Classic cause is too much water. Older more mature seedlings can cope with it better than very young ones. Try cutting back on the soil moisture.

Dave

Thanks Dave - I'll do that. I have been bottom watering them, and am not very experienced with that.

I had problems with the water I was using, I now only start plants with distilled water... tap water has chlorine that can build up in the potting mix.

Too much water can tie up the root's ability to assimilate nitrogen (the micro amounts of which are already in the potting mix), and you said that your older plants have no issues, yes?

I'd say first evaluate the water you are using, and if you are using tap water that you'd call "hard water" then the PH of it could be too high as well as laced with chlorine. It's a minor detail, but you can buy pH lowering drops from any local garden center, and adjust your distilled water to something around 6, and see if that doesn't clear it up. Small plants just can't use much fertilizer at this stage in their life.

I have never checked the PH of my water, but have been meaning to. I have well water, not city water, so chlorine isnt a problem, but it is pretty "hard".

At this point most of the yellowed petunia leaves have reverted to a healthy green, so I suspect Dave was right on the money with his advice - But I do need to check the ph of the water. - Thanks

Like mandolls we live on wells and have very hard water but have never had a problem with using it for plants.

All our greenhouse plants and garden plants are raised on hard water and while we do get some lime build-up in the soil that is still far healthier for the plants than the salts that accumulate from softened water. Thank heavens we don't have to worry about the chlorine issue.

Dave

PS: glad to hear the plants are doing better.