Effects of inoculation with Bradyrhizobium and urea application on nitrogen fixation and growth of yam bean (Pachyrhizus erosus) as affected by phosphorus fertilisers in an acid soil

A glasshouse pot-experiment was conducted to evaluate the effects of Bradyrhizobium strains (NFB 442 and NFB 450) and N fertiliser applied as urea at different levels (0, 50, 100 and 200 kg/ha N) on nodulation, N-2 fixation, P and N uptake and dry matter production of yam bean grown in an infertile soil, as affected by different soluble P fertilisers (single superphosphate-SSP; triple superphosphate-TSP; and fosmag thermophosphate-FTP) applied at 70 kg/ha P.A control treatment without P fertiliser was included. A hydromorphic podzol soil, representative of the coastal highplains of north-east Brazil, containing native rhizobial strains, was used. All P fertilisers increased dry matter production, number and weight of nodules, nitrogenase activity and P and N uptake. Of the P fertilisers tested, FTP combined with Bradyrhizobium inoculation showed the best results, similar to SSP plus 200 kg/ha N. Urea applied up to 200 kg/ha N did not appear to inhibit nodulation and nitrogenase activity on yam bean. Nitrogen application had no effect on dry matter production but inhibited nodulation in the absence of P fertiliser. We conclude that there is no need to add mineral nitrogen to yam bean when plants are inoculated with selected Bradyrhizobium strains.