Agriculture Reference
In-Depth Information
Table 11.8.
Chemical compatibility of the mixture of some common fertilizers in fertigation: I, incompatible;
C, compatible. (Source: Cadahía, 1998.)
NO
3
NH
4
Urea
(NH
4
)
2
SO
4
(NH
4
)
2
HPO
4
(NH
4
)H
2
PO
4
KCL
K
2
SO
4
KNO
3
Ca(NO
3
)
2
NO
3
NH
4
-
Urea
C
-
(NH
4
)
2
SO
4
C
C
-
(NH
4
)
2
HPO
4
C
C
C
-
(NH
4
)H
2
PO
4
C
C
C
C
-
KCL
C
C
C
C
C
-
K
2
SO
4
C
C
C
C
C
C
-
KNO
3
C
C
C
C
C
C
C
-
Ca(NO
3
)
2
C
C
I
I
I
C
I
C
-
Parameters of fertigation with soilless crops
Pathogens in the drainage waters
The proper management of fertigation
requires the periodic analysis of the nutrient
solution to assess its goodness of fit to the
requirements of the crop and to perform nec-
essary adjustments to its composition. In
addition, it is necessary to frequently monitor
(automated or manual) the pH and EC (elec-
trical conductivity) of the nutrient solution
and the leachate, to prevent any anomaly.
In practice, when a computer is avail-
able, a certain threshold of EC of the nutri-
ent solution is fixed, for instance 2.5 dS m
−1
,
modulated as a function of the solar radi-
ation, decreasing it by 0.1 dS m
−1
for each
30 W m
−2
of solar radiation that exceeds
400 W m
−2
(Urban, 1997b). Obviously, these
rules must adapt to the specific conditions
of each operation.
Normally, the pH is not regulated but a
certain value is fixed. The EC and pH sen-
sors must be duplicated, at least, to prevent
an eventual failure. In addition, the system
must be fitted with alarms.
Other complementary analyses are car-
ried out on the substrate solution (extracted
with a syringe) and the drainage, to correct
the nutrient solution.
Analysis of vegetable tissue and sap
provide information on the nutrients that
are really absorbed by the plants (Cadahía,
1998). The analysis of the conducting tis-
sues is usually preferable to that of the
leaves, whose composition varies slowly. In
these analyses, the time variations are more
relevant than the absolute values (Morard
et al
., 1991).
The recirculation of the drainage water
requires the use of good quality water and
its disinfection, to suppress pathogens (bac-
teria, fungi and virus) in the recirculation
water.
The use of ozone, UV sterilization, ther-
mal treatment and ultrafiltration are effec-
tive to a varying extent, but the last technique
has the drawback that only 70-80% of the
drainage water is recovered (Dasberg, 1999b).
The use of bleach in recirculation water dis-
infection gives good results. Treatment with
UV radiation is effective, but it is necessary
to pre-filter the water so the radiation pene-
trates well (Dasberg, 1999b).
Automation
The use of computers, with several degrees
of automation to manage the fertigation, is
growing among greenhouse growers. It
must be expected, in the future, that these
systems will become integrated with cli-
mate control systems, in those green-
houses in which the technological level
allows for it, for combined optimization
(see Chapter 12).
11.8
Summary
•
Water fulfils a number of basic func-
tions for plants. Transpiration may
involve up to a maximum of 98% of the
total amount of water absorbed by cul-
tivated plants in their life cycle.
