Donkova, Radka, Kostadinka Nedyalkova, Jonita Perfanova, and Hristo Valchovski. 2016. “Response of Cowpea (Vigna Unguiculata L.) to Inoculation With Nitrogen Fixing Bacteria”. Bulgarian Journal of Soil Science, Agrochemistry and Ecology 50 (1): 21-25. https://agriacad.eu/ojs/index.php/bjssae/article/view/2035.