Experiments were conducted in 1972 on ‘Vansoy’ soybeans [Glyclne max (L.) Merr.] to investigate the effects of deep‐placed nitrogen fertilizer and organic matter additions on seed yields, nodule dry weights, and seasonal amounts of nitrogen fixed/ha. Treatments included applying 112 kg NH3‐N/ha, (N); passing the anhydrous applicator tines through the soil without added N, (T); applying 4.5 metric tons/ha of shredded corn cobs as an organic matter source (OM); N+OM; T+OM; and control, (C). Seed yields were not affected by treatments, but seasonal amounts of N2 fixed, estimated by an acetylene reduction assay, were stimulated by OM, T, and T+OM and decreased by applied N. Regression analyses were used to separate N2 fixation responses into effects on nodule mass/plant and fixation/unit of nodule mass. Increased fixation following OM and T+OM treatments could be fully accounted for by larger amounts of nodule mass/plant, which fixed less N2/unit of nodule mass than control plants. NH3‐N reduced N2 fixed/ha by 39 to 48% below control plots, by reducing both nodule mass and nodule efficiency. Application of N+OM increased nodule mass slightly but reduced nodule efficiency and seasonal N2 fixation. The attempt to maintain nodule mass in the presence of deep‐placed NH3‐N through the addition of organic matter was successful but a seed yield response was not obtained.