We are classifying the soil fertility for the delta, for which we need updates or modifications of the system developed by Sanchez et al (2003) from around countries.
In India, land capability classification has been done. But I am sure there is so soil fertility capability classification system for the entire country. However you may refer to: Adhikari 2010 and Bera 2014 (files Attached) for regional assessment study.
It is suggested for use of the technique, soil-test crop response based application of fertilizers and manures for sustainable crop production system. Further, it is also required to control the soil acidity, or alkalinity or degradation through use of suitable soil amendments. I think, this approach would serve the purpose for soil fertility capability classification.
I add hereby a Report of the European Soil Bureau. It gives an overview of the inventory methods used in the EU by different Member States, to classifiy soils (and their fertility) and the methodology used. My country was the first in Europe to produce a complete set of maps of soil resources at a scale of 1/20.000 for Belgium (digitally as well now). I think you may have a usefull resource (the report added) here to get inspired on developing the best method for the Region of Interest you have in mind. You mention 'The Delta', but there are many delta's in Asia, so I cannot advise you on that except that a river delta is regional scale, Maybe it is the Mekog Delta for example?
The methodlogy developed for Belgium might be appropriate for the Mekong, since Belgium is a country of only 30.000 km² (but with 11 million inhabitants or about 370 inhabitants/km²), that's quite comparable to the size of the Mekong delta and its population density.