ABSTRACT
Tropical savannahs cover 45% of the land area in Latin America, or 243 million hectares (Mha), mainly in Brazil (200 Mha), Colombia (20 Mha), and Venezuela (12 Mha). The soils are mainly Oxisols and Ultisols, which are characterized by low nutrient reserves, high acidity (pH 4.0-4.8), high aluminium (AI) saturation (up to 90%), high phosphorus (P) fixing capacity (Sanchez and Logan, 1992), and a low capacity to supply P, K, Mg, and S. In addition to soil chemical constraints, these soils also exhibit high bulk density, high resistance to root penetration, low rates of water infiltration, low water-holding capacity, and low structural stability (Amezquita, 1998a, b; Phiri et aI., 2001a). Because of these chemical and physical constraints, these soils are more susceptible to degradation than most soils, often degrading within 5 years of being opened up for agricultural production (Thomas and Ayarza, 1999). Furthermore, the traditional technologies in use on these soils are unsustainable. Soil organic matter (SaM) contents have
gradually decreased, reducing soil fertility (da Silva et aI., 1994), and excessive seedbed preparation has led to the destruction of the favorable physical soil structure, accompanied by subsoil compaction and soil sealing that eventually promoted severe erosion, even on gentle slopes (Klink et aI., 1993). Soil preparation and cultivation techniques, therefore, need to be devised to reverse degradation while maintaining high yields. A highly successful strategy for intensifying agricultural production sustainability and reversing problems of degradation involves the integration of crop/livestock systems (agropastoralism) (Rao et aI., 1993; Thomas et aI., 1995). This strategy is based on the assumption that a beneficial synergistic effect on production and on soil occurs when annual and perennial species are combined (Lal, 1991; Spain, 1990). Available nutrients are used more efficiently and the chemical, physical, and biological properties are improved.
