The choice is yours as long as it is a well balanced nutrient fertilizer, preferably with micronutrients added. As to the amount to use, that depends on type brand you choose to use - be it organic or synthetic. So following the label directions is best.

I find the key with feeding asparagus well is frequency - at least 3x per year. It needs regularity at the various stages of its life cycle - pre-emergence, post harvest growth to fern, and dormancy.

Some prefer using lots of composted manures, other will use a granular synthetic 13-13-13 or similar, and others use, others use a combination of both. If your bed was not well amended prior to planting then I'd opt for several inches of aged manured compost.

Dave

perfect! aged composted manure is something I have plenty of access to.

If spears are already up isn't it a bit late to fertilize? I am asking because I always thought you were supposed to fertilize before they emerge.