It's not possible to say that inoculating your soybean seed will increase the yield you will get by 3 or 6 or more bushels an acre or by a definite percentage because fertility varies so much from field to field. But the experience of many farmers and many experiment station tests indicate that inoculation is well worth the small cost and may bring substantial returns.
Norman, A. G.
"Inoculate Your Soybeans,"
Farm Science Reporter: Vol. 4
, Article 7.
Available at: https://lib.dr.iastate.edu/farmsciencereporter/vol4/iss1/7