The podzol, or ashy soil, was defined at the end of the last century by the Russian school, under the leadership of Dokuchaev, as a zonal soil characteristic of the boreal zone of the taiga, i.e. of the coniferous forests with ericaceae. The profile consists of three horizons very different in their colour, morphology and other properties: the AO horizon is a brown or black mor or raw humus, the A2 horizon is ashy and without structure, and the B horizon is strongly coloured by the accumulation of amorphous organic and mineral compounds (particularly hydroxides of aluminium and iron); this is the spodic horizon of the American classification. The way in which this soil develops can be summarised quite simply: the very active organic compounds produced by the litter and the mor cause the silicates, including certain clay minerals, to weather biochemically by complexolysis (Razzaghe-Karimi 1974, Razzaghe-Karimi & Robert 1975, see Ch. 1); all the complexed materials migrate leaving a residue of fine quartz below the mor (the ashy A2 horizon) and accumulate in the B, forming the spodic horizon (or more often, horizons) when both a dark humus-rich Bh and a rusty coloured, sesquioxide-rich Bs horizon are formed.
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