I think I have a deficiency of some sort in my plant. Four days ago I noticed a slight discoloration in some of the lower sets of leaves. I figured it had probably eaten through the nutrients already in the soil, so I decided to give my plants it's first feeding. The discoloration has now begun moving up the plant, and the lowest sets of leaves are dead. I've read through descriptions of nutrient deficiencies but most of them are pretty ambiguous and I'm having trouble pinning down what is the likely culprit. Nute burn seems pretty unlikely, since the problem started suddenly a couple of days ago, before I gave the plant anything. So basically my question is: does anyone have any idea what causes this sort of discoloration? Thanks.