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Effect of Available Nitrogen on Symbiotic Fixation¹

ABSTRACT

Nitrogen absorption from the atmosphere and from inorganic sources was measured and compared in a number of leguminous plants when varying quantities of inorganic nitrogen were applied. Inorganic nitrogen absorption, apart from the atmospheric nitrogen, was measured by the use of tagged fertilizer. The plants were grown in solution culture in exploded vermiculite for 10 weeks. Nitrogen fertilizer was added in small increments at weekly intervals.

The magnitude of nitrogen fixation was proportional to the total growth and nitrogen uptake. Nitrogen fixation decreased and fertilizer absorption increased with increase in the quantity of available nitrogen. In no instance, however, was fixation completely inhibited or was all of the available nitrogen absorbed by the legume plant. The lower rates of addition of inorganic nitrogen served to increase growth and nitrogen absorption more than they served to diminish fixation. High increments of nitrogen fertilizer had less influence than the low increments in increasing plant growth but a greater tendency to replace the fixation process.

¹ Journal Paper No. J2715 of the Iowa Agr. Exp. Sta., Ames, Iowa. Project 1070. Contribution of the Agronomy Department. Presented before Div. III of the Soil Science Society of America, St. Paul, Minn., Nov. 9, 1954.

² Graduate Assistant and Associate Professor of Soils, respectively. Isotopic analyses of nitrogen samples were made through the cooperation of Dr. Harry J. Svec of the Institute of Atomic Research, Ames, Iowa.

Received for publication December 15, 1954.