Soil acidity is one of the greatest problems faced in the management of agricultural lands around the world, mainly in the tropics, where phosphorus and nitrogen are also limiting factors. In the case of nitrogen, biological nitrogen fixation is the most economical and ecological alternative to decrease the use and impact caused by nitrogen fertilizers. Two "in vitro" experiments and one under greenhouse conditions using four Bradyrhizobium strains (BR 4406, BR 29, SEMIA 587 and INPA 03-11B) were carried out at the Soil Science Department, at Federal University of Lavras from July 1998 to July 1999, aiming to verify the effect of three pH values (5.0; 6.0 and 6.9) on growth of Bradyrhizobium strains in YM medium, soybean symbiosis and peat survival. In the first experiment, Bradyrhizobium strains had diffentiated behaviour at the three pHs of YM medium, and had the highest colony forming unities and exopolysaccharide production at pH 6.0. In the second experiment, nodule number (NN), nitrogenase (Nase) activity, dry matter of nodules (NDM), roots (RDM) and shoots (SDM) of soybean were not affected by the different pHs of the culture medium used to produce peat inoculants. INPA 03-11B had similar efficiency to BR 29 and SEMIA 587, recommended as inoculant strains for soybean, on the production of NN, Nase activity, NDM, RDM and SDM. In the third experiment, all strains had the same cell survival at pH 6 and 6.9, except BR 29, which had the highest survival at pH 6.0. The best behaviour of Bradyrhizobium strains at pH 6.0 in culture medium and in peat survival indicated the possibility to use this pH in the peat inoculant production as a means of acid adaptation to this soil stress condition in the tropics.
Biological nitrogen fixation; soybean; peat based inoculant; acid tolerance; exopolysaccharide production