I've not tried to apply classical soils methods to measuring the CEC or the ECEC of biochars, but can imagine some potential difficulties.  First, biochars are typically alkaline and the abundance of Ca, Mg, K etc will depend on the feed stock.  Given the the relatively low combustion temperatures typically used these cations, along with Fe and Al, will be retained and be the basis of the exchangeable cation complement.  A portion of these metal ions will be in soluble forms.

Consequently,  the appropriate procedures for the measurement of CEC or ECEC in biochars are those recommended for alkaline soils that (may) have soluble  salts. 

Good question and I appreciate the good answer of Dr .Milham.Can we we expect some more answers from the scientists who have worked on biochars of diverse origin.What could be the reason for high water holding capacity/ power of biochars?

Generally pyrolysis temperature and hold time govern the properties of biochar, in relation to the feedstock material composition. Higher temperature pyrolysis attributed to higher pH and Electrical conductivity. As well FTIR analysis revealed presence of various charged groups (representing alkaline) which may lead to increase in CEC of the biochar.

In practical terms, we really face difficulty in determining CEC of the fresh/pure biochar due to sticking of the sample by normal Acetate centrifugation method.

I guess, some experts working on this area will definitely contribute on this query who have already published their contributions.

Regards,

Rishikesh Singh

Similarly to soil, the NH4Ac pH 7 methodology has proven to be useful as it allows for harmonized comparison between different types of biochar irrespective of feedstock and production conditions. This is partially with the IBI has included it in their biochar standards material. Also similar to soil, this methodology does not always reflect "field conditions". I suppose the question is appropriate for what? 

Is a good question and I m according with David Guerena. As the biochar is going to be used as soil amendment, whe use the same methodology that for soil characterization (NH4Ac pH7). This made possible to compare with CIC of soils and other organic amendments

Thank you all  for the comments.

Here is my concern: biochars are light (bulk density is about 03 g/cm3), and have many micro- and nano-pores. NH4OAC may be trapped physically in these pores, and when flushed with methanol or ethanol (and perhaps, KCl), as the method proposes, would give a positive error. We have determined the CEC of many biochars using the NH4OAC, pH 7 method, and most values we got were all  above 20 cmol/kg. I do not believe biochars can have that much negative charge (carboxylate groups) at pH7. Phenolate would not occur until pH around 9. Any thoughts?