506 Handbook of herbs and spices
medium available NPK usually 100 kg N, 50 kg P 2 O 5 and 50 kg K 2 O/ha are applied
for standard oil yields. The entire quantity of the phosphorus and potash along with
50 kg N/ha is applied as a basal dose at the time of planting. Remaining quantity of
N is applied in three equal parts. The first dose is applied after first weeding (5–6
weeks after planting) and second dose after 9–10 weeks of planting and third dose
after first harvest.
Working on the different species of Mentha Singh et al. (1989) reported that
herbage and oil yields of Mentha arvensis (Japanese mint), M. piperita (Peppermint)
and M. spicata (Spearmint) increased significantly with N fertilization up to 100 kg
N/ha and those of M. citrata (Bergamot mint) with up to 150 kg N/ha. Plant height,
leaf:stem ratio and leaf area index increased with N application and oil content
decreased in all the species. Economic optimum doses of N for M. arvensis, M.
piperita and M. spicata were 167, 157 and 145 kg/ha, respectively and their oil yields
expected from the response equation were 190, 103 and 50 kg/ha, respectively.
Field investigations were carried out by Randhawa et. al. (1984) to ascertain the
optimum row spacing and nitrogen requirements of M. spicata, at Punjab Agricultural
University, Ludhiana, India. The treatments consisted of all combinations of three
row to row (30, 45 and 60 cm) spacing and four nitrogen (0, 50, 100 and 150 kg/ha)
levels. The results of three years study showed that in order to get higher herb and
oil yields this crop should be spaced at 30 cm between rows and supplied with
100 kg N/ha.
31.3.4 Irrigation and drainage
The water requirement of this crop is very high (above 1000 mm/annum). During the
winter months (December–March) irrigations are generally required at 10–15 day
intervals whereas during summer months (April–June) irrigations may be applied at
an interval of 7–10 days. Waterlogging nevertheless, has to be avoided by providing
adequate drainage both for irrigation and rain water.
Ram et al. (1992) investigated the effect of irrigation on the yield and oil quality
of mints including spearmint. The result of a field experiment conducted by these
authors on spearmint var. MSS-5, and M. arvensis var. Hybrid-77 and CIMAP/MAM-
11 under five levels of irrigation (0.4, 0.6, 0.8, 1.0 and 1.2 IW:CPE ratio), revealed
that both the Mentha species, regardless of their varieties, produced maximum herb
and essential oil yields at 1.2 IW:CPE ratio. While the carvone content of spearmint
var. MSS-5 remained almost constant, the menthol content of the essential oil of the
two M. arvensis varieties considerably increased with irrigation levels up to 1.2
IW:CPE ratio during the first harvest. At the second harvest the menthol content of
both the varieties of the M. arvensis decreased with the irrigation levels. The carvone
content of MSS-5 variety of spearmint was maximum at 0.8 IW:CPE ratio.
31.3.5 Interculture and weed control
Like all other mints, the fields of spearmint are also affected by weed infestation and
competition. The weeds, if not controlled in time can even cause a 60–80% reduction
in yields. The critical period of weed interference in spearmint is found to be between
30–50 days after planting and 15–30 days after first harvest. Usually 2–3 manual
weedings are needed to keep the weed growth under check. The weed menace can be
minimized by resorting to suitable rotation involving crops like paddy. When paddy