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Nodulating and Nonnodulating Soybean Isolines: II. Response to Applied Nitrogen and Modified Soil Conditions¹

Nodulating and nonnodulating soybean isolines field-tested for 7 years produced essentially equal N yields when adequate combined N was available. The amount of N fixed symbiotically depended largely upon the availability of soil N, water, or both, and ranged from 1 to 142 pounds per acre, that is, 1 to 74% of the total N uptake, respectively. The amount of symbiotic N fixation decreased rapidly with increases in fertilizer N. Nodule weight, number, and size were directly related to increased N fixation and inversely related to increased increments of applied N. The correlation of symbiotic N fixation with nodule mass was r = 0.9. Except for a severe moisture stress, the nonnodulating line responded linearly to applied N, with 3.2 pounds of fertilizer N equivalent to 1 pound of plant N. Short-term rotation studies indicated that, when no fertilizer N was used, the residual effect of a nodulated soybean crop in the previous year was 33 pounds of N per acre.

² Agronomist, Crops Research Division, ARS, USDA, and Professor of Agronomy, Iowa State University of Science and Technology, Ames, Iowa. Suggestions from L. R. Frederick, Professor of Agronomy, Iowa State University are gratefully acknowledged.

Received for publication June 16, 1965.

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