From what I'm seeing in all of your photos in your diary, the bottom leaves are acting quite naturally and dying off. This is completely normal and certainly not a magnesium deficiency (that doesn't start on lower leaves!)... but I'm also going to say that at her age it's past time to be giving her some decent grow nutes - not just the silica. Don't shock her by giving a full dose the first time you feed - go a little slowly and give her 1/2 strength for the first feed - you can increase it next week ... I think you'll see she really appreciates it!
As for those bottom leaves, you can cut them off and let the plant put all its energy into the top leaves and bud sites. She's a very pretty plant and is going to do you proud! Just give her some food!
Good luck!