What is the way to treat a deficiency?

Ok i got a question.
Im reading about deficiencies and there are some deficiencies that they occured due more than one case?
Lets say that you have a plant that start showing a MG deficiency,now i have read that mg def can be caused by
1)low ph
2)A very wet, cold and/or acidic root environment
3)Limited root system and a heavy load on the plants
4)High EC in the medium, inhibited evaporation
5)to much potassium or calcium

ok can understand that possibly a normal grower would possibly is testing his watering ph and runoff so lets leave that out and the second (A very wet, cold and/or acidic root environment) its probably traceable by a normal grower with some reading but if this guy is sure by the tests that he does not have low ph or bad root enviroment how can he possibly check for the other three
like how can he check for too much potassium or calcium??
or how can he check and know if his root system is to small for his plant?

I wish you can get my point ...

off topic but cant believe that name hadnt been taken before on here lol

and to answer for you... change your substrate as in transplant to better substrate after rinsing off bad soil...

thats the easy way since you have so many compounding issues

u can check nute concentration with a tds meter. it will tell u the total amt of everything in the water but not how much of each nute.

as for checking the roots, try starting out w small containers so u can see when the roots start growing out of the bottom, then transplant. i wouldnt go digging up the soil or anything tho.