Agriculture Reference
In-Depth Information
•
between 4.4 and 7.0 kg of K t
−1
of
quality (Geisenberg and Stewart, 1986).
Equally, the balance between other nutrients,
especially between calcium (when its supply
is required) and potassium, and magnesium
is necessary, as well as between the different
forms of nitrogen (nitric/ammoniacal).
In drip irrigation, the amount of salts
in the water must be limited, if possible,
to 2 g l
−1
(which is not feasible, in some
cases, when saline water is used), to
decrease possible dripper blockage prob-
lems. When using good quality water, it is
a usual practice to add sodium chloride
(common salt) to the water, up to the indi-
cated limit, to improve tomato quality,
because the soluble solids content
increases with salinity which contributes
to the improvement of its internal quality,
although the fruit size is reduced.
A good irrigation efficiency is, logically,
required for efficient fertigation and also
contributes to a significant reduction in the
environmental impact of fertilizer (nitrogen,
especially) residues.
Foliar fertilization, in tomato crops, is
usually limited to microelements, when
deficiencies are forecasted or observed.
Leaf analysis (of the limb, petiole or the
whole leaf) is a good auxiliary index on
which to base the scheduling of fertiliza-
tion, being more common than sap analy-
sis, as the latter displays a wider variability
and requires more thorough sampling
(Chapman, 1973; Van Eysinga and Snilde,
1981; Morard, 1984).
In greenhouse crops, low soil temper-
atures (15°C) in winter may limit absorp-
tion of nutrients, especially phosphorus
(Wittwer, 1969) and nitrates (Cornillon,
1977). On the other hand, high tempera-
tures favour nutrient absorption, although
the nutrient uptake per harvest unit is not
affected, as previously thought (Nisen
et al
., 1988).
harvest;
between 1.2 and 3.2 kg of Ca t
•
−1
of har-
vest; and
between 0.3 and 1.1 kg of Mg t
•
−1
of
harvest.
The differences in nutrient uptakes are
influenced by the type of pruning and,
especially, by the timing of the removal of
the axillary shoot. It is advisable to prune
shoots as soon as possible to minimize the
wasteful uptake of nutrients by the crop
(Castilla, 1985).
The scheduling of fertilizer applica-
tion must rely on the type of fertilizer
used, on the irrigation technique and on
the soil conditions, among other factors. In
sandy soils, with low water storage capac-
ity, supplies must be frequent with the
irrigation (conventional), whereas in heavy
soils it is only necessary to apply part of
the nitrogen as a top dressing (Geisenberg
and Stewart, 1986).
With surface irrigation, the most com-
mon practice is to apply the phosphorus
with the pre-planting fertilization, for
example when applying manure (around
30 t ha
−1
), and at a time when half of the
potassium is applied. The rest of the potas-
sium and nitrogen are applied in alternate
weeks after transplanting until 1 month
before the end of the cycle (Nisen
et al
.,
1988). With drip irrigation, all fertilizers
can be applied by fertigation, although it is
common that at least part of the phospho-
rus is applied with the manure.
In drip irrigation, it is essential to know
the absorption rhythm of the mineral ele-
ments in order to schedule fertilization
(Zuang, 1982). In Mediterranean unheated
greenhouse crops for autumn-spring cycles,
fertilization rates higher than 0.3 g N m
−2
day
−1
do not seem advisable (Castilla, 1985).
The fertilizer's content of the irrigation
water is, in some cases, notable and must be
taken into account for the fertilization
schedule.
Nitrogen excesses negatively affect fruit
quality, and maintaining an N:K ratio at 1:2
(or even 1:3) during the fruit enlargement
stage, with drip irrigation, favours their
11.7.7
Fertigation of soilless crops
The nutrient cycle in soilless crops
Soilless crops, with free drainage, have sim-
ilar problems to soil cultivation regarding
