Toncheva, R., H. Pchelarova, and E. Zlatareva. 2014. “Comparative Effect of Using Poultry Manure and Sewage Sludge As Soil Improvers”. Bulgarian Journal of Soil Science, Agrochemistry and Ecology 48 (3-4): 31-36. https://agriacad.eu/ojs/index.php/bjssae/article/view/2283.