Agriculture Reference
In-Depth Information
Table 11.4.
Tolerance level of some crops to salts (dS m
−1
), expressed as the expected yield
(in percentage of the maximum yield). (Source: Ayers and Westcot, 1976.)
Percentage of maximum yield
100%
90%
80%
50%
Crop
EC
w
EC
e
EC
w
EC
e
EC
w
EC
e
EC
w
EC
e
Max.
EC
e
a
Climbing bean
0.7
1.0
1.0
1.5
1.5
2.3
2.4
3.6
6.5
Broccoli
1.9
2.8
2.6
3.9
3.7
5.5
5.5
8.2
13.5
Melon
1.5
2.2
2.4
3.6
3.8
5.7
6.1
9.1
16.0
Cucumber
1.7
2.5
2.2
3.3
2.9
4.4
4.2
6.3
10.0
Potato
1.1
1.7
1.7
2.5
2.5
3.8
3.9
5.9
10.0
Lettuce
0.9
1.3
1.4
2.1
2.1
3.2
3.4
5.2
9.0
Onion
0.8
1.2
1.2
1.8
1.8
3.2
2.9
4.3
8.0
Pepper
1.0
1.5
1.5
2.2
2.2
3.3
3.4
5.1
8.5
Spinach
1.3
2.0
2.2
3.3
3.5
4.9
5.7
8.6
15.0
Strawberry
0.7
1.0
0.9
1.3
1.2
1.8
1.7
2.3
4.0
Tomato
1.7
2.5
2.3
3.5
3.4
5.0
5.0
7.6
12.5
a
Max.
EC
e
, maximum electric conductivity tolerable of the soil's saturated extract for that specific crop.
leaching that would follow the use of
the previous formula (Eqn 11.10), some
authors propose that the higher of the terms,
E
a
or (1 −
LF
) of the formula (Eqn 11.10) is
deleted, although this would result in an
incomplete control of the salts in the less
irrigated zones of the plot. The drainage of
these salts would be entrusted, after the irri-
gation campaign, to the rain or to possible
complementary irrigations (soil disinfec-
tion, pre-sowing).
have received wide use and nowadays are
common in HFLI systems. Their detailed
use is described in several papers (e.g.
Castilla and Montalvo, 1998).
In order to use the soil moisture meas-
urements for irrigation scheduling, it is nec-
essary to know the desired moisture
thresholds, expressed in water volumetric
content or in matrix potential at the rooted
depth. The upper limit of soil moisture is
close to field capacity, and the lower limit is
the 'allowable soil water depletion' (see
section 11.4.2). The lower limit indicates
when to start irrigating and the upper limit
when to stop it. The difference between the
two limits indicates the maximum amount
of water than can be supplied. Once the
limits are set, irrigation management can be
done manually or automatically.
When installing the moisture sensors
the fact that soil moisture is usually very
heterogeneous must be taken into account.
This heterogeneity necessitates placement
of the sensors in representative places, and
replicating the number of sensors at least
twice or three times, to obtain representa-
tive measurements.
For common greenhouse vegetable
crops, when a single sensor is being used, it
must be located between 10 and 20 cm deep,
10-15 cm from the base of the plant, and
Methods based on soil parameters
Soil moisture sensors can be used as a
unique method to schedule irrigation or in
combination with the water balance meth-
ods, with plant sensors or, even, as a com-
plement to an irrigation strategy based on
experience.
The evaluation of the volumetric water
content in the soil using reflectometry (TDR)
techniques is rarely used, difficult to oper-
ate and expensive.
In practice the measurement of the
water tension in the soil (which is equal
to the absolute value of matrix potential) is
the most affordable procedure. The use of
plaster blocks is not common, among other
reasons due the need for a good calibration
depending on the composition of the soil's
