Singh, L N, et al. “Efficacy of Urea in Integration With Azolla and Vermicompost in Rainfed Rice (Oryza Sativa) Production and Their Residual Effect on Soil Properties*”. The Indian Journal of Agricultural Sciences, vol. 75, no. 1, Jan. 2005, https://epubs.icar.org.in/index.php/IJAgS/article/view/9041.