Skills developed: determining the cation exchange capacity (CEC) and base satura

Question

Skills developed: determining the cation exchange capacity (CEC) and base saturation of the soil. The cation exchange capacity (CEC) of a Soil is strongly affected by the nature and amount of mineral and organic colloid presenting the soil. Soils with large amounts of clay and organic matter have a higher CEC than sandy soils low in organic matter. Examples of CEC values for different textures in a Mollisols are as follows: Suppose that a soil contains 900 ppm Ca^2+ (mg/kg soil) 540 ppm AI^3+ (mg/kg soil) 585 ppm K^+ (mg/kg soil) 300 ppm Mg^2+ (mg/kg soil) Calculate the meq/100 g soil for each cation.

Explanation / Answer

Answer :

Note some important conversions to be used in required conversions.

1) 1 g = 1000 mg

2) 1 ppm = 1 mg/Kg.

3) Equivalent weight of Metal cation = Atomic weight of metal / # of electrons transferred

so, 1 equivalent cation = Atomic weight of respective metal / Charge on cation.

4) 1 equivalent = 1000 miliequivalent.

5) 1 g/eq = 1000 mg/1000 meq = 1 mg/meq.

6) 1 ppm = 1 mg/Kg = 1mg/1000 g = 0.1 mg / 100 g

7) [Mn+] in (meq/100 g) = Concentration in (mg/100g) / Equivalent weight in mg/meq.

a) Let us first calculate equivalent weights of each cation involved in mg/meq.

i) Ca2+ : Atomic mass of Ca metal = 40.08 g/mole. Charge of Ca2+ = 2 units.

Equivalent weigt of Ca2+ = 40.08 / 2 = 20.04 g/eq = 20.04 mg/meq. (Conversion 5)

ii) Al3+ : Atmoic weight of Al = 27 g/mole. Charge on Al3+ = 3 units.

Equivalent weight of Al3+ = 27/3 = 9 g/eq = 9 mg/meq.

iii) K+ : Atomic mass of K = 39.1 g/mole, Charge on = 1

Equivalent weight of K+ = 39.1 / 1 = 39.1 g/eq = 39.1 mg/meq.

iv) Mg2+ : Atomic weight of Mg = 24.3 g/mole. Charge = 2

Equivalent weight of Mg2+ = 24.3 / 2 = 12.15 g/eq = 12.15 mg/meq.

b)

i) For i) Ca2+ :

i) [Ca2+]= 900 ppm = 900 mg/Kg = 900 mg/ 1000 g = 90 mg/ 100 g.

By conversion formula (7)

[Ca2+] in (meq/100 g) = 90 / 20.04 = 4.49 meq/100g.

ii) [Al3+] = 540 ppm = 54 mg/100 g

So, [Al3+] in (meq/100g) = 54 / 9 = 6 meq/100g.

iii) [K+] = 585 ppm = 58.5 mg/100g.

So, [K+] in (meq/100g) = 58.5 / 39.1 = 1.5 meq/100g.

iv) [Mg2+] = 300 ppm = 30 mg/100g.

So,[Mg2+] in (meq/100g) = 30 / 12.15 = 2.47 meq/100g.

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