AGRONOMY AND CROP PRODUCTION - Onions and other Vegetable Alliums - page 295

Fig. 6.13. The effects of water stress caused by the depletion of soil water and/or
soil solution salinity on the water stress coefficient, K S , which quantifies the stress-
caused decrease in crop evapotranspiration (see Eqns 6.11, 6.14, 6.15 and 6.16).
(NB: this graph and the equations (6.15 and 6.16) on which it is based are only
approximate and are unlikely to hold when salinity and water depletion from the
soil exceed levels needed to reduce yields by 50%) (based on Allen et al ., 1998,
Fig. 44, with data from their Table 23 and the effects of soil type on available water
given in their Example 36.)
decrease in yield per unit relative deficit in E TC , during the bulbing phase is 1.5
(see Fig. 6.14b; Duranbos and Kassan, 1979; Al-Jamal et al. , 2000). Over the
whole growing season, K Y for onions is 1.1 (Duranbos and Kassan, 1979).
The larger yield response factor for the bulbing period reflects the fact that
yields are particularly sensitive to water stress at this stage (Bosch-Serra and
Currah, 2002).
WATER STRESS CAUSED BY SALINITY The tolerances of crops to salts in the soil have
been characterized in terms of the electrical conductivity of the solution from
soil saturation extracts, EC e , which first reduces yield below its full potential
(EC ethreshold ) and as the slope (b) of the reduction in crop yield with increase in
salinity beyond this threshold (Allen et al. , 1998). Onions have an EC ethreshold of
1.2 dS/m (deciSiemens/m) at 25°C and a 16% reduction in yield per 1 dS/m
increase in conductivity beyond the threshold, making them, next to beans, the
most sensitive common field crop in this respect (Allen et al. , 1998, Table 23).