Agriculture Reference
In-Depth Information
species, range between 10 and 30°C, with
daytime values higher than the night values.
It is preferable (for economic reasons) to
maintain lower temperatures at night, not
only because the largest greenhouse energy
losses happen at night (around 75%), but
also because the lower temperature will also
reduce respiration losses (Hanan, 1998).
In crops such as tomato, a night tem-
perature around 15°C limits the losses by
respiration and may be considered optimal.
However, when the night temperature is
much lower than 15°C, as the case may be
when the greenhouse is located in low alti-
tude subtropical latitudes, it becomes a lim-
iting factor (Jensen and Malter, 1995).
The optimal temperatures usually
decrease with the age of the plant, being
higher during germination and the first stages
of development. Different parts of the plant
might have different temperature optima: for
example the tomato plant growing point (at
the top) would benefit from a higher temper-
ature whereas the fruit (below) would rather
be at a lower temperature. When the avail-
able radiation or the air CO
2
content increase,
the optimal temperature, from a photosyn-
thetic point of view, also increases. Other
physiological processes may have different
optimal temperature values, such as, for
instance, the distribution of assimilates.
When temperatures are lower than opti-
mal, normally, the quality of the product
decreases, which may occur in winter in
unheated greenhouses.
The capacity of most horticultural spe-
cies to integrate the temperature on 24 h
periods, or longer, within a range of 10-25°C,
means that if the average temperature of the
period is maintained (24 h) the growth
won't change (Hanan, 1998), which allows
for flexible heating management to reduce
costs.
The concept of the thermal integral,
applied to longer periods, allows for the
prediction of the crop's phenology with the
aim of scheduling the harvest (Mauromicale
et al
., 1988). The thermal integral is based
on the hypothesis that the lower the tem-
perature the slower the growth rate and
development of the plants will be (see
Appendix 1 section A.2.1).
Within the range of horticultural
species, we may distinguish three types
of thermal requirements: (i) low demand,
such as for lettuce, strawberry, endive,
carnation, whose day/night thermal levels
are around 10-25°C during the day/7-10°C
during the night; (ii) medium demand, such
as for tomato, beans, pepper, aubergine,
courgette, with day/night thermal levels
around 16-30°C during the day/13-18°C
during the night; and (iii) high demand,
which require values of 20-35°C during
the day/18-24°C during the night, such as
cucurbitaceous crops (melon, watermelon,
cucumber) and some ornamentals.
3.4.3
Soil temperature
Close to the surface, the soil temperature
follows a very similar pattern of develop-
ment to the air temperature (i.e. sinusoidal
shape and slightly delayed in relation to the
air; Plate 8). The extreme values are buff-
ered with the depth of the soil.
The type of irrigation system used
influences the soil temperature; on the one
hand by the water temperature itself and
on the other hand by its effect on water
evaporation from the soil and plants, and,
therefore, the energy balance (Berninger,
1989).
The soil, as well as the substrate in soilless
crops, or the pots in ornamentals, are the
heat sink of the greenhouse, that is they are
centres of thermal inertia. The crop has lit-
tle importance with regard to thermal iner-
tia compared with the soil. A 10 cm soil
layer has five to eight times more thermal
capacity than the mass of a normal crop
(Berninger, 1989).
During the night the soil returns part of
the energy it has stored during the day back
to the greenhouse. Use of white mulch, used
to reflect radiation, limits the daytime heat-
ing of the soil and, thus, reduces the ther-
mal inertia of the soil.
