Agriculture Reference
In-Depth Information
large-sized particles), between 10 and
30 kPa (for soil with medium texture, i.e.
medium-sized particles) and between 20
and 40 kPa (for fine textured soil), measured
in the maximum root density zone. These
values are for guidance, and must be
adjusted depending on each case's specific
conditions.
Normally, during the autumn and the
winter in the Mediterranean coast the
matrix tension must be maintained
between 20 and 40 cb, because very low
values (10-20 cb) can generate problems
of root asphyxia; when the evaporative
demand increases, in spring and summer,
it is advisable to maintain it between 15
and 30 cb.
Good tensiometer management, besides
fixing the irrigation frequency, allows the
quantities of applied water that are used to
be checked thus avoiding unnecessary
leaching and wastage of water.
11.5.1
Introduction
In herbaceous plants the water constitutes
normally more than 80% of their fresh
weight.
11.5.2
Characterization of the water
in the plant
The water content and availability in the
plant can be characterized by direct or indi-
rect methods. Among the direct indicators
are the
relative water content
, which quan-
tifies the water content of a plant tissue, in
relation to its maximum possible value, and
the measurement of the
plant water poten-
tial
. Neither of these direct methods is of
practical application.
Among the indirect indicators of the
plant water status, besides visual symptoms
(leaf rolling, colour changes, wilting) that
only appear under severe water stress con-
ditions, we can include: (i) the stomatal
conductance; (ii) changes in stem diameter;
(iii) the sap flux; and (iv) the plant tempera-
ture (Gallardo and Thompson, 2003a).
In response to dehydration of the
phloem caused by transpiration and later
rehydratation, the stems and trunks of
plants experience contractions and dila-
tions in 24 h cycles. Quantifying these is a
good indicator of the plant's water status
(Huguet
et al
., 1992).
The sap flux that ascends through the
stem (due to transpiration) is another indi-
cator of the water status of the plant.
When the plant's water status is good,
with a normal transpiration that cools the
plant as water evaporates, the plant's tem-
perature is usually lower than the surround-
ing air. If water stress occurs, transpiration
is limited by stomatal closure increasing the
temperature (Jackson, 1982); this can be
measured using an infrared thermometer.
The 'crop water stress index' (CWSI) has
been proposed as an indicator of the water
status of the plant (Idso
et al
., 1981) (see
Appendix 1 section A.8.1).
Use of moisture sensors in substrates
In order to measure the moisture content of
substrates very sensitive tensiometers are
used, whose measuring limits are, normally,
between 1 and 10 kPa of matrix tension and
with special ceramic capsules as the 'lap-
tometers' (Terés, 2000), which give a very
fast reading. They must be installed so that
there is good contact between substrate and
sensor, which is difficult in porous sub-
strates, locating them at the right depth,
given the variability of water content in the
substrates.
11.4.4
Quality of the irrigation water
In greenhouse crops the assessment of the
irrigation water quality must not be limited
to the conventional parameters (see Appendix
1 section A.8), but must include evaluation
of: (i) the solid elements content (if drip irri-
gation is used); and (ii) its temperature (par-
ticularly where significant volumes of water
are applied (i.e. surface irrigation) in
unheated greenhouses during the cold sea-
son, to avoid a thermal shock due to low
water temperature).
