Saline and Sodic Soils

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Saline and Sodic Soils Chapter 10
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This one.
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Percent yield on y-axis and increasing level of
salinity on x-axis.
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OK, you can have natural salinity, especially
where salts may concentrate, like in a seep area,
but you can also salinize a soil with long-term
irrigation. This has ruined many an acre, around
the world. There are also sodium-affected soils
and a combination.
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Say you add a bit of salt with an irrigation.
Next time you irrigate, add more water than just
enough to wet the root zone and water percolates
deeper, dissolving and leaching the salt from
the previous irrigation. Of course, to make
this design work, you need good internal drainage.
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For salinity For sodicity
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EC increase with concentration of electrolyte,
right? Pure water is a poor conductor of
electricity. The units of measurement are dS /
m. Work at the USDA Salinity Lab in California
showed that conductivity is (on a log-log basis)
related to ionic strength, and the latter is
something of a measure of electrolyte
concentration.
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This is an older measure of sodicity that has
been replace in soil testing labs by an easier
but indirect measure (sodium adsorption ration,
next slide). Note the similarity of ESP with BS.
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The above threshold may seem surprising but the
basis is that even a 15 saturation of the CEC
with Na means that there is little Ca- or MgCO3
in the soil. Since both Ca2 and Mg2 are
strongly preferred to Na adsorption, if
there was appreciable Ca- or MgCO3 present, the
ESP would be lower. Further, in the absence of
solid phase Ca- or MgCO3, the pH is not
controlled by carbonate equilibria (keeping it
about 8.5) so that it increases to plant-bad
values.
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The basis of this is something called the Gapon
model, which is empirical, NaEx / (CaEx MgEx)
k Na / (Ca2 Mg2)1/2 The left hand
side is close to ESP, NaEx / (CaEx MgEx NaEx
) NaEx / CEC
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lt 4.0 lt 13 lt 4.0 13 4.0 13
gt
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Sodic Saline and Sodic
Not Affected Saline
SAR 13 ESP 15
EC 4
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Salt deposited on surface of soil when water
evaporates white.
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8.5
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Na is a dispersing cation. It is not tightly
held to negatively charged colloids (figure left
is supposed to show 2 parallel colloidal
particles, one pair with tightly held Ca2 and
one With loosely held Na, off-setting the
particle negative charge. Notice that for the
Ca2, the repulsive negative charges are shielded
by a concentrated Ca2 zone but not so for
the diffuse Na zone. So, Ca2 is flocculating,
Na dispersing.
However, if the concentration of cation charge is
sufficiently high, even with Na (figure to
right), the negative charge of adjacent particles
is effectively shielded so that particles can
closely approach, i.e., are flocculated. But
decrease that high concentration and they
disperse.
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No, colloids dispersed and hydraulic
conductivity too small.
No, here too. See previous slide.
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2Na Ca2 ? Ca2 2Na
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Notice, also, that a solid phase of CaCO3 forms,
thereby dropping pH to 8.5. So, there is a use
for gypsum besides drywall.