Morphology and Genesis of Podzols

Geographically, podzols cover probably the largest habitable area of the earth's land surface. They extend, according to the map of Glinka (5) , from the subarctic region, at the boundary line of forest vegetation, through the temperate zone, up to a few degrees north of the Mediterranean in Europe, to about the parallel 50° north latitude in Asia (at the Pacific coast it extends as far south as the parallel 40°) and North America (at the Atlantic coast it extends as far south as the parallel 40° north latitude). According to estimates of the Russian Bureau of Soils (3 ) , more than 10 million of the 21 million square kilometers of the area in U.S.S.R. are classified as podzolized. This does not include the close to a million square kilometers of forest steppe soils which are also a sub-type or transition type of podzols. According to Glinka (5) , the climatic conditions of the podzol zone, where a wide range of meteorlogical data is averaged, are: 500 to 570 mm. of rainfall and a mean annual temperature of 3.6° C. Within this zone any temperature rise is accompanied by a rise in the annual precipitation and vice versa—any temperature lowering is accompanied by a lowering in the annual precipitation. In other words, the net results of these two primary factors of climate, which include also evaporation and humidity, are a definite leaching process and a biotic environment characteristic of this climatic region, namely forests and meadows. One of the outstanding morphological features of a mature podzol profile is the bleached A2 horizon, and this is perhaps the reason why this zone has been studied much more than any other soil zone, save the chernozem. Another one widely discerned is the physical property of compactness in the B horizon with the characteristic coloration of the iron and humus which accumulate there, sometimes accompanied with concretions or ortstein formation. Of the physical properties which are typical for podzols, structure is outstanding. In mature podzols the A horizon is practically structureless, or of a loose structure which becomes platy towards the B horizon and nutty in the B. Color is also a distinguishing property of the podzol profile. The A1 horizon is gray or brownish gray with a sprinkling of white dots here and there. The A2 horizon is lighter in color, sometimes almost ashgray to white, which gives to the podzol profile the distinctive morphological characteristic spoken of above. When it comes to the B horizon the color changes again from a light reddish brown to a dark reddish brown, dependingon the relative quantities of iron and humus. Chemically, podzols are very distinct with respect to the distribution of the soil constituents in the profile. The A horizon is impoverished of bases and sesquioxides and relatively enriched with SiO2, whereas the B horizon is enriched with sesquioxides and some bases. The SiO2 content is lower in B than in A. The complex capable of base exchange—organic and inorganic—is partially unsaturated in all the horizons. There is a high coefficient of unsaturation in A, especially in A2, with a decrease in B. The total cation exchange capacity is high in A1, low in A-2, and high again in B. Biologically, the podzol horizons have not been investigated to any great extent, but from the meager data available and from the knowledge we have on the environmental conditions in the podzol profile, it is known that A2 is rich in fungi and poor in bacteria. On the other hand, A1 and B are lower in fungi, especially the B, and high in bacteria. Thus the fungi are sandwiched between bacteria, as pointed out by Williams (27) , who builds his speculative ideas about the podzol process on the theory of fungus activity and the formation of crenic and apocrenic acids, with the separation of silica gels, which in a large measure are responsible for the light coloration of the A2 horizon.