Chapter 9

SALT- AFFECTED SOILS

I. ORIGIN AND DISTRIBUTION OF SALTS.

Salt-affected soils often occur on irrigated lands, especially in arid and semiarid regions,
where annual precipitation is insufficient to meet the evaporation needs of plants. As a
result, salts are not leached from the soil, but accumulated.

A. Mineral weathering without leaching.

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During the weathering process, both physical and chemical, salts were released from
rocks and minerals of the earth’s crust.

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In humid areas, soluble salts are carried down through the soil profile by percolating
rainwater and ultimately are transported to sea.
In arid regions, leaching is very limited. Therefore, salts tend to accumulate.

B. Dissolution of Fossil salts.

Ground water or drainage water from irrigation system can dissolve appreciable amount
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of saline deposit, and the resultant water can be very high in salt content.

C. Atmospheric deposition.

Rainfall may contain as high as 50 to 200 mg/liter of salts near the sea coast. This amount
decreases rapidly, however as rain moves island. The rain near the seacoast is high in Na,
Cl- and Mg, whereas island precipitation is dominated by Ca- and Mg-SO4. Over several
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thousand years, this atmospheric deposition becomes significant.

D. Salt movement with water.

If downward movement of water is not a limiting factor then salt will move downward to
the wetting front. However if plants take up most of water, then salt will accumulate near
the surface and becomes detrimental to plants.

E. Upward movement with capillary water.

Water from groundwater tables within a few meters of the surface can move up by
capillary action to the soil surface, where it evaporates and leaves behind its salts. The
amount of salt accumulations can be very large in the top 15cm of soil.

II. IRRIGATION WATER QUALITY.

A. Salt in the soil tends to match that of irrigation water.

When you apply irrigation water over a period of time, salt content of that irrigated land
tends to be similar to the salt content of the water. As you can see in fig. 11.3 the upper

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profile of the soil has essentially the same salt content as the irrigation water.

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1. Amount of water effects.

Furthermore, when the leaching fraction of water, which is the portion of water passing
through the root zone, increases then the depth of the soil profile where the salt content is
relatively low also increases. (see the upper portion of fig. 11.3)
 2. Evapotranspiration effect.
For the same season, evaporation and transpiration have a great effect on salt
accumulation. The greater the evapotranspiration, the smaller the leaching fraction of
water and consequently, the higher the salt content of the soil solution.

B. Total dissolved solids (TDS) -- early method.

In the early days, the salinity of irrigation waters as expressed in terms of total dissolved
solids (TDS). The TDS was determined evaporating a known volume of water to dryness

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and weighing the residue.
Most irrigation waters contain less than 1000 ppm of TDS, and ground water usually

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contains higher TDS than surface water.

C. ELECTRICAL CONDUCTIVITY – Recent method.

At present, electrical conductivity (EC) is used as a measure of salinity of irrigation

water.
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Ohm meter

Electrode

Solution (irrigation water)

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To determine the EC, the solution is placed between 2 electrodes of constant geometry
and constant distance of separation. When a constant electrical potential (voltage) is
applied, the amount of current produced is a linear function of the concentration of total
dissolved salt (or electrolytes) in the solution.

At a constant potential the current is inversely proportional to the solution’s resistance or

directly proportional to the solution’s conductance. The unit of conductance is 1/Ohm or
mho (Ω-1).
The measured conductance is the resultant of the solution’s salt concentration and the
electrode geometry. The effects of electrode geometry are incorporated into a term called
cell constant. The cell constant is commonly obtained by calibration with KCl solutions
of known concentration.

Specific conductivity = measured conductivity x cell constant

EX: Cell constant = 2.0 cm-1
Measured conductance = 0.5 mΩ-1 (mS)

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Then the specific conductivity = 0.5 x 2 = 1 mΩ-1cm-1
= 1dSm-1

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The basic unit of EC is mho/cm, which is too large for most natural waters, therefore
mΩ-1cm-1 is often used. In SI units, the unit of conductivity is siemens/meter. Sometimes,
µΩ-1cm-1 is also used, especially in old literature.
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D. RELATIONSHIP AMONG INDICATORS OF WATER QUALITY.

1. EC to salt concentration. There are several empirical relationships for

converting one type of water quality measurement to another. For solutions in the EC
range from 0.1 to 5 dS m-1.
Sum of cations (mmolc/L) = EC (dS m-1) x 10
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Ionic strength (I) = 0.013 x EC (dS m-1)

And TDS (mg/liter or ppm) = EC (dS m-1) x 640

Example: an irrigation water containing

3 mmolc/L Ca2+
2 mmolc/L Mg2+
3 mmolc/L Na2+
(
8 mmolc/L total cations) ===> EC ≈ 0.8 dSm-1
And TDS ≈ 510 mg/l.
 2. Hardness: the conc. of Ca2+, Mg2+, expressed as CaCO3 equivalent in
ppm (mg/L).

