While compost is wonderful, it is not soil, and does not supply everything plants need to grow. Additionally, it will shrink significantly over the course of the season as the microorganisms in it continue to do their work. Your annuals would be very happy planted in a mix of 70 percent native soil and 30 percent compost.
Thank you Douglas.
It is not uncommon to get "volunteer" annuals growing out of compost heaps -- squash, pumpkins, tomatoes, etc. It can be done if you like. Nobody has ever pointed out a specific reason to me as to why it couldn't work.
I tried it years ago and it didn't work. I didn't know compost from dirt.
I planted some annuals in compost only by mistake this year & they all died. Won't do that again!