Question
We're spading up an old barn area for a garden. The soil is basically just well-rotted manure that is spongy in texture. Do you know what plants might do well here?

Answer
Cow manure can have alot of salt in it.  Horse manure is better for not burning plants.  However, the chances are NOT good that anything will do well unless you dilute it 3 fold by rototilling into it loamy soil from underneath or brought in.  Why don't you experiment first before paying to haul in tons of loamy soil.  Get a pot and fill it with what seems to be the common mix found in old barn yard and put a flower transplant from local nursery and also plant a bean or pea seed.  This will determine if plants will do well and seeds will germinate.  Do not overwater and do not fertilize.
Good luck.
Bryan