Some soils can be recovered through the use of fertilizers, while others that are badly eroded, or that are fully covered by invasive species such as grasses or ferns, need more drastic rehabilitation techniques. There are also situations of extreme degradation where soils cannot be recovered at all. The recovery of the soil's productive capacity is frequently very expensive; thus the techniques used must produce financial returns in order to ensure their adoptability by the local farmers (Montagnini 2002).
Tree plantations and tree-crop combinations represent particularly desirable land-use alternatives for deforested lands with poor natural forest regeneration due to long distance to sources of propagules or intense site degradation. Among the latter, low soil fertility, soil compaction after abandonment from cattle grazing, and invasion by grasses and other aggressive vegetation can be serious obstacles to both forest regeneration and conventional agriculture (Lugo 1988; Nepstad et al. 1991). As the area in degraded lands spreads out, emphasis is increasing on the use of tree species that can grow in such conditions and yield economic products (timber, fuelwood, and other) as well as environmental benefits (soil conservation, watershed protection) (Evans 1992, 1999). An example of the impacts of trees on soil properties and their use in land rehabilitation projects in Latin America is presented in Box 6.13.
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