Evaluation of soil fertility in smallholder agroforestry systems and pastures in western Amazonia

Agroforestry systems are often mentioned as a type of sustainable agriculture that is appropriate for the edapho-climatic conditions of Amazonia. However, long-term studies on the sustainability of this land use do not exist. This study evaluates the soil fertility in smallholder agroforestry systems in Western Amazonia, and compares the fertility of land under agroforestry with the fertility of neighboring land under pasture and native forest. Smallholdings located in Nova California, Rondônia, Brazil, were selected, with two different soil types. Soil pH, organic C, P, K, Ca, Mg and Al were determined. The soils of the agroforestry system maintained their improved chemical characteristics that originated in the burn, especially with respect to increased levels of exchangeable Ca and Mg and reduction of exchangeable Al, while maintaining stable levels of organic C, even when compared to adjacent primary forest soils. However, in the agroforestry system K and P fell to extremely low levels, below those considered to be critical for Amazonian soils. This reduction can reasonably be attributed to nutrient exports by consecutive harvests of cupuassu (Theobroma grandiflorum, Sterculiaceae) and pejibaye (Bactris gasipaes, Palmae) fruits. These nutrients can limit sustainable fruit yields in the agroforestry systems.