The potential of agroforestry as a practical means of sustaining soil fertility

The maintenance of fertility through soil biological processes requires management practices which control the quantity, quality and timing of decomposition of plant residues. Agroforestry is one of the major practical managment methods which have the potential to bring this about. It is widely applicable, through many different kinds of practice, has substantial effects on the soil, and is relatively free from supply constraints. Trees restore soil fertility through their potential to increase the supply of organic materials and nutrients, to reduce nutrient losses, and to control the quality and timing of inputs. A first approximation to a plant/soil organic matter cycle under agroforestry is presented, showing that if the assumptions on which it is based can be verified, agroforestry has the potential for the design of land use systems that combine production of annual crops with maintenance of soil fertility. The significance of work on different fractions of organic matter, and on root residues, is discussed; these studies suggest benefits from mixtures of woody and herbaceous litter. Results of work on soil rest period (fallow) requirements are summarized; it has yet to be shown whether spatially-based agroforestry systems can be more efficient, in terms of land requirements, than crop/fallow rotations. Current research in agroforestry suffers from over-emphasis on a pragmatic approach at the expense of an understanding of basic processes. Research is needed on the decomposition of woody and herbaceous residues, singly and in combination, and their effects on soil properties

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