Saturday, 15 July 2006

The Peculiarities of Rendzina Genesis.

Mihail A. Kutrovskiy and Vladimir F. Val'Kov. Rostov State Univ, B. Sadovaya Str., 105, Rostov-on-Don, 344006, Russia

Rendzina soils originate from limestone and marle, and despite global bioclimatic diversity of nature, possess some common features, such as, for example, little absolute geologic age. All of them are formations of post-glacial period, in territories with intense denudation. Rendzinas with residual soil thicker than 10-16 ft (3-5 m) represented by eluvium of limestone rocks are hardly ever met. Territories are commonly well-drained, providing either constant water irrigation or deep penetration of water into soil layers. Soil-formation always includes sod-soil forming process. Then, depending on the bioclimatic conditions, a stage of humus-accumulating soils of different manifestation degree is typical. Humus-accumulating layer is formed out not only by means of herbaceous vegetation, but also exists in forests with no sod. Thus it should be assumed that humus-accumulating layer of rendzinas is formed out: a) in herbaceous cenoses, b) in forest cenoses with herbaceous cover, c) in forest cenoses with no sod (although participation of sod in the initial humus formation is impossible to deny). The biogeocenotic diversity of vegetation in different bioclimatic zones varies from boreal to tropical with various humidity conditions. Leaching and decarbonization are the principal conditions of eluvium formation and clayey mass accumulation, originally found in limestone and marle. Nevertheless limestone decarbonization cannot be considered the only way of forming the silica-alumina clayey part of carbonaceous rocks eluvium. Carbonates washing-out is too slow a process even in humid conditions. The estimation of soil-formation balance illustrates that mineral soil layer is formed not only due to carbonates dissolution and insoluble elements cumulation, but also by means of introducing silica-alumina products from outside. Important are the lateral and intra-soil migration ways of clay suspensions and colloidal solutions at any stage of rendzina development, as well as accumulation of atmosphere dust. The residual-eluvial and atmogenic origin of silica-alumina material in rendzinas is proved by stable heavy granulometric composition of the mineral part of soils. Sandy rendzinas are met in no bioclimatic conditions. Rendzinas are perhaps the only intrazonal type of soil formation which in its evolution may be included into the zonal soil formation, the soils of rendzic origin always featuring the traits of rendzinas. Soil formation in different natural zones goes along the following scheme: rendzinas of litosol type (AD) underdeveloped rendzinas (A+AD) typical rendzinas (A+AD+D) rendzinas formed out of rocks eluvium (A+AB+CD+D) rendzinas close to zonal residual-carbonate soils. Typical rendzinas based on eluvium may be leached and calcite-saturated at different degree. Nevertheless in humid and arid-humid conditions being leached from carbonates is the essential condition of transition to zonal type of soil formation. Rendzinas at any stage of evolution posses a unique ecobiological specificity of biocenoces and agricultural plants.

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