The analysis of a soil indicates that its composition by weight is 22% clay mine

Question

The analysis of a soil indicates that its composition by weight is 22% clay minerals of which 5% is smectites (150 meq/100 g at pH 6; 155 meq/100 g at pH 7) and 95% is kaolinite (3 meq/100 g at pH 6; 6 meq/100 g at pH 7). In addition, the soil contains 2.1% organic matter (250 meq/100 g at pH 6; 290 meq/100 g at pH 7). Assuming that these are the only fractions that provide negative charge to this soil calculate the corresponding CEC (in meq/100 g) at pH values of 6 and 7. (Review Example Problem, page 23 of 7^th edition textbook, page 26 of 8, h edition textbook).

Explanation / Answer

Answer-

CEC= Cation exchange capacity, it is the ability of soil to hold exchangeable cations. It gives the idea about soil fertility. More CEC means soil are more nutritous and fertile. As given in the question the CEC value of smectite mineral group at pH =6, is 150meq/100gm and 155meq/100gm at pH=7, means the CEC also depend upon the pH value of soil. As the acidity increases the CEC will be decreases and vice versa. Same rule will be applied on kaolinite mineral group. But the value of CEC of organic content are more than clay mineral group, as we can see the value of CEC in organic matter is high among all constituents at pH=6 and pH=7.

Therefore, we can calculate the average CEC value for given clay minerals and organic content at pH=6, are as follows;

CEC= (150+3+250)/100 = (403)/100= 4.03 meq/100gm

And at pH= 7, as follows

CEC= (155+6+290)/100=(451)/100= 4.51 meq/100gm

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