SHUKLA, ARVIND K, et al. “Nitrogen Use Efficiencies Using Leaf Colour Chart in Rice (Oryza Sativa)-Wheat (Triticum Aestivum) Cropping System”. The Indian Journal of Agricultural Sciences, vol. 76, no. 11, Jan. 2011, https://epubs.icar.org.in/index.php/IJAgS/article/view/3259.