Samir A. Haddad, The amount of biological nitrogen fixation can be a function of the number and weight of root nodules which support the rhizobia bacteria nitrogen factory. To determine this you will need to target a plant sampling at about half the soybean maturity say 50 to 60 days. Take up the whole plant root ball and gently rise off the soil. The nodules are then counted weighted and then dried. The Number fresh and dry weight of the plant nodule should be proportion to effectiveness of the biological nitrification. Biological nitrogen fixation can be associated with soybean plant yield and influence quality attributes. This process can be quite laborious and time consuming and you should contact researchers who are directly working with biological nitrogen fixation to get their insights. Sincerely, PRH