Nikova, Ivona, and Venera Tsolova. “Mobility of Trace Elements in Arable Soils from the Bobov Dol Valley, Bulgaria”. Bulgarian Journal of Soil Science, Agrochemistry and Ecology, vol. 51, no. 2, June 2017, pp. 12-20, https://agriacad.eu/ojs/index.php/bjssae/article/view/1936.