AbstractGrowth and nodulation response of soybean (Glycine max (L.) Merr.) to various single nitrogen sources in solution culture is confounded by unequal shifts in solution pH. A recirculating ion exchange system was designed in which a cation exchange resin (Amberlite IRC 50) was used to control the pH of solutions in which soybeans were grown. Nutrient solution pH levels were established at range extremes of 9.0 to 3.7 with 100% Ca2+ or H+ forms of resin, respectively. Intermediate pH levels were established by varying the ratio of Ca2+ to H+ forms of resin. The system is capable of maintaining pH within 0.5 to 0.9 units of the initial pH over a two‐week growth period of soybeans with either nitrate‐ or urea‐N sources. In the absence of the resin column, pH of the urea nutrient solution rapidly declined to less than pH 4 which resulted in depressed plant nodule development. The optimum pH range for nodule mass and N2 fixation (measured by acetylene reduction) was between 5.2 and 7.0 with urea nutrition. Both nitrate‐ and ammonium‐N sources were inhibitory to acetylene reduction in comparison with urea which allowed extensive nodule development and activity.