Cabbage seedlings - some leaves wilting and curling up

My wife planted some cabbage seeds a few weeks ago and they have been growing nicely.

However, we are starting to notice that some of the leaves (not all) on many of the seedlings are curling up and dying. This doesn't affect every leaf on the affected seedlings though, just one or two.

This is both happening on seedlings which she is hardening off outside (bringing them in at night) and also some which are currently sitting on a window sill which has a radiator below it (the radiator is on for a couple of hours twice a day - this is the same window sill where they germinated inside an unheated propagator.).

However, the worst affected are those which are being hardened off outside.

Neither of us have grown cabbages before so we are unsure of the cause.

The fact that those outside are worse off makes me think it's a temperature problem, but I also wonder if it's a watering issue (they do get plenty of water as they sit inside a gravel tray which has some water at the bottom).

The lower leaves are mainly affected, but sometimes just the upper leaves.

The seedlings are about 3 to 4 inches tall on average.

They are in a seed tray with a reasonable amount of space between each seedling (since they were transplanted from their tiny inserts about a week or so ago).

Thanks for any advice anyone can offer.

 

Yes, the compost is definitely soggy. I can't think of a way to dry it out either short of leaving the trays out in the sun (or, in my area, the cloud and a stiff breeze). .

 

Thanks, will do.