Agriculture Reference
In-Depth Information
transplanting effort needed to achieve a particular plant population. In
addition to loose-filled trays, 'peat blocks' - cuboid blocks of compressed peat -
of various sizes are satisfactory for raising such transplants. Blocks formed to
a 27 mm cube by compressing 50 ml of peat-based compost have proved
adequate, and little benefit has been found from larger peat blocks.
For loose-filled trays, various compost media may be used, provided attention
is paid to nutrient requirements. A mixture of finely milled sphagnum peat mixed
with 10% by volume of sand containing ground magnesium carbonate, calcium
carbonate, calcium phosphate, potassium nitrate and fritted trace elements has
been developed (Hiron and Symmons, 1985). Because of the small volume of
compost, extra potassium nitrate liquid feed is needed weekly from the first true
leaf stage onwards. Alternatively, small granules of a slow-release fertilizer can be
incorporated in the compost. Peat blocks contain more reserves of nutrients, and
therefore less frequent liquid feeding is satisfactory.
Seedlings at the 1-2 true leaf stage, satisfactory for planting, are produced
in about 200 day-degrees > 6°C from sowing. A sequence of temperatures
starting with germination at 18-21°C, progressing down to 10°C then
followed by 2 weeks of hardening at ambient temperatures can produce such
seedlings 6 weeks after a mid-February sowing in the UK. If soil or weather
conditions are not suitable for transplanting, modular transplants can be
stored for up to 2 weeks at 0.5°C with 95% RH in the dark, without loss of
viability. Before transplanting it is beneficial to give such modules a thorough
watering with liquid feed. Irrigation after transplanting is desirable, although
not always essential. Dried-out peat blocks can harden and entrap the seedling
roots. To avoid this, the tops of the blocks must be covered by soil after planting.
Fertilizer and post-emergence herbicidal requirements are similar to those
for a directly seeded bulb crop, but herbicidal treatments immediately after
transplanting should be avoided to allow the leaves to 'harden' and develop a
normal, waxy coat. The variable costs incurred in establishing a crop in this
way in the UK have been calculated as being about 3.5 times greater than for a
directly sown crop (Senior, 1982). Higher yields, better quality (e.g. larger
mean bulb size), earlier harvesting and more reliable results may make the
extra cost worthwhile in areas where the growing season for bulb onions is
short.
Sets
Sets are small bulbs, typically weighing 2-3 g when fresh, produced by
growing a crop from seed sown at the very high density of 1000-2000
plants/m
2
. They are < 25 mm diameter and are planted to develop into larger
bulbs. Because of their size, sets make a more robust plant at emergence than
seeds, and do not need such meticulous seedbed preparation. They are
therefore easier to grow successfully and are widely used by amateur and
small-scale growers. Sets also have a shorter growing season than plants from
seeds and are used where rapid, early-season production is important.
