My seedlings are not doing so well. Their leaves have begun to yellow and I'm not sure what the problem is. Do they need to be closer to the light? Should I fertilize them? They are mostly tomato plants & some pepper plants too.
Can you post a picture? Or if you can't do that, can you tell us a little more about them, like how big are they, have you ever fertilized them, what kind of lighting do they have now, etc. If they're good size seedlings and you haven't fertilized them then that definitely could be the problem, but yellowing leaves can also be a sign of overwatering or other things. If it's lack of light, then they'll also get really leggy as they try to stretch for the light--do they look like that?
Chlorosis! I knew I'd remember the word. It popped into my head, alas, while I was looking at my 'Brandywine' seedlings last night. In one pot the 3 seedlings are yellow and in the other they're green. I used the same potting soil for both pots. Go figure. It might be iron, it might be boron. I gave 'em a drink of Miracle-Gro tomato food, which is 18-18-21, but highly diluted. The book I'm using says that the seed itself contains enough food for the seed leaves, but when the true leaves start coming the plant depends on the nutrients in the potting mix. So there'd better be some! You want the plant to pour its energy into strong roots at this point, so don't feed them anything whose 1st number (of the 3) is the highest -- that will promote leaf growth instead.
you want to wait till the soil is dry to the touch to water. if the plant has grown its "true leaves" and has not yet been fed this can be the problem. i personally as a pro gardener dont feed with anything higher than the 10-10-10 or other variations. i try to keep it low to prevent fert burning.