Russia

Remarkable features of tundra soil are the "polygon" or "me dallion" type and the "spot" type. The former, tesselated soils, are due to cracking of the surface layer during drying with conse quent drainage of water along the cracks, removing fine particles and sometimes depositing stones, so that sandy and occasionally stony streaks separate the polygons. "Spotty" tundra may be due to compression of the semi-liquid mass of earth between the frozen sub-soil and the newly freezing surface; this semi-liquid mass breaks through to the surface and forms spots deprived of vegeta tion; in other places it is apparently due to the action of wind. Peat formation is observed at the highest latitudes in the Arctic regions ; except where the ice and snow cover is permanent. The frozen sub-soil and the severe climate make agriculture impossible here and the tundra is the land of the wild reindeer and the nomad reindeer breeder, who takes his herds to the south during summer and to the north during winter and supplements his dependence on the reindeer for food, transport and clothing by a little fishing in the summer and by occasional hunting. Huts erected on this frozen sub-soil sometimes cause thawing with a development of a consequent upward fountain, the water of which may freeze into solid ice. Remains of birches and spruces have been found beneath peat hillocks on the tundra.

The forest stretched much further northwards at some post glacial stage (the "Atlantic" phase in western Europe), when the climate was warmer than it is now, but the advance of tundra on forest has been encouraged by human destruction of the thinner fringe of "island" forest to the north, which helped Sphagnum bog to swamp the forest fringe. Towards the west, under the influence of the Atlantic, the tundra belt is narrow ; it increases in width towards the east under the influence of the continental Asiatic winter, and thrusts much farther south in Kamchatka, partly be cause of the height of the land and partly under the influence of cold ocean currents from the north ; oceanic influences thus playing an entirely different role in the east. South of the tundra bog begins a zone of scattered dwarf spruce and birch trees, transitional forms to the trees of the taiga zone. In Asiatic Russia a belt of vast marshy plain covered with dwarf birch lies south of the tundra belt. Along the river valleys, better protected from the wind, which induces winter evaporation and thus hinders their growth, and also better drained so that sub-soil ice is not so near the surface, the forest pushes northward. In the immediate vicinity of the streams, forest is denuded by the blocks of ice brought down in spring.

Podzol.

The coniferous forest (taiga) belt and much of the mixed coniferous and deciduous belt are characterised by pod zolised soils. Podzol or ash-coloured soils are developed in condi tions where the moistening of the upper soil horizons is sufficient to ensure the percolation of water into deeper horizons, and where there is sufficient, but not excessive evaporation so that the per colating solutions are drawn upwards. Forest conditions, which prevent rapid or excessive evaporation and which at the same time ensure moistening of the soil are peculiarly favourable to podzol formation, though this type of soil has also been found in meadow and meadow-moor regions. Owing to strong transpiration, the sub-soil under forest is drier than that under herbaceous vegetation, and the level of sub-soil water is therefore lower under forest. At the same time the upper horizon under forests is mois tened by better preservation of the snow covering and by lower evaporation under the shade of leaves and forest bedding.

The main morphological characteristic of podzol is the presence of two horizons, an eluvial upper horizon A, which is comparatively deprived of colour and leached, with sandy particles, and an under lying eluvial horizon B, more intensely coloured, having an in creased clayey content and chemically enriched with hydrates of sesqui-oxides and with humus. The upper horizon may be divided into a layer containing undecomposed organic matter (e.g., hyphae of fungi), a greyish humus layer and a whitish horizon almost de prived of humus and enriched with silicic acid. The B horizon may have an upper coffee-coloured layer, with precipitated humus substances and a lower more yellow or rust-coloured layer with precipitation of iron hydroxide. This horizon may sometimes con sist of ortstein, a compact indurated ferro-humus layer. The pro cess of leaching to which the upper horizon of the soil is subjected is known as podzolisation and varies in degree, sometimes being indicated by almost imperceptible spots and at the other extreme by a compact whitish streak of a schistose character. The colloidal nature of humus plays an important part in the development of podzolisation. The type of parent rock is also important, the mor phology of clayey, sandy-loamy and sandy podzolised soils vary ing greatly. The exact processes of podzol formation are still a subject of dispute; for an account of various theories see S. S. Neustruev, "Genesis of Soils" 1927, Pt. III. of Russian Pedological Investigations. This podzol zone is very extensive and broadens out in Asiatic Russia so that it reaches from the tundra in the north to the plateau region in the south.