feeding brassicas

So I thinned out my brassicas into 7.5cm pots to grow them on prior to transplanting later in September/October. They have grown on well but the compost I used, while nice stuff, is cheap and I suspect it will have little by was of feed in it. I will not be putting them in until the ground gets a bit more by way of moisture so they are likely to be in the pots for at least another four weeks or more. I was wondering if a weak liquid feed would be in order? They have been in the pots just about three weeks now and are well on to producing new leaves.

I am presuming these are spring cabbage/caulis.

If the leaves are nice and green do not feed them but keep them well watered. If they are a bit pale/yellow/blue then a bit of feed will perk them up.

If they are fed too much nitrogen at this stage they will get soft and leafy, not like to be transplanted and struggle to survive a hard Winter.

Feed them as soon as there are signs of Spring with chicken pellets and they will grow away fast.

A pinch of sulphate of ammonia might help.