PH problem?

Hi everyone,

I'm a first timer who's been lurking for about 2 months. I'm 5 weeks into a soil grow (Roots Organic) and I have a plant suffering from obvious Mg deficiency to the extent that I think I might lose the plant. When I figured out what it was I put some Cal-Mg in the normal watering cycle but haven't seen much difference. When I measured the PH at watering I noticed the (Cal-Mg treated)drinking water I was using would be 6.3-6.4 before going in but the runoff was consistently 5.3-5.4. I'm thinking this acidity is preventing the roots from taking in the Cal-Mg. Make sense? If so what should I do? Thanks.

Lower ph Will start to lockout calcium and magnesium and 5.3 is pretty low for soil. What soil mix are you using?

The plant looks really bad. Sorry I'm still a bit reluctant to post pics. I know 5.3 is low (and likely locking out the Calcium and Magnesium) but considering it's going in at 6.4 I don't want to change it and I don't want to flush either. I had an episode with some fungus gnats so I'm freakishly paranoid about overwatering. Soil is Roots Organic and nutrients are Advanced 3 part - Grow and Micro. They are in 4.75G Giant pots and they're getting about 3L of water every 5 days. So confused, all of the other plants look really healthy.