OP- when you say the soil is "old", what do you mean? Has it been previously used?
If so, what was the previous cultivar?

Do you, or does anybody else in the dwelling, smoke tobacco?

Would it be possible to get a shot of the underside of an effected leaf?

Have you had any whitefly or aphid issues?

How quickly is this progressing? Days, weeks,...?

Are there any cankers where petitole meets meristem?

How old is this mom? When was the last time she was trimmed? Did you take cuts of any of your other moms at the same time?

I would quarantine that plant.
rhizome Reviewed by rhizome on . Magnesium (Mg) deficiency? Pics As best as I can tell from checking the two links below, one of my mothers to be has an Mg deficiency. The leaves feel hard and brittle. The yellowing, which turns brown starts at the tip mostly, but also the outer edges. I doubt it matters, but this is the same plant that is in with the shrooms in this post: http://boards.cannabis.com/plant-problems/134279-shrooms-weed.html. It starts in the older leaves, and eventually the bottom ones turn completely brown. There is interveinal chlorosis, and Rating: 5