Sustainability of crop rotation and ley pasture systems on the acid-soil savannas of South America - Amézquita Collazos, Edgar; Friesen, Dennis K.; Rivera Peña, Mariela; Rao, Idupulapati M.; Barrios, Edmundo; Jiménez Serna, Juan de la Cruz; Decaëns, Thibaud; Thomas, Richard J. (2002)