Agriculture Reference
In-Depth Information
availability of facilities for raising transplants is limited. However, in common
with many other vegetable crops having small seeds and delicate seedlings, a
high standard of husbandry is required to produce a satisfactory crop from direct
sowing. As discussed in detail in Chapter 4, onion seeds are slow to germinate
and emerge after sowing, and these rates depend on temperature and moisture.
For effective herbicidal weed control, emergence must be spread over a
short period (see Chapter 5). This requires uniform seedbed conditions as
defined by temperature, moisture availability and aeration; a uniform depth of
sowing, since the time to emergence will increase with sowing depth; and high-
quality seed with an innate capacity for rapid germination over a short spread
of time. The timing of rainfall or irrigation can strongly affect the percentage
and spread of emergence (see Fig. 4.13). Irrigation is best applied when
sufficient day-degrees have accumulated since sowing for most seeds to have
reached the stage immediately prior to radicle emergence (approximately 90
day-degrees > 1.4°C for onions) (Finch-Savage, 1990; Fig. 4.14). Irrigation is
usually essential for autumn or late summer sowing but, in many regions,
spring sowings rely on rainfall and existing soil moisture.
The importance of plant density in controlling bulb size has been described
above. Therefore, a predictable and high percentage of emergence, spread
uniformly over the field, is important in ensuring that bulbs develop to the right
size. Clearly, if the percentage emergence is spatially variable, plant density, and
hence the ultimate size of bulbs, will be variable. Furthermore, plant density
influences the maturity date of bulbs, with high densities reaching maturity
and being ready for harvest before low densities (see Chapter 4). Therefore, a
spatially variable percentage emergence will create difficulties in determining
the optimum time for harvesting, and in the timing of pre-harvest sprays of
maleic hydrazide (see Chapter 7). For all these reasons, spatially uniform
emergence with little spread in time is crucial for a successful onion crop.
Directly sown onion crops are normally drilled at a depth of about 2 cm.
The seeds are commonly drilled in rows about 30 cm apart on to beds six rows
wide. The quantity of seed required for sowing can be estimated using Eqn 6.6:
Target (plants/m
2
)/[seeds/g
Seeds (kg/ha) = 1000
(Eqn 6.6)
% laboratory germination
field factor]
There are usually 300-400 onion seeds/g and, with good-quality
commercial seed, the laboratory germination should exceed 90%. The 'field
factor' depends on seedbed conditions: for ideal conditions this is 0.9, i.e. 90% of
the viable seeds emerge; for average conditions 0.7 and, for a cold soil with poor
tilth, 0.5 (MAFF/ADAS, 1982). Various types of seed drill are suitable, and
precision drills that place seeds in the soil at regular intervals are widely used.
Trials comparing bulb onion crops carefully thinned to a uniform within-row
spacing with crops, in which the distance between plants within the row varied
randomly, showed no improvement in yield or uniformity of bulb size from a
uniform within-row spacing; both spacing arrangements had the same overall
