Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 317

8.7 Management of Sodic and Saline-Sodic Soils

8.7.1 Management of Sodic Soil

Sodic soils require different management than saline soils. Excess sodium must first
be replaced by another cation and then leached. Sodic soils are treated by replacing
the sodium with calcium from a soluble source. Gypsum (CaSO 4 . 2H 2 O) is con-
sidered the cheapest soluble calcium source for reclamation of sodic soils. Soil
application of gypsum should be applied according to the gypsum requirement of
the soil. Technically, soil reclamation is possible but may not always economically
feasible.
Chemical amendments available for reclamation are

(a) soluble calcium salts: Calcium chloride (Cacl 2 .2H 2 O), Gypsum

(CaSO 4 . 2H 2 O).
(b) Acids or acid formers: Elemental sulfur (S), sulfuric acid (H 2 SO 4 ), Iron sulfate
(FeSO 4 ).
(c) Insoluble calcium salts: Ground lime stone (CaCO 3 ).

On calcareous soils (soils with excess CaCO 3 present), elemental sulfur may be
added to furnish calcium indirectly. Sulfur is oxidized to sulfuric acid which reacts
with the calcium carbonate to form gypsum. Oxidation of elemental sulfur is slow,
so this method may be of limited value.
Three processes are occurred for the reclamation of sodic soils:

(i) Sodium ion (Na + ) is replaced by calcium ion (Ca ++ ) on the exchange complex
(ii) Soil hydraulic conductivity increases
(iii) Sodium salts are leached from the soil system.

The use of gypsum to increase water infiltration is a common and an old practice.
Surface application of gypsum reduces soil crusting thereby increases water infiltra-
tion and in turn, crop yield. Ayers and Westcot ( 1985 ) reported that for a potato crop,
gypsum applied and disked into the soil at rates as high as 10 Mg/ha/yr resulted
in greatly improved infiltration. Adding gypsum to the water at rates sufficient to
increase the calcium concentration by 2-3 mmol/l was also effective. Machines are
now available which make it relatively easy to apply the gypsum to the irrigation
water.

8.7.1.1 Cation Exchange Reaction
(a) Reclamation by Gypsum (Na + Is Replaced by Ca ++ )
Ca ++ replaces the Na + from the colloidal surface, and Na + with SO 4 - form Na 2 SO4
which is readily soluble in water and leach down or drain out from the field with
irrigation water.