what to feed leeks

I have leeks in a cat litter tray they don't seem to be growing any fatter .   Should I give them some sort of a feed if so what with.

Mine stay like grass for a while then, when I pot on, they start to grow a little thicker. About a month after that, I apply a liquid feed to give them a boost because they will have probably used up all the nutrients in the compost so, when I plant them out, most are usually about pencil-size in thickness. See other threads about leeks on here too.