Diseases and Management of Crops under Protected Cultivation

Water Requirement (lpd/plant)
(Diseases and Management of Crops under Protected Cultivation)
Figure 3: Water requirement of Cucumber under open field and shadenet
0.8
0.7
Open 35% 50% 75%
0.6
0.5
0.4
0.3
0.2
0.1
0
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88
Time (Days)
Figure 4: Water requirement of Summer Squash under open field and shadenet
4.3 Drip irrigation frequency
In conventional irrigation, soil water depletion must be maintained below certain thresholds
(available soil water depletion) in order to avoid crop transpiration reductions, that can induce yield
decrease. A proper irrigation frequency will avoid excessive depletion. In drip systems, good
management will always be based on very high irrigation frequency, even several times each day
(Villele, 1984), specially when using saline water, being the water storage role of the soil
unimportant relative to conventional irrigation methods.
Different plant and soil parameters have been suggested to schedule the irrigation
frequency. A wide range of plant based measurements more or less sophisticated (sap flow, stem
diameter, water potential, plant temperature...) have been suggested to detect stress, using the
plant as a biosensor. The leaf water potential method, reliable when used in conventional irrigation
systems, is not practical in drip irrigated vegetable crops. Plant temperature based methods of
water stress detection are more accurate in greenhouse than in open field (Stanghellini, 1993) but
they must be developed and locally adapted. The crop water stress index, based on the higher
temperature of the crop when suffering from water stress, has been suggested as a more reliable
method, but it is not easy to use. The spatial variability of soil-water contents in drip-irrigated soils
limits the interest of methods based on soil water content measurements ("available soil water
depletion"). The soil water matrix potential measurement, in drip irrigated. soils, using tensiometers
is a reliable way for monitoring soil water conditions in the wetted zone, in order to fix the irrigation
frequency and to confirm the adequacy of the applied water amount. Two tensiometers, at least,
should be placed in each observation point, installed at two depths, a few cm away from the
emitter, depending on the soil water distribution and rooting patterns. Other methods for
monitoring soil water potential, as gypsum blocks, need a good calibration depending on the
composition of the soil solution and have not spreaded.
Wetting the soil profile with preplanting conventional irrigations (for salt leaching, soil
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