Irrigation and Fertilization - Greenhouse Technology and Management - page 213

agriculture and defines the limit at 50 mg
of NO 3 − l −1 of water. Several areas of the
Mediterranean Basin have been declared
'vulnerable to nitrates' in accordance with
this directive. The leaching of nitrates
comes both from substrate cultivation
(open systems) as well as from soil cultiva-
tion, in this last case recently documented
by Thompson et al . (2002).
Phosphorus does not usually cause pol-
lution problems, except in exceptional cases
of soils with low phosphorus fixation capac-
ity or when large quantities of animal
manure are applied over many years.
The potassium leachate is, normally,
limited and does not cause important prob-
lems of environmental impact, as it is
retained in the soil in high proportions.
Other macronutrients, such as calcium
and magnesium, do not cause environmen-
tal problems, as they are natural compo-
nents of the soil, which retains them in large
quantities.

are high, or if the irrigation water contains
it in sufficient amount, the inputs must be
consequently corrected.
As a guide, Table 11.5 summarizes the
approximate nutrient uptakes of some horti-
cultural crops.
It is important to know the nutrient
absorption dynamics, to adapt the inputs to
the extraction rates, which vary through the
cycle and are influenced by the climate con-
ditions, especially by soil temperature and
radiation.
When the availability in the soil of
some nutrients is high, overconsumption
may occur (in potassium) or it may nega-
tively affect the quality (nitrogen) of the
fruits, in extreme cases being possible to
induce salinity, or even phytotoxicity. The
availability of nutrients must be balanced
and adapted to the plant requirements, to
avoid antagonisms and possible restrictions
to the nutrient absorption, which allow for
optimum fertilization. Therefore, it is fre-
quent to maintain predetermined relations
between all or some of the nutrients.

11.7.3 Nutrients extractions

11.7.4 Tolerance to salinity

It is necessary to know the fertility charac-
teristics and nutrient levels in the soil, mak-
ing the pertinent soil analysis, to schedule
fertilization.
Normally, if the nutrient level is good,
fertilization in practice is based on supply-
ing the crop's uptake, corrected for the use
efficiency, which allows for maintaining,
after the crop cycle, a proper fertility and
nutrient level. If the levels of any nutrient

The tolerance to salinity of the crops may be
assessed in several ways. The most exten-
sively used method (Ayers and Westcot,
1976) quantifies the tolerance (Table 11.4)
by the percentage of the maximum yield that
would be obtained for a certain level of elec-
tric conductivity of the saturated extract of
the soil ( EC e ) or the irrigation water ( EC w )

Table 11.5. Approximate nutrient uptake of some horticultural crops (compiled from very diverse sources).

Plant uptake (kg ha −1 )

Crop Yield (t ha −1 ) N P 2 O 5 K 2 O CaO MgO

Tomato 80 250 80 500 300 70

Pepper 40 180 60 180 160 50

Aubergine 50 250 40 300 150 25

Melon 60 230 80 400 300 70

Cucumber 200 320 160 600 250 100

Squash 40 70 70 390 - -

Lettuce 40 100 50 250 50 12

Green bean 45 150 15 60 30 6