Water Quality
effects associated with sodium
affected soils, are counteracted
by soil salinity. For purposes of the
DSS, soil infiltrability, operating
at the soil surface, and hydraulic
conductivity, operating within the
bulk of the soil, were selected as
indicators of the effect sodium
and salt content of irrigation
water, has on soil permeability
and other physical properties.
Even low sodium concentrations
at the soil surface can lead to crust
formation and reduced infiltrability
under rainfall or low salinity irriga-
tion applications. The implication
is that the infiltrability of irrigated
soils can be negatively impacted
by rainfall or overhead irrigation
with low salinity water during the
period of incomplete vegetative
cover when rain or overhead
irrigation droplets hit soil surface
particles and form surface crusts.
In line with the philosophy of using
conservative assumptions for Tier
1 evaluations, it is assumed that
low salinity rainfall will determine
the effect of soil surface sodicity
on soil infiltrability. For Tier 2
evaluations, the EC of the actual
water application (rain or irriga-
tion water) and the degree of
crop cover (which protects the soil
surface from crust forming drop-
lets) are used to calculate a worst
seasonal value. based on a calculated monthly
tolerable load.
Soluble salts move, redistribute
and accumulate in soils largely as
a result of water movement. The
distribution of sodium and salts
within a soil profile is thus largely
determined by soil and water
management practices which
affect water distribution within a
soil. The concentration of sodium
and salt within the different soil
layers are calculated to derive a
qualitative measure of the degree
to which the hydraulic conductivity
of sensitive soils would be affected
by the combined effects of soil
water salinity and soil sodicity. While some trace elements are
essential plant nutrients at low
concentrations, at high concen-
trations most of them become
either toxic to crop growth or to
humans or animals consuming
the produce grown on such trace
element enriched soils. Since
trace elements tend to accumulate
in soil, there is practically no safe
level for sustainable irrigation
on a continuous basis. The DSS
calculates the time it would take
for a trace element to accumu-
late to a protective accumulation
level in soil, as indicator of the
acceptability of trace element
concentrations in irrigation water.
Accumulation up to the threshold
load within a period of 200 years
or longer is considered ideal, while
periods of less than 100 years are
considered unacceptable.
While a high soil organic matter
content in soil, and by implica-
tion organic carbon in irrigation
water, is considered as advanta-
geous, excessive concentrations
of organic carbon in irrigation
water can give rise to the devel-
opment of anoxic conditions,
plant root stress and unpleasant
odours. It is therefore important
to ensure that short term overload
does not occur. The criteria used
to indicate the likelihood of unde-
sirable consequences linked to
the Chemical Oxygen Demand
(COD) of irrigation water, are thus
Suitability indicators of
crop yield and quality
The effect water constituents have
on crop yield is mostly indirect, in
that crop yield is affected by the
constituent concentration in the
soil. Six suitability indicators are
used in this regard, namely the
EC, B, Cl and Na concentrations
in the root zone, as well as nutrient
and pesticide contents. Three suit-
ability indicators are directed at
quantifying the effects on crop
quality, namely leaf scorching by
Na and Cl, microbial contamina-
tion of crops and nutrient effects.
The presence of salts within the
root zone reduces crop growth
by reducing the ability of plant
roots to absorb water from soil.
It is convention to measure and
express the root zone salinity of soil
as ECe (Electrical Conductivity of
a saturated soil extract). In addi-
tion to EC, crop growth is also
affected by the accumulation of
B, Cl and Na in the root
zone. The effects of the latter
three constituents are deemed
to be of a toxic nature, since
their effect on yield reduction is
more pronounced than would be
expected from the equivalent EC
concentration in the root zone.
A large body of data is avail-
able that links yield response
of different crops to soil EC and
concentration of B, Cl and Na in
the root zone. This body of data is
used by the DSS as crop specific
parameters to assess the effect
of EC, B, Cl and Na in the root
zone on crop yield.
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Local contact: Maskam Water, +27 21 981 6546 | [email protected] | maskamwater.com