Agriculture Reference
In-Depth Information
rotting and overheating. In moister climates the umbels are dried in bins under
a warm air draught but, to avoid damage, the temperature of the draught
should not exceed 32°C until the seed moisture content is less than 18%, 38°C
until less than 10% or 43°C when below 10%.
When the umbels can be crumbled in the hand they are ready for mechanical
threshing and cleaning. Seed should not be too dry at threshing otherwise the
seedcoats are liable to be cracked, and this results in lowered germination. The
trash can be separated from the cleaned seed by sinking the onion seeds in water
and floating off the trash. After this, seed must be rapidly re-dried by centrifuging
and drying under a forced draught to less than 12% moisture.
The principles underlying seed storage in order to maintain viability for a
long period are described in Chapter 4. Low moisture content, low storage
temperature and a high initial viability result in a long storage life. At high
temperature and humidity onion seed loses viability faster than most vegetable
seeds, and this is a serious problem in tropical areas. Commercially, seed is dried
to about 6.3% moisture content and sealed into moisture-proof cans or foil
packets. In these conditions it can remain fully viable for at least 3 years, even
at warm temperatures, and can therefore be safely marketed to tropical areas.
For long-term seed preservation in gene banks it is recommended that seeds are
stored at 5% moisture content and
18°C, and are regenerated when the
seedlot viability declines below 85% (Astley, 1990).
GARLIC PRODUCTION
The agronomy of garlic has been reviewed by Brewster and Rabinowitch
(1990), Burba (1993) and Messiaen et al. (1993). Garlic is most commonly
grown as an overwintered crop using strains locally adapted to produce high-
quality bulbs following barn storage at ambient temperatures. Bulbs are
separated into cloves just before planting. Cloves used for planting vary in
weight from 1 to 9 g, with 4-5 g being common. Plant size and final bulb weight
tend to increase with clove weight at planting. Cloves are planted with their
bases 2-3 cm deep, usually at a density of around 40 plants/m 2 when grown
with irrigation. The optimum plant density for producing the large bulbs that
obtain premium prices depends on the potential yield. For rainfed crops that
yield about 15 t/ha at the highest densities tested, a plant density of 14-18
plants/m 2 was found optimal for good-quality bulbs (Castillo et al. , 1996).
However, bulb yields in fertigation trials in Mexico approached 40 t/ha at high
plant densities and, in these conditions, 30-42 plants/m 2 was economically
optimal (Castellanos et al. , 2004). Higher final yields may occur at higher plant
densities, but bulbs may become too small. In California densities of 60/m 2 are
used to produce high-yielding processing crops, when bulb size is unimportant.
Well-drained soils should be used, since cloddy soils create difficulties with
mechanical harvesting. Phosphate fertilizer is frequently applied as a band
 
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