Agriculture Reference
In-Depth Information
area of the Central Valley, California, grapes are grown on soils of pH 3.4-4.5.
But a high organic matter content of 10-20% may ensure that most of the
Al
3
ions mobilized at such a low pH are complexed by organic ligands, which
renders them harmless to the roots. Liming to raise the soil pH is discussed in
section 5.5.3.
Other important chemical indicators of the quality of the soil environment
are
salinity
and
sodicity
.
7.2.2
Soil Salinity
Salinity
refers to the total concentration of dissolved salts in a given volume of
soil. The major ions contributing to salinity in the soil and in water used for ir-
rigation are Ca
2
, Mg
2
, Na
, Cl
, SO
4
2
, and HCO
3
, with low concentra-
tions of K
and NO
3
. Salinity is most conveniently measured by the
electrical
conductivity
(
EC
) of the medium—bulk soil, a soil extract, or a water sample.
7.2.2.1
Water Measurements
EC
is the
specific conductance
of the water, independent of sample size, and is mea-
sured with a conductivity meter. The total concentration of dissolved salts (
TDS
),
sometimes referred to as the “salinity hazard,” is related to
EC
by the equation
TDS
(mg/L)
640
EC
(dS/m)
(7.1)
This empirical equation, developed from the analysis of many surface and ground-
waters, holds for water with an
EC
up to 10 dS/m. Note that
EC
increases with
temperature and should be corrected to a standard temperature of 25°C. In Aus-
tralia, the
EC
of irrigation water is commonly given in “EC units,” which are
S/cm (1 dS/m
1000
S/cm).
In the western United States,
EC
classes of
0.25, 0.25-0.75, 0.75-2.25,
and
2.25 dS/m at 25°C have been used to define waters of
low
,
medium
,
high
,
and
very high
salinity. The
EC
of most irrigation waters lies between 0.15 and 1.5
dS/m. In soil, however, salts from the irrigation water become more concentrated
due to evapotranspiration. With time, the soil salinity may reach levels that are
detrimental to vine growth, unless a salt balance is achieved through irrigation
management (section 7.2.2.4).
7.2.2.2
Soil Measurements
The bulk
EC
of soil depends on both the conductivity of salts in solution and the
surface charges on clays and organic matter. The surface charges are immobile and
constant (in the short term), so the concentration of dissolved salts determines
soil salinity. A standard laboratory method of measuring
EC
involves mixing a
sieved soil sample with deionized water to make a “glistening paste,” then ex-
tracting the solution by vacuum filtration. The
EC
measured on this
saturation
extract
is called the
EC
e
.
EC
is also measured in a 1:5 soil water suspension, and
converted to
EC
e
values if necessary. A table for the conversion is given in ap-
pendix 12. Soils with an
EC
e
4 dS/m are classed as
saline
.
The bulk
EC
of soil in situ is measured using instruments such as an EM38
(box 7.2).
