It is true that certain weeds are indicative of certain soil components, ph, moisture content, etc. There are lots of interesting books and other literature on this topic.

Here is a good article on weeds as indicators of soil types and soil conditions. Change the soil and you may eradicate some weeds--but they are adaptable and find many soils suitable!

http://www.hort.purdue.edu/hort/courses/HORT417/supplemental_reading/WEEDS/WEEDS-INDICATORS.PDF

Also an interesting article on the no-till method and the weed seed bank.

Google: weed soil indicators. You will find lots of reading material to keep you busy for a while.
Ginger

Here is a link that might be useful: weed seed bank and tillage

Thanks Ginger! YOu got me pointed in the right direction!

Weeds are more apt to grow in crappy soil than good,balanced amended soil...

False, without a doubt. Sure, some weeds will indicate some soil types but 99% of the weeds I know will grow much better in good soil than bad. In real poor soil certain weeds do better because of lack of competition. Weeds like the same type of conditions as the 'good plants'. I think the reasoning of the statement comes from good balanced amended soil has been worked and weeded for some time, whereas poor soil hasn't and the weeds were let to grow and reseed.

Two stories that may flesh out this enquiry.
A man once famous for his knowledge of growing the kind of pasture thoroughbred horses thrive on, a blind man named Friend Sykes, was called in to advice on the purchase of some land. "Take me to the tallest thistle," he said to the prospective buyer. "But there are no thistles," the man replied. You may guess the rest.
If you browse a reference book on wild flowers such as the Peterson field guide you will notice a common reference that begins "grows in disturbed soil". What this means is that seeds of weeds can remain dormant for hundreds if not thousands of years and only come to life when you start to dig. Of weeds we used to say that one years seed is seven years weed.
It is common sense to study what type of plants (weeds or otherwise) grow naturally where you are and take the lead from that.

"Weeds are more apt to grow in crappy soil than good,balanced amended soil..."

Ive read this in gardening books before, but it hasnt been my experience in the garden, the weeds seem just as happy to grow in the amended areas as the poor sandy areas, but Im more likely to stay on top of the ones in the amended areas where Ive planted until the plants can outcompete them. the poor soil in my garden has been neglected for probably many years so the only thing growing there is the weeds, they just dont have any competition in those empty spaces.