Agriculture Reference
In-Depth Information
are started by sowing into small beds of well-manured soil followed by regular
watering. Dry grass mulches are often used to protect the soil surface from
damage by heavy rain, to conserve moisture and to suppress weeds (Currah
and Proctor, 1990).
Premature bulbing of transplants must be prevented. Seedlings that have
started to bulb at transplanting can continue to develop small bulbs, cease to
grow and enter bulb dormancy prematurely. This occurs where conditions in
the seedbed favour bulbing. In temperate regions, late-transplanted crops may
experience bulb-inducing photoperiods while still in the seedbed as the days
lengthen in spring. In the tropics, seeds sown in summer to provide transplants
for early-autumn transplanting are likely to experience high temperatures and
photoperiods that induce bulbing. In both situations the high planting density
used in seedbeds will increase the tendency to bulb (see Fig. 4.33). In Norway,
this problem is prevented in late-transplanted seedlings by decreasing the
photoperiod to 12 h during the last half of the transplant-raising period by the
use of blackout screens.
To ease handling during transplanting, shoots plus roots are frequently
trimmed to a length of 10-15 cm, but experiments have shown that this reduces
yields slightly. Transplants are normally planted with the base of the seedling
about 2.5 cm below the soil surface. Experiments with the large-bulbed, 'Sweet
Spanish' types showed that deeper planting, at 5 or 10 cm, made the bulbs more
elongated in shape and reduced the degree of internal doubling. This made the
crop more suitable for producing unsplit 'onion rings' when sliced across. The
same effect could be achieved by ridging soil around the onions as they grew
(Chipman and Thorpe, 1977).
Semi-automatic transplanting machines exist, but yields are lowered if
seedlings are not planted upright. Transplanted crops are frequently grown at
rather wide spacings, e.g. 20-40 plants/m
2
, using a within-row distance of
about 10 cm, either because they are to produce large bulbs, e.g. the 'Sweet
Spanish' types of North America, or because the layout of ridges and furrows
for irrigation or limitations of soil fertility make closer spacing unsuitable.
MULTI
-
SEEDED MODULE TRANSPLANTING
Experiments in Norway (Vik, 1974)
showed that satisfactory bulb crops were produced when groups of three to
seven seedlings were raised in small pots and transplanted as a cluster in April.
During bulbing, the plants pushed each other apart and the resulting bulbs
were not misshapen. Plastic trays containing 9-15 ml of loosely filled compost
are widely used for raising vegetable seedlings, and are suitable for raising
groups of five to six onion seedlings for transplanting. In the UK, the yield of
bulbs > 40 mm in diameter reaches a maximum of 45-55 t/ha when modules
containing five to six seedlings are transplanted at 10 modules/m
2
(Hiron,
1983). In these conditions, 60-70% of the bulbs are > 60 mm in diameter.
Using more seedlings per module, or planting modules at a higher density,
reduces mean bulb size. Clearly, using such multi-plant modules reduces the
