Tomato leaves are purple and crispy

After reading up on what causes the leaves of tomato seedlings to turn purple, I learned it was a phosphorous deficiency caused by (among other things) cool temperatures. I have heard it will go away on its own when the temps warm up.
HOWEVER, my purple leaves are starting to turn a bit crispy. When I touch them, they are very dry and want to flake away.
Does anyone know if this is also phosphorous deficiency, or if it is something else? Will the leaves turn purple and crispy if I water too much (which I suspect I may be doing)? The seeds were started in Jiffy peat pellets, and moved to Jiffy 2" peat pots with Miracle Grow organic potting mix, which I set in trays and water from the bottom. I have noticed that when the pots are very dry, the plants wilt, and when I water them, they usually perk up (unless they were too far gone). So I try to keep them constantly damp, but might this be TOO much water? --S.
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This is my biggest problem with the plants that I have indoors. I have started to evaluate them by their weight. If it feels light, I water it. Mostly this is in trays with other plants, and the surface is not level, so the water tends to run to one side or the other. I rotate the trays, and check to make sure that they feel heavy. Having beat the heat, and varmints, I don't want to lose them to inattention.
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