Although I’ve done the same things this year, as far as I know, a lot, not all, of my seedlings have developed these leaves of an unhealthy, pale green.
As usual, I started seeds, one to each 2″ soil cube made with Pro-Mix . Fertilized with a half-strength solution of Miracle Grow when true leaves appeared. (Seedlings are supposed to carry their own “food” to this point.)
My favorite theory is that the small soil cubes (like the basil below) dried out on the heat mat and then got flooded when I watered. That inconsistency would interfere with the plants ability to take up water and nutrients. It’s my favorite theory because I’m potting everything up, anyway. As usual, I do that when they have a couple sets of true leaves or earlier, if I see roots at the edge of the soil cubes.
Most of the web sites on this issue are written for mature plants and because the problem is on the newer leaves, most sources would indicate an iron deficiency. But why this year and not others?
Other theories are a magnesium deficiency, although that would be easy to treat with Epsom Salts, again, why this year only? And web sites that show pictures of magnesium deficiencies show more damage than just a poor color.
Other years, I’ve had damping off problems and fungus gnats, as well as mice that ate my seedlings, and a cat who ate them and upchucked green goo on my putty colored carpet (sigh). This is a new one.
So, as I was potting up, I cleaned all of the flats and the watering can with a 10% solution of bleach in water, just in case. I sprayed the plants with the same fertilizer solution — if for some reason they can’t get food through the soil, maybe they can get it through their leaves. It has small amounts of both iron and magnesium as well as the usual.
I’m also considering a test where I also spray a few plants with an Epson Salts solution, a teaspoon in a pint of water, because of anecdotal evidence that tomatoes love the stuff.
Doing several things at the same time isn’t a very scientific way of finding out the cause but mostly, I want my seedlings to thrive.