AGRONOMY AND CROP PRODUCTION - Onions and other Vegetable Alliums - page 300

irrigation was reported for sweet 'Vidalia' onions in Georgia, USA, in terms of
either yield or reduced bulb rot (Diaz-Perez et al. , 2004; Gitaitis et al. , 2004).
On a sandy-loam soil in Colorado, USA, yields from drip-lines at 8 cm
depth in the centre of 40 cm-wide four-row beds were lower than for sprinkler-
irrigated onions, unless large amounts of water in excess of ET C were used to
wet the soil profile to the full width of the beds. In the absence of this excess
water, soil at the root plate of the outer rows of onions was too dry for
adventitious root development (Al-Jamal et al. , 2001). It seems that the soil
texture, with its associated water conductivity, is a factor in determining the
success of drip-lines and their appropriate depth and spacing.
The results of experimental studies of onion crop responses to irrigation
are variable. On the one hand it is clear that in many arid zones where the crop
is grown, e.g. New Mexico, USA, virtually all the water needed is supplied by
irrigation (Corgan and Kedar, 1990). In other regions rainfall is sometimes
adequate and yields may actually be decreased by irrigation - for example, if
nitrogen is leached from the root zone (Brewster, 1990b).
Crop evapotranspiration, ET c , calculated using modified Penman-Monteith
equations (see Fundamentals of irrigation, above) has proved a reliable basis for
scheduling irrigations. Studies have generally shown increases in yield with
increases in irrigation applied of up to 100% of ET c (Kruse et al ., 1987; Shock et
al ., 2000; Al-Jamal et al ., 2001) or even 120% of ET c (see Fig. 6.16b; Martin de
Santa Olalla et al ., 1994). Excessive irrigation will result in leaching and nitrate
loss, and may promote rotting disease in storage (see Fig. 6.16a).
Water productivities in various irrigation studies are shown in Table 6.5.
The experiments in Table 6.5 were carried out in fairly arid climates, mostly at
a fairly low plant density, so the soil surface evaporation component of ET c (see
Fundamentals of irrigation, above) would be quite large, since much of the soil
would not be covered by leaves even at full crop development. In general, it
appears that drip irrigation is more efficient in water use than sprinkler or
furrow irrigation and that furrow irrigation measurements on farm fields were
well below experimental plot values. Furrow irrigation is likely to have the
greatest proportion of wet soil unshaded by leaves and the buried drip-lines are
likely to have the least, and this is probably the major cause of the differences in
water productivity. The high water productivity achieved by Bosch-Serra
probably results from the high crop density, resulting in a high proportion of
transpiration to evaporation in the overall ET c .
Irrigation and adequate N during leaf and early bulb growth ensures that
an onion crop reaches a high LAI as quickly as possible. This will tend to
promote rapid bulb ripening and good bulb quality for storage (see Fig. 4.33).
Irrigation and N applications late in bulbing may delay maturity and reduce
bulb quality by virtue of skin splitting and rotting, particularly if they follow a
period in which growth has been restricted by a lack of water or N. A cessation
of irrigation some 2 weeks before harvest has frequently been recommended as
promoting bulb ripening and ease of harvest (Brewster, 1990b).