint oil in any region requires the optimum use of fertilizer
and water to maintain herbage growth and delay maturity as long as possible so that the herbage
may be harvested with a minimum of flowers (Murray et al. 1988)
2.5 Harvesting time
Amount and composition of essential oil is strongly dependent on developmental stage of the
plant (ontogeny), and therefore harvesting time is one of the most important factors influencing
mint oil quality.
Harvesting a crop early or late resulted in a low yield of leaves as well as the essential oil content
because at an earlier or later stage of harvesting, the crop was immature or over mature resulting
in a poor yield of herb and oil content (Shah & Gupta 1989). According to Chalchat and coauthors
(1997), harvesting should be carried out after flowering to obtain oil that contains a high
amount of menthol. Ulseth (1994) found that the poorest oil quality was obtained from mints that
were in a stage of early blooming or were not yet blooming at all. Oil content in stems was very
poor and present only in younger parts (Croteau & Gershenon 1994). According to Clark and
Menary (1984), besides the timing of harvest, the numbers of harvests per year greatly influence
yield, and composition of oil. The essential oil from the first harvest was richer in menthol than