Question

A quantity of 100ml gypsum solution having 36 meq/l concentration as calcium, on reacting with 5gm of an alkali soil showed 32 meq/l of Ca+Mg concentration in the filtrate. Estimate the gypsum requirement in meq/100gm of soil.

• A. 6
• B. 8
• C. 12
• D. 16

Hint:

Find how much calcium disappeared from the solution after reacting with the soil, then scale that loss up to a 100 gm soil basis.

Answer 1

The Correct Option is B

Step 1: This is the standard gypsum requirement test for a sodic (alkali) soil. A known volume of gypsum solution, with a known calcium concentration, is shaken with a weighed soil sample. The soil's exchangeable sodium reacts with the calcium in solution and pulls some of that calcium onto the exchange sites, releasing sodium in its place. Whatever calcium (plus magnesium) is left over is what shows up in the filtrate.

Step 2: Work out the total calcium put into the system at the start. Volume of solution = 100 ml = 0.1 litre, and its concentration = 36 meq/l as calcium. So the calcium supplied = 36 meq/l x 0.1 l = 3.6 meq.

Step 3: Work out how much Ca+Mg remained in the filtrate after the soil reacted with it. Concentration in filtrate = 32 meq/l, and taking the same 0.1 litre volume, the amount left in solution = 32 meq/l x 0.1 l = 3.2 meq.

Step 4: The difference between what was added and what remained tells you how much calcium the soil actually took up, the amount consumed reacting with the alkali soil: 3.6 meq minus 3.2 meq = 0.4 meq. This 0.4 meq was consumed by the 5 gm soil sample used in the test.

Step 5: Scale this up to the standard reporting basis of meq per 100 gm of soil: gypsum requirement = (0.4 meq / 5 gm) x 100 gm = 8 meq/100 gm of soil.

Step 6: So the gypsum requirement of this alkali soil is 8 meq per 100 gm, which is option 2. The other values (6, 12, 16) would come from using the wrong concentration difference or the wrong soil weight in the scaling step, so they do not match the actual numbers given in the question.