Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 281

Movements of water influence salinity. Hence, salinity has been related to clear-
ing and irrigation development and results from changes of land use and water
movements in landscapes. Saline soils are the easiest of the salt-affected soils to
reclaim if good quality water is available and the site is well drained.

8.3.1.2 Sodic Soil

Sodicity refers to the amount of sodium in soils. Sodic soils are low in soluble salts
but high in sodium. This soil is also referred to as “black alkali soil.”

Characteristics:

• Exchangeable sodium percentage (ESP) is greater than 15 (or, sodium absorption

ratio, SAR value is greater than 13), reflecting greater amount of exchangeable
sodium.

• Electrical conductivity (EC) is less than 2 dS/m, indicating lower amounts of

soluble salts.

• The pH value is generally greater than 8.5.

The dominant cation is Na ++ with smaller amounts of Ca ++ ,Mg ++ Na + , and K +
ions.

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The HCO 3 - and CO 3 - are the dominant anions with smaller amounts of Cl - and
SO 4 - .
Sodic soils have exchangeable sodium percentages (ESP) of more than 15. This
means that sodium occupies more than 15% of the soil's cation exchange capacity
(CEC). High ESP is manifested by a dispersed clay system, which greatly reduce
both air and water entry into the soil system. Thus, sodic soils have restricted
water movement, are easy to get stuck in when wet, form lumpy seed beds and
often have unfavorable pH for crop growth. They can be reclaimed, but it may be
slow and expensive due to the lack of a stable soil structure, which slows water
drainage.
Sodic soil develops through a process whereby sodium ions build up in prefer-
ence to other soil cations (particularly calcium) on the exchange complex of the soil.
Increases in soil pH and decreases in calcium and magnesium usually accompany
this process. If sodium salts are the dominant type of salts present, a relatively small
amount of sodium salts can negatively affect soil structure and create a sodic soil
condition but may not necessarily have high electrical conductivities.
If a soil is highly sodic, a brownish-black crust sometimes forms on the sur-
face due to dispersion of soil organic matter. By the time darkened crusts are
visible on the soil surface, the problem is severe, and plant growth and soil qual-
ity is significantly impacted. Dispersion of soil particles often results in crusting
and poor emergence. Plants growing on sodic soils may appear stunted and often
show a burning or drying of tissue at the leaf edges, progressing inward between
veins.

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