>> N-P-K as a group.
I'm trying to recall the terminology I heard 1-2 times that distniguished THREE groups:
micro-nutrients (trace amounts like Mn, Boron, Selenium, probably Cu)
macro-nutrients (the big three NPK, though no one mentions CO2, O2 and water as nutrients!
and the in-betweeners that are needed in moderate amounts: Fe, Mg, S, Ca ....
I think they had some tidy Greek or Latin prefix for "in-between", like "hemi". I forget which group Si fell into. Maybe it's a trace requirement for most plants, but a "hemi" for grasses and bamboo.
