Donkova, Radka, et al. “Response of Cowpea (Vigna Unguiculata L.) to Inoculation With Nitrogen Fixing Bacteria”. Bulgarian Journal of Soil Science, Agrochemistry and Ecology, vol. 50, no. 1, Mar. 2016, pp. 21-25, https://agriacad.eu/ojs/index.php/bjssae/article/view/2035.