Improvement of low fertility soils (oxisols) for high productivity and sustainability of crop-livestock systems in tropical savannas of Colombia - Amézquita Collazos, Edgar; Rao, Idupulapati M.; Barrios, E.; Rondón, Marco Antonio; Ayarza, Miguel Angel; Hoyos Garcés, Phanor; Molina López, Diego Luis; Corrales, I.I. (International Center for Tropical Agriculture, 2007)