Japanese comments on NWI gSoil quality. Determination of effective cation exchange capacity (CEC) and exchangeable cations using acobaltihexammine trichloride solutionh


The effective cation exchange capacity (ECEC) is defined as the total amount of exchangeable cations, which are mostly sodium, potassium, calcium and magnesium (hereafter collectively termed as bases) in non-acidic soils and bases plus aluminum in acidic soils.  The ECEC has been determined by extracting bases and aluminium with ammonium acetate and potassium chloride, respectively.  To our understanding, the summation method has been accepted as a de facto standard for the ECEC.  It is true that the conventional ECEC method involving two extractions is not necessarily suitable for handling large numbers of soil samples and the ammonium acetate extraction method for exchangeable bases does not satisfactorily work in calcareous soils.  There is also a good reason that a simple and rapid method exists.  An alternative method for the ECEC, however, must be compatible with the conventional summation method, i. e., the ECEC value from a new method must be comparable with that from the conventional method.

In view of this compatibility consideration, we are afraid that there might be significant disagreement between the ECEC values from the two methods. We know that some complex cations including cobaltihexammine, copperethylenediamine and silverthiourea have extremely high affinity for negative charges on the layer of silicate minerals as evidenced by a series of experiments with smectites.  With regard to other cation exchangers of soils, however, the results of case studies as well as basic studies are still insufficient.  It is questionable whether cobaltihexammine could quantitatively replace cations on allophane and imogolite that are major cation exchangers in soils originated from volcanic ash.  Actually, there is a paper reporting that the amounts of bases extracted with silverthiourea were 10% smaller than those with ammonium acetate for volcanic ash-originated soils.  The exchange selectivity of cobaltihexammine with aluminium held by humic substance has not yet thoroughly examined.

With these concerns, we are obliged to disapprove the present NWI.  Much many primary and case studies are needed before we consider the proposed method as an NWI for IS.