Ran across this to day in the Sustainable Agriculture Research and Education site:
"Sweetclover residue is said to be allelopathic against kochia, Russia thistle, dandelion, perennial sowthistle, stinkweed and green foxtail. Repeated mowing of yellow sweetclover that is then left to mature is reported to have eradicated Canada thistle. Letting sweetclover bloom and go to seed dries out soil throughout the profile, depleting the root reserves of weeds. "
It is also an accumulator plant, bringing various minerals up from subsoils and making them available for other plants.It is supposed to be almost unmatched in its value as a green manure crop. Seed can remain viable in soil for 10 years.
One thing that maybe should be mentioned though is that sweet clovers contain coumarin which can be a problem especially in mouldy hay. SARE says pasture or DRY (clean) hay from first year plants are equivilent to alfalfa. The Manitoba Dept of Agr. suggests feeding such pasture in 6 week rotations with other pastures with absolutely no sweetclover, to allow the animals to rid their systems of it. There are some strains now with a low coumarin count.