"Weeds and What they Tell" by Ehrenfried E. Pfeiffer, orginially published by
the Bio-Dynamic Farming and Gardening Assn. (my copy was reprinted by Rodale
Press in 1970) provides lots of food for thought about soil /weed relationships.
Also there have been several articles in Science News in the past year which
reported on the effects of excess nitrogen. It seems to push toward weedy
species (from native species to such invaders as quack grass, for example).
Solar Farm Education