Agriculture Reference
In-Depth Information
Box 2.4 The Sodium Adsorption Ratio of Irrigation Water
Ions are charged forms of the elements that are released when compounds are
dissolved in water. The salts of the elements Na, Ca, and Mg in irrigation water
are almost completely ionized to produce the cations Na
+
, Ca
2+
, and Mg
2+
that are
electrically balanced by the anions Cl
−
, SO
4
2−
, and HCO
3
−
. he
SAR
of the water is
defined as
+
[
Na
]
(B2.4.1)
SAR
=
+
+
2
+
2
Ca
Mg
2
The ion concentrations in this equation, indicated by the square brackets, are
measured in millimoles of charge
1
per liter. Through a process of ion exchange,
the
SAR
determines the proportion of each cation on clay surfaces, which in turn
affects the way that the clay behaves in water (see box 4.3, chapter 4). In soil, the
critical factors are the proportion of Na
+
relative to Ca
2+
plus Mg
2+
and the total
concentration of dissolved salts. The proportion of exchangeable Na
+
relative to
Ca
2+
, Mg
2+
, and K
+
is usually expressed as the exchangeable sodium percentage.
With prolonged use of irrigation water of
SAR
≥6, the soil exchangeable sodium
percentage can rise to a value >6 and the soil becomes “sodic,” a condition that
predisposes to structural problems. The management of sodic soils is discussed
under “How to Improve Soil Drainage” in chapter 4.
1
Moles of charge are explained in box 3.4, chapter 3.
high
TDS
of reclaimed water (1000-1800 mg/L) can create problems after regular
use unless the soil is leached by rain or a substantial volume of low-salt irrigation
water is applied during winter to remove the excess salts. For example, salts seem to
be accumulating in some McLaren Vale vineyards that have been using reclaimed
water for a number of years (Arbuckle, 2013). One option to minimize salt build-up
is to blend or “shandy” reclaimed water with water of low
TDS
. Given current con-
cern about industrial wastewater polluting supplies of potable water, large wineries
have been installing their own treatment plants to treat their wastewater and pro-
duce reclaimed water for possible use in the vineyard or for irrigating woodlots and
pasture (see “Winery Waste and the Soil,” chapter 5).
Soil Organisms
Some pest and disease organisms of grapevines live for part or all of their life cycle
in the soil, for example, the root aphid phylloxera and parasitic nematodes. The
history of a site is especially relevant in indicating what tests should be carried out.
