Figure 2.2 represents a spatial hierarchy of agricultural systems within which farmers' decisions are made. The highest level in the hierarchy, supraregional systems, occupies the largest land area and can transcend national boundaries. Macroeconomic processes, as well as certain geological processes, are best understood at this level. The lowest level (soil systems) covers the smallest spatial unit and is the level at which specific biological processes such as nutrient uptake may be investigated. A basic rule in systems theory is that systems at level n are constrained and controlled by systems at level n + 1, and in turn they constrain systems at level n - 1 (Allen and Starr, 1982).
Farmers (operating at the farming system level) thus have to take the environment provided by the village or landscape level as a constraint in their decisions regarding soil fertility practices. Likewise, the village or landscape scale is constrained by regional system variables that operate themselves within the confines of supraregional variables. Consequently, farmers integrate a wide range of ecological, social and economic parameters belonging to levels higher than the farming system in their decisions to adopt soil fertility management and agroforestry practices.
Decisions made at the farming system scale have repercussions at the same scale, as well as at lower and higher scales in the hierarchy. These are mediated through various economic and biological processes, such as nutrient cycling and the market mechanism. Because these processes transcend farm boundaries, it is helpful to establish a distinction between the economic processes that occur at the farm scale and those that are manifest at the landscape or watershed and global scales. Even though soil fertility management and agroforestry practices are very localized interventions on farmers' fields, it is essential for scientists to realize that the key processes of relevance to their adoption occur at the farm, regional and global scales.
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