I test the ph of soil by measuring the runoff...water your plant...measure the runoff with a ph meter...
I also do not agree with britewire about your ph...the bible and every other reference to growing insoil states to keep your soil at 6.5...

I am not trying to piss in anyone's cornflakes, but I think that is bad advice...Sorry
Britewire and I have debated about this in the past, but I think the majority of experience growers anywhere, will tell you emphatically, to use a ph of 6.5

5.8-6.0 for hydro, even down to 5.4

But if you follow britewire's advice and 'I' am right, then you fry your plants; Because low ph is more acidic and I think that is the root of your problem...that can create a mg deficiency...
another thing is My way you save your plants with no adverse effects...

follow britewire's ph advice and you might be OK???

One thing for sure is No-one other than britewire that I know of set their PH to 5.8 in soil...No-one.

Too many chiefs here now...good luck, and let me know if you need anymore help.

By the way; the advice I gave you pretty much comes right out of Jorge's "The Indoor bible"...there is not a better more up to date source for Indoor horticulture than this book. lw