EFFECTS OF INOCULATION WITH BRADYRHIZOBIUM AND UREA APPLICATION ON NITROGEN-FIXATION AND GROWTH OF YAM BEAN (PACHYRHIZUS-EROSUS) AS AFFECTED BY PHOSPHORUS FERTILIZERS IN AN ACID SOIL

A glasshouse pot-experiment was conducted to evaluate the effects of B radyrhizobium strains (NFB 442 and NFB 450) and N fertiliser applied a s 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 gr own in an infertile soil, as affected by different soluble P fertilise rs (single superphosphate-SSP; triple superphosphate-TSP; and fosmag t hermophosphate-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 rhiz obial strains, was used. All P fertilisers increased dry matter produc tion, number and weight of nodules, nitrogenase activity and P and N u ptake. Of the P fertilisers tested, FTP combined with Bradyrhizobium i noculation showed the best results, similar to SSP plus 200 kg/ha N. U rea 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 st rains.