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The Effect of Ammonium Nitrate Applications to Field Soils on Nodulation, Seed Yield, and Nitrogen and Oil Content of the Seed of Soybeans¹

ABSTRACT

A 2-year field study was conducted to determine the possibility of supplementing soil and symbiotically-fixed nitrogen of soybeans with nitrogen fertilizer.

In 1947, during a hot dry growing season marked responses were obtained from added nitrogen. The number of nodules per plant decreased 80 to 90%, there were appreciable increases in seed yields, nitrogen content of the seed increased, and oil content decreased.

In 1949, with adequate rainfall, moderate temperatures, and 30 to 40 days additional growing season there was little to no response to added nitrogen. The number of nodules per plant on the untreated plots was larger than in 1947, and the largest application of ammonium nitrate resulted in only a 35% decrease in number of nodules. The yield of seed increased very little, and no change occurred in the nitrogen and oil contents of the seed.

A comparison of three dates of side-dressing with plow-down treatments indicated that the most effective utilization occurred with plow-down applications.

This experiment helps explain some of the variation in response of soybeans to nitrogen fertilizers. Rainfall and temperature conditions during the growing season apparently have a direct influence on the sufficiency of symbiotically-fixed nitrogen for maximum yields, thus indirectly influencing the response to added nitrogen.

¹ Contribution from the Department of Agronomy, Illinois Agricultural Experiment Station, Urbana, Ill. Published with the approval of the Director. Presented before Section IV, Soil Science Society of America, August 30, 1951.

A major part of this work was submitted by the senior author in a thesis for a partial fulfillment of the requirements for the Master of Science Degree in the Graduate College. This investigation was supported by the International Minerals and Chemical Corporation.

² Assistant in and Associate Professor, Soil Fertility, University of Illinois, respectively. The authors gratefully acknowledge the assistance of R. E. Hay, E. O. Schneider, and D. M. Smith, the farm cooperator, with this experiment.