I am a litle bit confused with the calculation of soil CEC (by summing up the major cations).
Imagine you have a soil sample in which the contents of Ca is 5 meq/100g (100 mg Ca/100g soil), Mg is 2 meq/100g (24 mg Mg/100g soil), Na is 2meq/100g (46 mg Na/100g soil), K is 3 meq/100g (117 mg K/100g soil), and Al is 1.5 meq/100g (13.5 mg Al/100g). Then the CEC would be: 5+2+2+3+1.5=13.5 meq/100g.
According to many soil textbooks, meq/100g is numerically the same as cmol/kg. So, it can be said that the CEC of this soil sample is 13.5 cmol/kg.
If you first change the unit of cations (from meq/100g to cmol/kg), and sum up later, the result of CEC would be:
CEC = 2.5 cmol Ca/kg + 1 cmol Mg/kg + 2 cmol Na/kg + 3 cmol K/kg + 0.5 cmol Al/kg = 9 cmol/kg. Note that the contents of Ca, Mg, and Al (in cmol/kg) are reduced respectively by factors of 2, 2, and 3 because Ca and Mg are divalent, and Al is a trivalent cation.
How to explain the difference ? Please give your comments. Thanks
Dear Colleague,
the exact tem for "cmol/kg" is "cmol(+)/kg; that means, "charge" of any cation must be accepted. Hence, you can calculate as hitherto (previously) in units mval/100g, or change units to "cmol(+)/kg " without any numerical change of your results.
With Best Wishes,
Karel FIALA, alias Petr MRAK.
Dear Colleague,
the exact tem for "cmol/kg" is "cmol(+)/kg; that means, "charge" of any cation must be accepted. Hence, you can calculate as hitherto (previously) in units mval/100g, or change units to "cmol(+)/kg " without any numerical change of your results.
With Best Wishes,
Karel FIALA, alias Petr MRAK.
Well,
(According to many soil textbooks, meq/100g is numerically the same as cmol/kg. So, it can be said that the CEC of this soil sample is 13.5 cmol/kg),
the wright of the abovementioned sentence is only refer to univalent cation not for divalent, and trivalent cation.
HERE, the right solution is the second one , CEC = 9 cmol/kg.
Sincerely.
This book helps you for calculating exchangeable bases with ( cmolc / kg soil) : Procedure for soil analysis , Fifth edition , edited by : L.P. Van Reeuwijk 1995.
Practically, the soil analytical data of the exchangeable cations extracted are usually converted before summing them up for CEC.
- Badges
- Science topic
- Similar topics
- Agricultural Science
- Soil Science
- Soil Analysis