Pigeon pea obtains N for growth by N2 fixation although yield generally is not improved by either the inoculation of Rhizobium or by the application of N fertilizer in Puerto Rico. Sixteen strains of Rhizobium spp., different in geographical origin, were tested for N-fixing effectiveness, determined from comparisons with uninoculated controls, N controls and the standard strain 176A22. Inoculated treatments showed significant differences in nodulation, plant dry weight and %N. Several strain x plant combinations had higher N content than the N treatment, reflecting the ubiquity of effective strains and the possible lack of response of pigeon pea to inoculation or N fertilization. Strains superior in N 2 fixation were selected for testing for symbiotic effectiveness under field conditions.

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