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LWR 107
Soils in Dry Regions
SOIL ALKALINITY
Causes of Alkalinity: Natural Vs Anthropogenic
Characteristics and Problems of Alkaline Soils
Development of Salt-Affected Soils
Measuring Salinity and Sodicity
Classes of Salt-Affected Soils
Reclamation of Saline-Sodic and Sodic Soils
Alkaline soils?
 pH greater than 7.5
 Presence of large quantities of Calcium,
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magnesium, potassium and sodium
Typical in the low rainfall areas- low leaching,
and deserted areas. Physical weathering
dominates
In some areas, we can find alkaline soils in small
areas within production fields
Poor management may cause zones of alkalinity
Typical alkaline soils contains Ca/Mg- and Nacarbonates
How the pH increases?
Clay-Ca+2 + H2O -------- Clay-H+ Ca+2 + 2OHOH release in this reaction increases OH in the soil, so
if pH=14-(pOH) so pH increases!
 Usually in this kind of soil, the %BS is dominated by
Ca/Mg in saline soils, and Na in sodic soils.
 Usually %BS is very high (close to 100%)
 Consequences of high availability of calcium
 P deficiency
 Micronutrient deficiency
 High osmotic tension, water stress!
Soil Alkalinity
 Soil physical conditions can be severely
affected
 Sodic soils, dominated by sodium are
structure less and very disperse.
 Lack of porosity and difficult root penetration
 Water stress in most plants, due to an
increase in soil osmotic potential
Soil alkalinity
 Basic definitions
 Electrical conductivity: the presence of salts in
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the soil generates electrical flux of current when
an anode and cathode are connected.
Electrical conductivity (mmhos/cm-1) correlate
electricity Vs salt content in a soil extract.
Sodicity: Measured as %Na in exchangeable
sites (CEC). ESP =exchangeable Na %
ESP = [Na]/CEC x 100
Sodium activity ratio = Na/ 0.5 (Ca) + 0.5 Mg^1/2
Types of Alkaline soils
TYPE Soil
EC
(mmhos/cm)
ESP (%)
SAR
SALINE
<4
0-15
0-12
SODIC
0-4
>15
>12
SALINESODIC
No saline
no sodic
>4
>15
>12
0-4
0-15
0-12
How to manage alkaline soils
 Difficult to manage
 Before identify the type of management we have
to identify the type of alkaline soil
 Every type (saline, sodic, saline-sodic) has a
different management.
 In general any practice that reduces salt content
will be needed .
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Adding amendment (gypsum, OM)
Leaching salts with clean water
Irrigation techniques
Utilizing salt tolerant crops
Types of Alkaline soils
TYPE Soil
pH
Typical ions
Management
SALINE
<8.5
Ca, Mg, K
(chlorides
and sulfates)
Irrigation
Gypsum
SODIC
>>8.5 can
reach 10
>>Sodium
SALINESODIC
No saline
no sodic
>8.5
Ca, Mg, K
and >>> Na
Ca. Mg, K
Very diff.
Add water
Gypsum
>7<8.5
none
Things to remember:
 Three types of salt-affected soils are saline, sodic and
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saline-sodic soils.
Salt-affected soils may inhibit seed germination, retard
plant growth, decrease soil physical properties and/or
cause irrigation difficulties.
Saline soils often can be reclaimed by leaching salts
from the plant root zone.
Sodic soils often can be reclaimed by replacing soil
sodium with calcium by adding a calcium-based soil
amendment (gypsum).
Sodic soils respond to continued use of good irrigation
water, good irrigation methods and good cropping
practices.
Alkaline soils can be black, why?
Because the high pH levels and high
salt levels the organic matter
becomes very disperse. The disperse
humus moves upward in the capillary
water flow, and when the water
evaporates leaves a very intense
black color on the soil surface.
So, not always black soils are good
soils for food production.
Remember
To be prepared for a comprehensive
final exam
To check my homepage for class notes