Leek and shallot 373
the pseudostems have a higher length to breadth ratio. Also the degree of
blanching increases with density, especially for plants from the centre of beds (Brewster,
1994).
22.3.8 Planting
Leek seeds are generally sown directly into fields at rates of 10–15 seeds per 30 cm
row. Emerging seedlings are thinned to 10 cm apart. Transplanting of leeks is also
done and is often necessary for obtaining mid-summer through to autumn harvests.
Leeks are also sown directly or transplanted into trenches 15 cm wide and 15 cm
deep. As the leek plant grows, the trench is filled in. This results in the formation of
long white stems, a desirable characteristic for marketing leeks. The deeper the leeks
are trenched or hilled, the longer the tender white portion of the leaf stem becomes
(Decoteau, 2000).
The crop can be established more cheaply than transplanting by direct sowing into
beds in the spring. The viability and vigour of leek seed is highly variable and high-
quality seed is important for direct sowing. Besides being cheaper, direct sowing
tends to result in crops with less dirt in the leaf axils and with fewer bent pseudostems,
but the length of blanched sheath tends to be shorter than transplanted crops, and
direct-sown crops are more prone to bulbiness. In Bulgaria, leeks are grown from
transplants, sown mid-March and mid-April, and also by direct seeding at 8 kg seed/
ha without thinning. Direct seeding results in a higher total yield and is considerably
cheaper. For both methods the earlier sowing date produced considerably higher
yields and larger plants (Milanov, 1972).
22.3.9 Manuring and fertilization
Because the leek is larger than the onion, its requirements for manure and fertilizer
are higher. A crop of 30 t/ha removes 100 kg of nitrogen, 60 kg of P 2 O 5 , and 130 kg
of potash from the soil. The diameter and length of bulbs are increased by nitrogen
fertilization (McCollum, 1976). Kaniszewski (1986) reported the highest yield of
leeks with a preplanting application of 200 kg of N/ha under irrigated and non-
irrigated conditions in dry years. In wet years, split application of 600 kg of N
recorded maximum yields. Randy Baker (1991) stated that leeks require about 200–
250 kg N (nitrogen) per hectare, preferably in three instalments – one-third pre-plant
incorporated, one-third as a side dressing, and one-third as a top dressing when the
leaves are dry. Phosphate requirements of leeks are not very substantial and applications
of 50–100 kg P 2 O 5 per hectare are adequate. Potash requirements are also low and
150–200 kg K 2 O per hectare as sulfate of potash are adequate.
22.3.10 Irrigation and mulching
Uninterrupted growth is required for quality leeks and irrigation is often necessary in
areas where moisture stress occurs. Randy Baker (1991) reported that, depending on
weather conditions, a post-planting irrigation is desirable to ensure rapid establishment.
Further irrigation will be necessary if rainfall is deficient during the hot summer days
when rapid growth should take place. In Belgium two-year trials with leek cultivars
Proka and Catalina, sown in December and harvested in July, grown either in the
open or under polyethylene or PVC tunnels 4 m wide and 1.7 m high or 8 m wide and