Ex. 40 ppm Ca2+ = 100 ppm CaCO3

24 ppm Mg2+ = 100 ppm CaCO3
200 ppm CaCO3

3. Alkalinity or acid-neutralizing capacity is the concentration of (HCO3- +

CO32-) in irrigation water, that is usually determined by titration with a standard acid to
the methyl red endpoint (around pH = 4.5).

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a water with high EC
low hardness Poor quality water

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high alkalinity why?

a water with high EC

high hardness good quality water
low alkalinity why?

E. Sodium hazard .
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1. High Na in irrigation water leads to high exch-Na in soil unless divalent such as
Ca2+ and Mg2+ are also high in water.
2. High Na soils cause crust, disperse, and swell. All these phenomena interfere
with water movement through the soil such as the reduction in infiltration rate and
in aeration.
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The reason is that Na is held by relatively weak force, Thus, the diffuse double layer with
Na+ as the counter ion is very wide, which causes the soil to disperse.
3. High Na in water also upsets the balance among cations and interferes with cation
uptake by plants.
4. Sodium-adsorption ratio (SAR) characterizes the relation sodium status of
irrigation waters and soil solutions:
SAR = Na+ / ( Ca + Mg )½
2

All concentrations are in mmolc/L . A water would not be good for irrigation if its SAR ≥
15.
 5. Exchangeable- sodium ratio (ESR)

The exchangeable sodium status of soils can be predicted quite well from the SAR and a
Gapon-type exchange equation of the form:

ESR = NaX = KG Na+ = KG *SAR

2+ 2+
CaX + MgX ( Ca +Mg )½
2
ESR = KG *SAR

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All exch. ion concentrations are in mmolc /g or mmolc /100g and KG is commonly 0.010
to 0.015 (liter/mmole)1/2.

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F. Bicarbonate hazard.

Another hazard that irrigation water may cause is bicarbonate.

1. High bicarbonate causes high pH and CaCO3 precipitation.

Ca2+ + 2HCO3- CaCO3 + H2O + CO2 ↑

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2. Lowers solution Ca conc., and concentration of most micronutrients, especially
Fe. Fe deficiency in plants may result.
3. Increases SAR in solution and ESR in soils.

CaX + 2Na+ + 2HCO3- 2NaX + CaCO3(S) + H2O + CO2↑

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III. CLASSIFICATION OF SALT-AFFECTED SOILS.

A. Saline soils. High in salt content, its saturation paste extract would have: an
EC ≥ 2 dSm-1 and its pH < 8.5 .

B. Sodic soil. High in sodium content, when its has SAR > 15, its pH > 8.5 (9-9.5).
If a soil is high in both salt content (EC>2 dSm-1 ) and Na (SAR > 15), it is called
C. saline-sodic soil.
You may wonder why SAR is used as the criterion for sodic soil classification, why
not use ESP or ESR.
 Well (a). because there is a good relationship between SAR and ESR: knowing SAR,
ESR can be estimated to a good degree of accuracy.
(b). SAR is easier to determine and has less interference.
(c) . ESR or ESP is harder to determine and has several potential interferences.

IV. SALT TOLERANT PLANTS.

The main effect of soluble salts on plants is osmotic effect, since high salt levels
make it difficult for the plant to obtain water for growth. Boron toxicity is also another

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potential problem. Sea water has about 5 mg/L B.
Remember:

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OP (osmotic potential in bars) = EC (dS/m) * (-0.36)

The relative growth of plants in the presence of salinity has been termed their salt
tolerance. The salt tolerance varies drastically from one plant species to another, even
from one plant variety to another within a species. Barley is probably one of the most
salt-tolerant plants, where as field bean and green bean are least salt –tolerant plants.
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In order to minimize the salt injury, seedlings should be planted on the side of bed, as
you can see in fig. 11.5 Page. 293.
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V. RECLAMATION.
A. Leaching action. For saline soils, the main requirement is reclaiming the soils in
that sufficient water must pass through the plant root zone to lower the salt
concentration as you can see in Fig. 11.6 (p. 298), the passage of one meter of
leaching water per meter of soil depth removes approximately 80% of soluble
salts.
LR (leaching requirement = amount of drainage water/amount of irrigation water

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LR = EC iw/EC dw

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B. Role of CaSO4.2H2O. For sodic soils, a divalent cation (usually Ca2+) must be
present in water of the reclamation process.
Gypsum is usually used for this purpose:

NaX + CaSO4 CaX + Na2SO4 leached out.