Changes to criterion for identification of Spodosols

Please can somebody help me to understand the why Carbon (but also Al and Fe) extracted in pyrophophate was abandoned for spodic horizon identification after the 1975 ST ?

In case, please let me know relevant documents where this choice has been made.
Thanks



Edited 1 time(s). Last edit at 03/29/2017 03:36PM by Ken Scheffe.