The maintenance of soil Fertility in Amazonian managed systems

Most of Brazilian Amazonia faces important limitations for conventional agriculture and pastures due to a generally poor chemical fertility as well as the region's environmental conditions, especially high temperature and moisture. Without proper management, degradation of the soil and resulting unsustainability of agricultural and ranching production occur within a few years, leading to land abandonment. Use of perennial crops, especially those based on native tree species, would be instrumental in order to achieve best management such as that which assure recycling processes similar to those in the primary forest. Recommended alternative land uses are those producing high soil organic matter, recycling of nutrients, substantial agricultural production, and economic viability. These include agroforestry systems, enrichment of second growth with valuable native timber or fruit species, accelerated fallow regrowth via enrichment plantings, sequential agroforestry with slash-and-mulch, and diversified forest plantations. Improvement of agricultural soils can be based on lessons learned from the study of processes involved in the formation and maintenance of the rich "dark earths" (terra preta), which owe their high carbon content and fertility in part to high content of charcoal. Adding powdered charcoal combined with selected nutrients can increase soil carbon in modern agriculture. Considering that limitations to expansion of intensified land uses in Amazonia are serious, regional development should emphasize the natural forest, which can maintain itself without external inputs of nutrients. Instead of creating conditions to further expand deforestation, these forests may be used as they stand to provide a variety of valuable environmental services that could offer a sustainable basis for development of Amazonia. © Copyright 2009 by the American Geophysical Union