s:2321:"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 nutrients
are efficiently cycled with very small inputs and outputs from the system. In most agricultural systems the
opposite happens. Agroforestry encompasses the continuum between these extremes and emerging hard
data is showing that successful agroforestry systems increase nutrient inputs enhance internal Øows
decrease nutrient losses and provide environmental beneÆts–when the competition for growth resources
between the tree and the crop component is well managed. The three main determinants for overcoming
rural poverty in Africa are (i) reversing soil fertility depletion (ii) intensifying and diversifying land use
with high-value products and (iii) providing an enabling policy environment for the smallholder farming
sector.  Agroforestry  practices  can  improve  food  production  in  a  sustainable  way  through  their
contribution to soil fertility replenishment. The use of organic inputs as a source of biologically-Æxed
nitrogen  together  with  deep  nitrate  that  is  captured  by  trees  plays  a  major  role  in  nitrogen
replenishment. The combination of commercial phosphorus fertilizers with available organic resources
may  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 more
rewarding economically in addition to diversifying and increasing rural incomes and improving food
security. In the most heavily populated areas of East Africa where farm size is extremely small the
number of trees on farms is increasing as farmers seek to reduce labour demands compatible with the drift
of some members of the family into the towns to earn off-farm income. Contrary to the concept that
population pressure promotes deforestation there is evidence that demonstrates that there are conditions
under which increasing tree planting is occurring on farms in the tropics through successful agroforestry
as human population density increases 
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