Calopogonium mucunoides Desv. is able to form nodules, root organs in which biological nitrogen fixation takes place, after a symbiotic interaction with soil bacteria known as rhizobia. Such distinct advantage of some legume species faces different environmental abiotic factors such as acid and alkaline pH conditions of soil. Nodulation and symbiotic nitrogen fixation response in different pH ranges were determined under greenhouse conditions. Plants were cultivated in vermiculite and treated with nutritive solution adjusted to pH 4.0, 5.5, and 7.0, and after three months, the following variables were measured: nodule number, nitrogen fixation, tissue protein, amino acids, total ureides, allantoin, and allantoate. The number of nodules and nitrogen fixation were enhanced under acidic conditions, but nitrogenase activity was drastically decreased at pH 7.0. Acidic conditions decreased the amount of protein, amino acids, total ureides, allantoate, and allantoin in leaves and nodules, but at pH 5.5, only protein content was decreased. Symbiosis with C. mucunoides and biological nitrogen fixation were kept under acidic conditions, but it was negatively affected under conditions that are near to neutral. It is reasonable to conclude that the association between C. mucunoides and nitrogen-fixing bacteria from ''Cerrado'' evolved in this context, which is supported by the difficulties faced to keep interaction under pH near neutrality.

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