Agriculture Reference
In-Depth Information
In general though, sets should be 1-2 cm in diameter; those < 16 mm in
diameter rarely flower. Larger sets can bolt, particularly if exposed to long
periods at cool temperatures (7-12°C) before they start to bulb. Large sets also
have a greater tendency to produce split, double bulbs.
Set-planting densities of 30-80/m
2
are typical, with the exact density
depending on the bulb size desired. Ideally, sets should be planted upright and
only just covered with soil. When planted by machine, the sets are dropped into
shallow furrows and are not uniformly upright. Yield reductions of 10-30%
have been attributed to non-upright planting when machine-planting has
been compared with hand-planting. Firming of soil over the planting furrows
with a press wheel following the planting of sets can improve the emergence
and yield. Possibly, improved capillary movement of water to the base of the sets
in firmed soil promotes sprouting.
Post-establishment culture
Once an onion crop has been planted, has emerged and is satisfactorily
established, the grower should try to promote a rapid but even rate of growth.
With spring-sown crops at high latitudes the length of the growing season is
limited, and any delays in growth may reduce yields or delay bulbing until
photoperiods and temperatures cease to be conducive to its completion,
resulting in non-ripening, thick-necked plants (see Fig. 6.11). In autumn-sown
crops, checks to growth act like delays in sowing date and result in smaller
bulbs and lower yields. This is because Leaf Area Indices (LAIs) will be lower at
the time when inductive photoperiods cause leaf growth to cease because of
bulbing. Uneven growth rates late in development can lead to split skins on
bulbs that resume growth after the hard, dry outer skins have already started
to form. Factors partially within the grower's control include weed competition
(see Chapter 5), water supply and nitrogen nutrition.
Irrigation requirements
Several features of the root systems of edible alliums are important in
conditioning their response to both soil water and nutrients (de Melo, 2003):
they have a shallow rooting depth, with nearly all roots within 40 cm of the
surface, and individual roots are comparatively thick and sparsely branched
(see Figs 2.15 and 2.16). As seedlings develop, adventitious roots arising from
the stem base supersede the primary root and give rise to most of the root
system. The shallow, sparse root system means that a relatively small volume of
soil water is available before evapotranspiration rates become limited by soil
water supply. Among the commonly grown crops only a few other vegetables -
namely, celery, spinach, lettuce and radish - are as limited as onions and garlic
in this respect (see Allen
et al.
, 1998, Table 22). To keep these crops growing at
