Plants that your going to eat the leaves of - cabbage, kale, lettuce need a 'folial feed' ie something high in nitrogen. I feed my leaf veg with seaweed extract, once at the start and once at the end, and twice with Growmore in between.
not too late to sow Beetroot, ive just started some more, should be able to pull it September. You should be able to get some turnips in as well :D