Question

11. A similar analysis was performed for the same exchangeable cations. However, this time the results were reported as mg of the cation per kg of soil as follows: Na = 350 mg/kg K = 1000 mg/kg Ca = 2000 mg/kg Mg = 1000 mg/kg Calculate the CEC (5 pts) Please show your work and do not forget the units

Answer 1

Given data: The concentrations of cations Na⁺, K⁺, Ca²⁺, Mg⁺² are given in mg/Kg i.e in ppm unit. since 1Kg = 10⁶ mg. or 1 mg = 10^-6 g hence mg/Kg is equal to ppm unit.

Hence we have, Na⁺ 350 ppm, K⁺ =1000 ppm, Ca²⁺ = 2000 ppm, Mg⁺² = 1000 ppm.

We know the molecular weights of all these ions 23, 39, 40, 24 respectively and also we know that for Na nad K being monovalent cations have same equivalent weights also and for Ca, Mg they are halved as these are divalent cations. hence the equivalent weights are as follows

Na⁺ = 23 eq/g , K⁺ =39 eq/g , Ca²⁺ = 20 eq/g , Mg⁺² =12 eq/g.

Equivallents are reported as eqv/gram but CEC is in miliequivalent/gm i.e meq / g.

To convert them into meq/g we have to multiply all by 10 ( because to make it per 100 gr we have to divide with 100 and to make it miliequivalent have to multiply by 1000 that means multiply by 1000/100 = 10 )

Na⁺ = 230 meq/100g , K⁺ =390 meq/100g , Ca²⁺ = 200 meq/100g , Mg⁺² =120 meq/100g.

Now let us calculate indivisual's CEC value, formula,

For Na⁺ ; 350/230 = 1.52

For K; 1000/390 = 2.56

For Ca2+ : 2000/200 = 10

For Mg2+; 1000/120 = 8.33

CEC of soil = 1.52+2.56+10+8.33 = 22.41 meq/100 g

CEC of the soil is 22.41 meq/g.

The CEC Cation Exchange Capacity is the term used to describe the holding capacity of the soil for cations. It also can be described as the capacity for soli to exchange cations for other.

Hence CEC is 22.41 meq/100 g