Agriculture Reference
In-Depth Information
used. For greenhouse crops the tolerance to
salinity can be quantified (Sonneveld, 1988)
by means of the irrigation water salinity
threshold below which there is no problem,
and the percentage of yield decrease experi-
enced by the crop per unit increase of
salinity in the irrigation water, above the
threshold value. This method is more useful
for substrate crops. Table 11.6 summarizes
the data in this respect. The specific growing
conditions (cultivar, evaporative demand,
management, microclimate) may affect
these threshold values (Cohen, 2003). In
Mediterranean greenhouses, Magán (2003)
estimated the salinity threshold value of the
nutrient solution to decrease the fresh weight
tomato harvest between 4 and 5 dS m
−1
.
The importance of the water quality to
minimize the leaching fraction is enormous,
influencing its environmental impact poten-
tial. Poor quality water will require consid-
erable leaching and, as a consequence, will
generate more negative impact than good
quality water.
The control centre of a localized irriga-
tion facility must have the necessary equip-
ment to fertigate. This involves the use of
soluble or liquid fertilizers, allowing for
adjustable dosing and fractioning of the
inputs which optimizes their use.
Criteria for fertigation
Traditionally, the fertigation criterion of
supplying the nutrients as a function of the
expected uptake by the plants prevailed
(section 11.7.3).
Nowadays, the criterion of providing
nutrients based on an ionically balanced
physiological solution, used in soilless
crops, is extending to conventional soil cul-
tivation, when a suitable automated irriga-
tion head is available.
In soilless cultivation the correction of
the nutrient solution is performed based on
its analysis. In soil cultivation, the classic
method of analysing the saturated soil
extract is being replaced by the use of suc-
tion probes, with which a sample of the soil
solution is extracted for analysis. However,
information on the ideal nutrient levels to
use with this method is still scarce.
11.7.5
Fertigation
In HFLI, the restriction of root development,
to only a part of the soil colonized by the
roots, dictates the need to locate the nutri-
ents in this soil to the volume occupied by
the roots to ease their absorption. Therefore,
the nutrients, normally dissolved in the irri-
gation water, are supplied by a localized
irrigation system. This practice of joint
application of irrigation and fertilization is
known as fertigation.
a soil-grown tomato crop
Depending on each case's specific condi-
tions (soil fertility, climate, irrigation type),
there is notable variation in tomato fertiliza-
tion (Castilla, 1995). Preliminary analysis of
the soil is necessary.
In general, fertilizers are applied
depending on the crop's estimated nutrient
uptake. Although the variability in nutrient
uptake is enormous (Zuang, 1982; Castilla
et al
., 1990a), values that refer to harvest
unit are in general lower, as estimated by
different authors (Ward, 1964; Maher, 1976;
Bar-Yosef
et al
., 1980; Zuang, 1982; Castilla
and Fereres, 1990):
Table 11.6.
Tolerance of some vegetables to salinity
in greenhouses. Threshold value of the irrigation
water
EC
w
(dS m
−1
at 25°C) below which there is no
problem and percentage of yield decrease per unit
increase of
EC
w
(Sonneveld, 1988).
Threshold value
EC
w
(dS m
−1
)
Yield decrease
by salinity (%)
Tomato
1.8
9
Pepper
0.5
17
•
between 2.1 and 3.8 kg of N t
−1
of
Cucumber
1.5
15
harvest;
between 0.3 and 0.7 kg of P t
Green bean
0.5
20
•
−1
of
Lettuce
0.6
5
harvest;
