Agriculture Reference
In-Depth Information
The field capacity and permanent wilt-
ing point concepts are idealized. The best
way to define them, although not perfect, is
by the water tension in the soil (e.g. 0.33
atmospheres for the FC and 15 atmospheres
for the PWP).
percentage of readily available water (RAW).
In tomato, for instance, for an open field
well-irrigated crop the allowable soil water
depletion has been established between 30
and 50% of the readily available water
(Castilla, 1995).
Readily available water (RAW)
11.4.3
Measurement of the soil
water content
The readily available water is the difference
between the water content at field capacity
and at permanent wilting point.
Introduction
RAW = FC - PWP (11.1)
Tables 11.1 and 11.2 show the FC, PWP and
RAW values for different soils.
When considering substrates other than
soil, due to their special characteristics FC
and PWP are not used. The RAW is found
under very limited ranges of matrix tension
(see Chapter 10).
The soil water content can be measured by
direct procedures, taking soil samples and
drying them in a stove to calculate their
gravimetric moisture (in weight), but this is
a time-consuming process that does not
allow for continuous data monitoring.
If the gravimetric moisture has to be
transformed into volumetric moisture, the
apparent density of the soil is required.
Several indirect procedures allow for
the evaluation of the volumetric moisture
in the soil. Among them, we can highlight
the neutron probe method (nowadays in
disuse) and time domain reflectometry
(TDR) and frequency domain reflectometry
(FDR), which, to some extent, will be dis-
cussed later on. A full description of all
available methods can be found in Raviv
and Lieth (2008).
The continuous monitoring of the
matrix potential is carried out, normally, by
tensiometers, although electric resistance
sensors can also be used. Monitoring the
osmotic potential is commonly practised by
measuring the EC of the soil or substrate
solution. A practical review of their use in
greenhouses can be found in Thompson and
Gallardo (2003).
Allowable soil water depletion
Although plants can extract available water
retained in the soil or substrate, as the avail-
able water decreases the plant has difficul-
ties in extracting all the required water and
starts suffering water stress. This threshold
moisture value is the allowable soil water
depletion and varies depending on the
soil conditions, evaporative demand and
crop development stage. The allowable soil
water depletion is usually represented as a
Table 11.2.
Readily available water (RAW, in
mm m
−1
), as a function of the soil matrix tension
for different soils.
Soil matrix tension
(atmospheres)
Soil texture
0.2
0.5
2.5
16
Description of soil moisture sensors
Clay
180
150
80
0
Soil moisture sensors measure the volumet-
ric water content of the soil or matrix poten-
tial. The matrix potential is very close to the
total water potential, if the soil is not saline.
Silty-clay
190
170
100
0
Loam
200
150
70
0
Silt-loam
250
190
50
0
Silty-clay-loam
160
120
70
0
Sandy-clay-loam
140
110
60
0
Sandy-loam
130
80
30
0
m a t r i x
p o t e n t i a l
s e n s o r s
Tensiometers.
Tensiometers are cheap, sim-
ple, easy to use, and require minimum
Fine sand-silt
140
110
50
0
Fine sand-medium
60
30
20
0
