copy/paste from the Kitchen Sink thing:

“Much previous research has been done on this issue,” Board notes. “The general finding is that beneficial effects are only seen in cases where soybeans have not been grown in a particular field for five or more years. It is very difficult for the introduced rhizobium – inoculated onto the seed – to outcompete
the native rhizobi-um in the soil (in most cases this will be rhizobium USDA 123). Cases of positive results for introduced rhi-zobium have only occurred under greenhouse conditions and/or altered field conditions where competition from the native strain was removed.”