Actually, you should have added it a month ago. Bulbing action has started by now (it's triggered by day-length).

I amend my soil just before planting with compost, blood meal, bone meal, and wood ashes---the last three at the rate of 1 cup per row foot. Long about May I side-dress with the same amendments (less the compost) at the same rate.

I generally side-dress with 5-10-5 or 10-10-10 as soon as growth starts in spring and again about 6 weeks later. It's too late now - almost time for harvest!