Nodulation of Soybeans Grown Hydroponically on Urea¹

Abstract

Soybeans [ Glycine max (L.) Merr.] were grown hydroponically to determine the effects of urea‐ and nitrate‐N sources on nodulation. Urea allowed effective nodule development and function, as evidenced by nodule mass, acetylene reduction, and difference between the total Kjeldahl N gain per plant and urea uptake measurements. Plants grown in solutions with as high as 18 mM urea‐N produced nodules capable of N 2 fixation (C 2 H 2 reduction assay) while NO 3 concentrations low as 2 mM inhibited nodulation. The amount of N fixed by nodules of plants grown on urea varied from 27 to 71% of total plant N, depending on urea concentrations of the nutrient solutions. Urea uptake by the plants was slower than NO 3 uptake, regardless of urea concentration supplied. Therefore, internal concentrations of N or N‐components in the plant were likely lower with urea than with NO 3 nutrition which may have partially accounted for the degree and effectiveness of nodulation. Growth on urea is a convenient hydroponic method of producing healthy, vigorous, nodulated soybeans that are capable of N 2 fixation.