reading soil through plant species

I'm trying to get a handle on What the various type of weeds tell me about the soil. Or lack of a specific weed as well. The land I live on has a lot of wild onion, but no clover or dandelion.

Are there any resources that could help me learn what these types of things are telling me?

No idea where you would find such info. But you could give these guys a try and see if you have a overlap with 4 or so "weeds"
http://practicalplants.org/wiki/Allium_cepa_aggregatum maybe your soil is well drain and alkaline??

This thread on the PRI forums has lots of ideas.