Russia

In the Bryansk district, on the cretaceous marl and phosphorite sand, near the southern limit of the coniferous forest, spruce is found with an admixture of oak, Norway maple, poplar, birch and elm. Along the river valleys of the steppe are interesting south ward-extending strips of forest, oak growing on the high right bank and Scotch pine on the low sandy left bank. On some soils where sand lies above clay, the spruce, which is tolerant of shade, grows among the taller pines. Pine forests occur on loess covered chalk on the right bank of the Volga and in Voronezh, Kursk and Khar kov, and with them are certain rare plants, possibly relics of the Ice Age. A general characteristic of the oak is that it does not tolerate complete podzolisation and requires greater warmth than spruce, thus it penetrates north into the coniferous zone along river valleys where the soil is warmer and less podzolised and gives way to spruce on the interfluvial clay-lands. The earliest agricul tural development in Russia took place in the region of the heavy clay soil of Moscow, as the first period of relying on forest wealth for trade gave way to dependence on local cultivation of crops in view of the closing of the trade channel to the Black Sea and Caspian by Mongol invaders. Agriculture involved forest clearing by cutting or burning, more frequently the latter because the charcoal helped to make the soil productive. Before a good yield could be obtained, many years of careful cultivation and manuring were necessary, and the population constantly tended to drift to the more fertile black earth region, while of late years the population on the clay soils (podzol) has been more and more diverted from agriculture to industry.

Cereals, chiefly barley, rye and oats, with flax and hemp, pota toes and cabbages and recently grass and lucerne are the chief crops. Some wheat is grown, but not so successfully as farther south. Intensity of cultivation thins out markedly to the north and in Siberia hardly exists, the long severe winter and the liability to recurrent frost in spring being unfavourable. Timber exploita tion and hunting of fur-bearing animals are of ancient importance. Cattle breeding and dairying are developed in many parts. Human settlement in the northern hunting, fishing and forest-products belt is usually along the river valleys and the shores of lakes and seas, the peat bog watersheds being uninhabited. On the glacial soils of the central watershed the valleys are humid and cold and settle ment is on the warmer, drier morainic hills, where the season for ripening crops is 10 days longer than in the valley bottoms. The podzol zone of Asiatic Russia is sparsely peopled in view of the severity of climatic conditions, and averages one inhabitant per square kilometer. This is a misleading way to put it since settle ment is usually concentrated on the alluvial river strips and vast expanses of virgin forest have no settlements. Hunting and fishing are the main occupations, with cattle-breeding in the bog meadow belt lying east and west of the great bend of the Lena, on the alka line podzol. Berry-bearing plants are found everywhere in this

zone, whortleberry and arctic bramble spreading far into the north, while raspberries and currants form a luxurious undergrowth in many coniferous forest regions.

Black Earth.

The chernozyom (black earth) formations lie south of the podzol forest zone. Their origin was a subject of much discussion in the early days of soil study, though Glinka states that the Russian peasant had long ago arrived at the conclusion now generally held that chernozyom has been developed through the modification of an original material by the decay of animal and vegetable remains. Dokuchaiev in The Russian Chernozyom (1883) enunciated the view now generally accepted. Owing to the wide extent of chernozyom on loess, loess-like loams and loess like clays it was once thought that chernozyom was a humus coloured loess. But chernozyom is now known to occur on granites and andesites, on chalk, on Permian, Jurassic and other limestones, on Upper Permian marls, on Tertiary siliceous clays, and even on sands and sandstones, provided that the latter are rich in complex minerals.

The essentials for the formation of chernozyom are high evap oration with consequent desiccation of the soil during summer (high temperature with a high saturation-deficit) and long freez ing of the soil in winter. During winter, water accumulates in the soil and provides for luxuriant spring vegetation, this moisture being exhausted by the end of May or the middle of June. The resulting drought retards the decay of the steppe grass (stipa), whose strongly developed root-systems provide abundant humus. The essential characteristic of chernozyom is its rich humus con tent, thick and rich chernozyom yielding zo to 13% or more in southern latitudes, and 6 to o% in northern latitudes, 6 to o% is also the average content of ordinary chernozyom, while northern and southern chernozyoms yield 4 to 6%. The map shows that the central belt has the greatest humus content and that there is a decrease to north and south. During the intense summer evapora tion there is a powerful ascending current of water in the soil, gypsum and calcium carbonates being deposited in separate horizons.

Morphologically a chernozyom profile consists of an upper hu mus horizon and a horizon of carbonate accumulation. "Thick" when applied to chernozyom denotes thickness of the humus hori zon, though the limit of the underlying horizon is sometimes hardly distinguishable owing to its containing wedges, spots and veins of humus. It should be noted that chernozyom is found on mountain slopes (e.g., in Turkestan and Trans-Caucasia), the height at which it will occur being determined by the climatic conditions at the foot of the mountain. If forested podzol occurs in this region, no black earth will exist on the mountain, but if the mountain rises from desert or desert steppe, chernozyom will occur on the mountain provided the altitude is sufficient.