s:2273:"TI Trees, soils and food security 
AU Sanchez P A J 
AU Buresh R J 
AU Leakey R R B 
AB Trees have a different impact on soil properties than annual crops, because of their longer residence time,larger biomass accumulation, and longer-lasting, more extensive root systems. In natural forests nutrientsare efficiently cycled with very small inputs and outputs from the system. In most agricultural systems theopposite happens. Agroforestry encompasses the continuum between these extremes, and emerging harddata is showing that successful agroforestry systems increase nutrient inputs, enhance internal Øows,decrease nutrient losses and provide environmental beneÆts–when the competition for growth resourcesbetween the tree and the crop component is well managed. The three main determinants for overcomingrural poverty in Africa are (i) reversing soil fertility depletion, (ii) intensifying and diversifying land usewith high-value products, and (iii) providing an enabling policy environment for the smallholder farmingsector. Agroforestry practices can improve food production in a sustainable way through theircontribution to soil fertility replenishment. The use of organic inputs as a source of biologically-Æxednitrogen, together with deep nitrate that is captured by trees, plays a major role in nitrogenreplenishment. The combination of commercial phosphorus fertilizers with available organic resourcesmay be the key to increasing and sustaining phosphorus capital. High-value trees–` Cinderella 'species–can Æt in speciÆc niches on farms, thereby making the system ecologically stable and morerewarding economically, in addition to diversifying and increasing rural incomes and improving foodsecurity. In the most heavily populated areas of East Africa, where farm size is extremely small, thenumber of trees on farms is increasing as farmers seek to reduce labour demands, compatible with the driftof some members of the family into the towns to earn off-farm income. Contrary to the concept thatpopulation pressure promotes deforestation, there is evidence that demonstrates that there are conditionsunder which increasing tree planting is occurring on farms in the tropics through successful agroforestryas human population density increases 
";