Sisworo, Elsje L., et al. “USING THE N-15 METHOD TO DETERMINE N-SOIL, N-GREEN MANURE, AND N-UREA AVAILABILITY AFTER SIX SEASONS IN AN ALLEY CROPPING SYSTEM”. Jurnal Ilmiah Aplikasi Isotop Dan Radiasi, vol. 2, no. 2, Sept. 2013, pp. 23-34, doi:10.17146/jair.2006.2.2.571.