Plant pot feed

I have plants lingering in pots this year where they would normally have been planted out a while back - even in Cornwall the cold is holding things back.

What do folks recommend to revitalise compost when it starts to flag a bit - preferably organic?

Seedlings and young plants in question - tomatoes (for green growth, not setting fruit), carrots, parsnips, swedes, turnips, all squashes.

I have to hand comfrey liquor and worm juice; both high nitrogen.

Thanks - that's what I was thinking.

Re: Swedes and Turnips - they haven't formed their true leaves yet (my first ones grew too leggy as I was on holiday and they germinated and grew straggly while I was away) and my concern is survival against slugs.