Agriculture Reference
In-Depth Information
Box 7.2
Field Measurements of Soil Salinity
The EM38 instrument works on the principle of electromagnetic induction. A
transmitter in the instrument generates “loops” of electrical current in the soil, the
magnitude of which is directly proportional to the soil
EC
in the vicinity of the
loop. Secondary electromagnetic fields generated by the current flows are
intercepted by a receiver, and the summed signal is transformed into a voltage that
is proportional to the depth-weighted
EC
soil
. The effective depth of measurement is
approximately 1-2 m, depending on whether the EM38 is placed horizontally or
vertically, respectively. An example of an EM38 placed in a vertical position on the
soil is shown in figure B7.2.1. The EM38 reading is influenced by soil texture and
water content, so the instrument should be calibrated against
EC
e
measurements
(Rhoades and Miyamoto 1990). A depth-weighted value of
EC
e
, calculated from
the
EC
soil
for the EM38 in a horizontal position, can be used as a single index of
soil salinity to a 1-m depth.
Figure B7.2.1
An EM38 salinity
sensor. Photograph by
the author.
Grapevine Tolerance of Salinity
The main effect of salinity on vines is through the uptake of Na
or Cl
ions.
Not only can these ions adversely affect growth, but high concentrations in the
grapes are also detrimental to wine quality. The Office International de la Vigne
et Vin (OIV) sets 60 mg /L as the upper limit for Na
in wine. Grapevines are
moderately sensitive to salt as shown by the data in table 7.3. More specifically,
Na and Cl concentrations in the soil's saturation extract should be
3 cmols
charge/L (corresponding to
690 and
1050 mg/L of Na and Cl, respectively).
High concentrations of B are sometimes associated with saline soils—ideally, the
B concentration in the saturation extract should be
1 mg/L, and certainly not
3 mg/L.
As indicated in table 7.3, some rootstocks, especially those bred from
V.
berlandieri
, are more tolerant of salinity than
V. vinifera
itself. In Australia and
South Africa, the rootstocks 140-Ru (Ruggeri), 1103P (Paulsen), and 99-R
(Richter), which have a
V. berlandieri
parent, have shown moderate to very good
salt tolerance. Also, in the Australian irrigated areas, the rootstock Ramsey is par-
ticularly effective at excluding Na from the berries of grafted scions. Toxicity in
own-rooted vines is likely at leaf concentrations
0.4% Na and
0.7% Cl.
7.2.2.3
