I'd consider sun/shade conditions first. Verbenas and Mexican heather tend to be sun-loving, xeric plants, while impatiens prefer a shadier location and moister soil conditions. If these are all growing together, I'd be very surprised if they all thrived under the same light/moisture conditions.

Your weeds often give a clearer picture of whether your soil is acid or alkaline.

At the risk of deeply offending ( ;-) ): do you get the little daisies (Bellis perennis) growing in your lawn? If you do - they like an acid soil, moderately damp and with some shade to do really well.

Impatiens also prefer soil on the neutral to acid side and Verbenas usually look a bit manky come autumn.

I second Vetiverts suggestion of using weeds as indicators of soil types/conditions...this is a list of weeds & other plants that I've found works for soils around my place:

Borage = Fertile
Buttercup = Acid + lack of drainage
Chickweed = Fertile
Cleavers = Fertile
Clover = Lacking Nitrogen
Coltsfoot = Poor drainage
Comfrey = Damp soil
Cow Parsley = Too much nitrogen
Dandelion = Heavy, fertile clumpy soil
Dock = Poor damp soil
Galinsoga = Good humus
Horsetail (Equisetum) = Acid soil + poorly drained
Plantain = Acid soil
Polygonum = Heavy wet soil
Sorrel = Acid soil, lacing calcium
Spurge (Euphorbia) = Alkaline soil
Stinging Nettle = Good soil
Vetch = Lack of Nitrogen

I got this from "Organic Gardening  A Guide by Organic NZ". Edited by Valerie Freeman. It's aimed at NZ soils, but I guess as these plants can be found worldwide they grow in pretty much the same conditions. It hasn't yet been wrong!

Hope this helps,
GF

So if my lawn gets clover (low nitrogen), dandelions (fertile, heavy soil) and plantains (acidic soil) what does this say about my soil? My lawn does get all of the above.

The soil isn't low in nitrogen as the green turf can attest, I do have clay soil, but it is under 4-6" of sandy loam topsoil so it isn't particularly heavy and it's ph is 6.7 which is only very mildly acidic, certainly not acid enough to warrant adjustment.

I don't have any of the others in the list growing so I can't comment on those, but I don't see that the 3 I do have really indicate anything other than they are common weeds in my area.