N. P. Stamford, I. M. de M. B. Vieira, D. R. dos Santos, and C. E. de R. e S. Santos, “Selection of Bradyrhizobium strains for cowpea growing in an acid soil (LVA) of the semiarid region of Brazil”, PAB, vol. 25, no. 4, pp. 545–552, Apr. 2014.