EVOLUTION MODEL OF PEDOGENESIS

This comprehensive model of soil genesis is based on the observation that soils (S) evolve along two coacting pathways: progressive (P) and regressive (R). For all soils S=f(P, R). The progressive pathway includes conditions, processes, and factors that promote horizonation, developmental (assimilative) upbuilding, and/or subsurface deepening. The regressive pathway includes conditions, processes, and factors that promote haploidization, retardant (nonassimilative) upbuilding, and/or surface removal. The coacting P and R pathways may alternately predominate, one over the other, as a soil evolves. When one pathway predominates, component processes of the other pathway continue to function, but at subordinate levels. The soil evolution model captures the polygenetic character of soils and integrates the multiple processes of pedogenesis. The model also synthesizes the factorial/ functional, systems/process, and energy/ dynamic approaches to pedogenesis. Its utility as a research and practical tool is demonstrated by comparing the evolution of two hypothetical soils, from the late Pleistocene to the present. These soils represent compilations of documented morphologies,-profile attributes, and pedogenic processes. The examples underscore the concepts embodied in the model that all soils evolve and thus are polygenetic by degrees, but the degree of polygenesis may or may not be expressed by morphology.