Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 319

As a thumb rule, reclamation of a foot (12 in.) depth of sodic soil on one acre
requires 1.7 t of pure gypsum for each milli-equivalent of exchangeable sodium
present per 100 g of soil. The rate of gypsum needed for reclamation may be
determined by a laboratory test.
Once the gypsum is applied and incorporated, sufficient good quality water must
be added to leach the displaced sodium beyond the root zone. Reclamation of sodic
soils is slow because soil structure, once destroyed, is slow to improve. Growing a
salt-tolerant crop in the early stages of reclamation and disking in crop residues adds
organic matter which increases water infiltration and permeability, speeding up the
reclamation process.

8.7.1.2 Sample Examples

Example 8.7

A sodic soil has an ESP of 35 and CEC of 22 meq/100 g soil. Calculate the amount
of gypsum required to reduce the ESP to 15 at a depth of 40 cm.

Solution:
Change in ESP = 35 - 15 = 20

We know, ESP = (exchangeable Na/CEC) × 100

Ex. Na = (ESP × CEC)/100

= (20 × 22) / 100

4.4 meq/100 g soil
We know gypsum required for 1 ha-30 cm soil to change 1 meq ex. Na/100 gm
=

=

3.85 metric t (mt).
Thus, gypsum req. per hectare in this case, GR = (3.85 × 4.4 / 30) × 40 mt

22.58 mt
Considering safety factor for field application,
Field requirement, FR

=

= GR × safety factor for field application = 22.58 ×

1.25 = 28.23 mt (Ans.)

8.7.2 Management of Saline-Sodic Soil

Saline-sodic soils are characterized by the occurrence of sodium (Na + )tolevels
that can adversely affect several soil properties and growth of most crops. Some
practices suited to saline soils can be used for saline-sodic soils.
In reclamation of saline-sodic soils, the leaching of excess soluble salts must
be accompanied (or preceded) by the replacement of exchangeable sodium by cal-
cium. If the excess salts are leached and calcium does not replace the exchangeable
sodium, the soil will become sodic. Even nonirrigated sodic or saline-sodic soils
show dramatic improvement with gypsum application. Gypsum (15 t/acre) applied
to a saline-sodic soil in Kansas increased wheat yields an average 10 bushels per
acre over a 5-year period in an area with 28 inch average annual rainfall.