This document describes a method for determining the gypsum requirement of alkali soils to aid in soil reclamation. Key steps include: 1) Adding a saturated gypsum solution to a sample of soil and allowing them to equilibrate. 2) Titrating the calcium and magnesium left in the solution after equilibration using versenate titration to determine the amount adsorbed by the soil. 3) The amount adsorbed provides a measure of the gypsum requirement to replace sodium in the soil as an initial step in reclamation.

2. Presence of high percentage of exchangeable sodium in soils produced
alkali conditions- high pH and poor soil structure. Reclamation of such soils
involves the use of gypsum in the form of powder. A useful and rough measure of
exchangeable Ca (plus Mg) in soils and the amounts of gypsum required to
replace the sodium as an initial step in soil reclamation consists of adding a given
amount of saturated solution of gypsum to a weighed amount of soil and by
versenate titration, determining the combined Ca and Mg left in solution at
equilibrium. The amount of Ca adsorbed by the soil (initial Ca in solution – Ca
+Mg in solution after equilibration with soil) is a measure of the gypsum
requirement of the soil.
Theory/Principle/Background of the topic:

3.  Saturated gypsum solution :
Add 5 g of chemically pure CaSO4.2H2O to one litre of distilled water. Shake
vigorously for 10 min. using a mechanical shaker and filter through Whatman No. 1 paper.
 0.01N CaCl2 solution :
Dissolve exactly 0.5 g of AR grade CaCO3 powder in about 10 ml of 1:3 diluted HCl.
When completely dissolved, transfer to 1 litre volumetric flask and dilute to the mark with
distilled water. CaCl2 salt being highly hygroscopic should not be used.
 0.01N Versenate solution :
Dissolve 2.0 g of pure EDTA-disodium salt and 0.05 g of magnesium chloride (AR)
in about 50 ml of water and dilute to 1 litre. Titrate a portion of this against 0.01N CaCl2
solution to standardize.
 Erichrome black T indicator :
Dissolve 0.5 g of EBT dye and 4.5 g of hydroxylamine hydrochloride in 100 ml of
95% ethanol. Store in a stoppered bottle or flask
 Ammonium hydroxide –
ammonium chloride buffer : Dissolve 67.5 g of pure ammonium chloride in 570 ml of
conc. ammonia solution and dilute to 1 L. Adjust the pH at 10 using dil. HCl or dil. NH4OH.
Reagents:

4.  Weigh 5 gram of soil in a conical flask.
 Add 100 ml of saturated gypsum solution to it and shake for 5
minutes on a mechanical shaker or by hand, several times during a
period of 30 minutes.
 Filter, titrate 5 ml of filtrate against 0.01 N versenate solutions as
follows:
 Transfer 5 ml of filtrate into a china dish. Dilute to approximately
25 ml with distilled water. Add 10 drops of buffer and one drop of
the indicator.
 Titrate against versenate solution till colour changes from purple to
blue.
Procedure:

5.  Weight of soil taken = 5gm
 Volume of saturated gypsum solution added = 100ml
 Volume of saturated gypsum filtrate taken for Titration = 5ml
 Volume of 0.01N EDTA solution used for saturated gypsum = Xml Solution
 Concentration of Ca in saturated solution = X x 0.01x1000 = S me/l
5
 Volume of 0.01 N EDTA solution used for filtrate = Yml
 Concentration of Ca + Mg in filtrate = Y x 0.01x1000 = T me/l
5
 Gypsum requirement in milli-equivalent /100g of soil = 2(S-T) = G
 Gypsum requirement in tones /ha = G x10 x86 x2.25
 Dose of gypsum to be applied is decided on the basis of extent of deterioration and
crop grown. Generally it applied at the rate of 25 to 50% of gypsum requirement by
this method.
Calculation

6. Cautions :
All the standard solution should be made accurately and most
carefully
Result:
Gypsum requirement of the alkali soil is _____________.