PODZOL FORMATION

Podzol formation. Podzols are acidic soils of forested areas character ized by the pedogenic accumulation of iron (Fe) and aluminum (Al) in t he B horizon (podzolic B horizon). Podzols cover large areas of the we t temperate and boreal zones and are associated to coarse parent mater ials. Knowledge on podzol genesis is of considerable interest with res pect to the understanding, modelling and management of forested, agric ultural and aquatic ecosystems and to the forecasting of soil changes in response to environmental perturbations. Four theories, or conceptu al models, were formulated to describe podzol formation : the fulvate theory (Stobbe and Wright, 1959; McKeague at al., 1978), the proto-imo golite theory (Farmer at al., 1980; Anderson at al., 1982), the theory of chemical compartments (Ugolini and Dahlgren, 1987) and the Al poly mer theory (Browne, 1995). Each theory emphasizes the role of specific processes such as the formation and transport of organo-metallic comp lexes, proto-imogolite synthesis and imogolite precipitation, mineral weathering by carbonic acid (H2CO3) and the formation of polynuclear A l-silicate soluble complexes. However, the relative importance attribu ted to any of these processes varies as a function of the properties o f the sites explored by the authors of a given theory. The absence of an integrated theory of podzol genesis also reflects our incomplete un derstanding of podzol dynamics, the fact that important processes have been left out and the apparent opposition between theories.