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Effect of Anhydrous Ammonia and Organic Matter on Components of Nitrogen Fixation and Yield of Soybeans¹

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.

Key Words: Glycine max (L.) Merr. • Nodule efficiency • Acetylene reduction

² Research scientist, associate professor, and professor, respectively (Present address of senior author: Central Research and Development Dep., Exp. Stn., E. I. dn Pont de Nemours & Co., Inc., Wilmington, DE 19898.)

Received for publication February 26, 1975.