AGRONOMY AND CROP PRODUCTION - Onions and other Vegetable Alliums - page 298

potential of the sensors was monitored every 3 h and, if this fell below -20 kPa,
1.5 mm of irrigation was applied. Using this system the soil water potential at
20 cm depth was held close to -20 kPa, and the cumulative water applied plus
rainfall closely matched the evapotranspiration of the crop calculated using a
modified Penman-Monteith formula (see Fig. 6.15).
An earlier trial (Shock et al. , 2000) established that a water deficit of -20
kPa at a depth of 20 cm was about optimal. A lower water stress resulted in
more bulbs rotting in store in some seasons and less efficient use of water, with
more leaching. Yields - particularly of colossal grades - decreased at greater
water stress (see Fig. 6.16a).
In north-eastern Spain, at plant densities greater than 40 plants/m 2 ,
bolting was a serious problem that was associated with the high early LAIs of
high-density, fertigated crops (Bosch-Serra and Currah, 2002). Increased yields
from drip-line at fertigated onion crops have been reported from Poland (Rumpel
et al. 2003), but no improvement of drip-fertigation over conventional sprinkler
Fig. 6.15. Soil water potentials and cumulative total water received compared with
calculated crop evapotranspiration for sub-surface, drip-fertigated bulb onions
growing in the Treasure Valley of eastern Oregon, USA. (a) Measured soil water
potentials at depths of 20 cm (solid line) and 46 cm (broken line) for onions drip-
irrigated automatically when a sensor at 20 cm depth measured a soil water
potential of -20 kPa. (b) The cumulative water applied plus rainfall (solid line) and
the calculated crop evapotranspiration, ET C , (broken line) of this irrigation regime
(from Shock et al ., 2004. Courtesy of HortScience ).