The Punic Wars –Case for Salt in
Soils
“After the third punic war, the Romans stormed the
town and the army went from house to house
slaughtering the inhabitants in what is perhaps
the greatest systematic execution of noncombatants before World War II. Carthaginians
who weren't killed were sold into slavery. The
harbor and the city was demolished, and all the
surrounding countryside was sown with salt in
order to render it uninhabitable”
Richard Hooker (The Conquest of the Hellenistic Empires)
History of Western Civilization
By E.L. Skip Knox
ALKALINE AND SALT
AFFECTED SOILS
Soils of Dry Regions (Alkaline Soils)
Causes of Alkalinity
Characteristics of Alkaline Soils
Classes of Salt Affected Soils
Management of Soil Affected soils
Soils of Dry Regions
(Alkaline Soils)
 The accumulation of salt is one of several characteristics
of dry region soils.
 Soluble salts are minerals dissolved in water
 Accumulation of salts occur where evapotranspiration is
greater than precipitation
• Low precipitation means less leaching and more
accumulation of cations released from weathering.
• Soils in these regions are generally in the alkaline range
~pH 7 or above
 Dry region soils can be defined as follows:
– High pH (>7) (alkaline soils)
– High saline content (saline soils) –eg, NaCl, Na2CO3, NaHCO3
– High Sodium content (sodic soils)
Causes of alkalinity (High Soil pH)
Sources of alkalinity
•
•
The basic, OH- generating anions are carbonates (CO32-)
and bicarbonates (HCO3-)
Cations in soil solution and colloid surface (Ca2+, Mg2+,
K+, and Na+) do not produce acid nor alkaline in reaction
with water (Their effect in water is neutral (pH ~ 7)
1) Carbon dioxide and carbonates
2
CaCO3  Ca 2  CO3 .................(i )
CO3
2

 H 2O  HCO3  OH  .....(ii )

HCO3  H 2O  H 2CO3  OH  ....(iii )

H 2CO3  H 2O  CO2  ..................(iv)
CaCO3  H 2O  Ca 2  2OH   CO2 
Causes of alkalinity (Contd.)
2) Role of individual cations (eg. Na+ or Ca2+)
•
•
•
The particular cation associated with carbonate and
bicarbonate anion influences the pH.
If Na+ is prominent in soil, more OH- will be produced because
Na is more highly water soluble than Ca2+.
Na2CO3
2Na+ + CO32-
3) Role of soluble salts
•
•
High levels of neutral salts (CaSO4, Na2SO4, NaCl, and CaCl2)
lowers the pH by common ion effect.
E.g., increasing the concentration of Ca2+ or Na+ ions on the
right side of reaction drives the reaction to the left.
Characteristics of Alkaline Soils
1) Nutrient deficiencies
•
Availability of micronutrients such as Cu,
Zn, Fe, Mn, and B are influenced by soil
pH.
The lower the pH the more soluble and available they
are. So in alkaline conditions, plant growth may be
limited by deficiencies of these metals.
•
•
Phosphorus is commonly deficient in
alkaline soils because it is tied up with Ca
or Mg phosphate.
Only few micronutrients like Mo are more
available in alkaline conditions.
2) Cation exchange capacity
•
Alkaline soils have higher cation exchange capacity
because at low pH, soil charge becomes more
positive, whereas at high pH, negative charge
increases. (High pH stimulates pH dependent charge on the soil
colloids).
3) Calcium accumulation in subsurface layers.
•
Alkaline soils in low rainfall regions commonly have
layers of CaCO3, CaSO4 or other such layers that can
inhibit plant growth.
4) Soil water supply
•
The subsoil layers are always drier than in humid
regions resulting in much competition for water by
native plants. These soils therefore require greater
water management if cultivation is a priority.
Subsoil layer of Carbonate
Salt Affected Soils
(Sodic and Saline Soils)
Salt Affected Soils in the US
Where do saline and sodic
soils occur
1) They occur naturally in areas where
evaporation is more than precipitation
2) They occur in low flat areas with high
water table that may be subject to
seepage from higher elevations
3) They occur in areas where there is
irrigation
Development of Salt Affected
Soils
Measuring Salinity and
Sodicity
 Salinity is measured as:
– total dissolved solids, TDS (mg/L), or
– electrical conductivity, EC (dS/m)
 Sodicity is measured as
– exchangeable sodium percent (ESP), or
– Sodium adsorption ratio (SAR).
ESP 
Exchangeable Sodium
x 100
Cation ExchangeCapacity
[ Na  ]
SAR 
(0.5[Ca 2 ]  0.5[ Mg 2 ])12
Classification of Salt-Affected Soils
(Classification is based on Ec, ESP, pH)
1) Saline Soils
–
–
–
–
high dissolved salts in soil solution
(affects plant growth)
EC ≥ 4 dS/m (saturated paste)
Exch. Na < 15%
pH < 8.5
2) Saline-Sodic Soils
–
Same as saline soils except Exch. Na >
15%
3) Sodic Soils
–
–
–
–
do not have excess salts (EC < 4 dS/m)
Exch. Na >15
pH > 8.5
dispersion of soil colloids------->poor
permeability/drainage
Classification of Salt-Affected Soils
How salts accumulation affect
plant growth
 Plants respond to these soils in various ways. The two
common mechanisms are:
 Osmotic effects
– Salts lower osmotic potential of soil water
 Specific ion effects
– The kind of salt makes a difference in how plants respond to
salininty.
– Some ions eg. Na+, Cl-, HCO3- are toxic to some plants.
– Also high intake of Na can cause imbalance in uptake of other
cations (cations compete among themselves for uptake).
 Effects on soil physical properties
– Breakdown in structure (oxygen depletion)
– Breakdown in structure (poor water infiltration and percolation)
Management of Salt Affected Soils
(Soil Reclamation)
1) Saline soils
–
–
Establish drainage
Leach with water (Leaching Requirement)
2) Saline-sodic soils
–
–
–
Add Ca (usually gypsum CaSO4) to remove
exchangeable Na
Leach with high quality water to remove excess
salts
Leach with high Ca irrigation water
3) Sodic soils
–
–
–
Add gypsum
Deep tillage
Leach with high quality water when possible
In Summary
 Alkaline and salt affected soils occur in arid and
semiarid regions.
 They typically have high pH values and require
different management practices
 The three classes of soils are saline, salinesodic, sodic soils.
 They can be managed by leaching of excess
salts with good water, replacement of excess
ions, and sometimes using deep rooted
vegetation