Border's running out of steam

I think the annuals have drained most of the nutrients out of the soil even after a load of manure last spring and bone meal.Some of the cosmo's are only 2inch across where they were 4 a few weeks ago.What is the best feed to give them a boost until the first frost?

Don't forget to keep dead heading.  Plants lose a lot of energy on making seed so if you thwart them they will keep on flowering.