A special experiment was established in 1963 and initiated in 1964 at Eerika, Tartu County, Estonia (58°22’ N, 26°36’ E) to study pedogenesis and its continuous development under grass-herbaceous vegetation on reddish-brown calcareous till which was practically free from organic carbon (0.6 g kg–1) and nitrogen (0.2 g kg–1). The results of the study of three earlier decades have been discussed earlier. This paper deals with the processes of synchronous production and soil formation as well as with pedogenetic activity during the fourth decade of the experiment and during the total period of 40 years. An intensive humus-accumulative process, wavy and cyclic in intensity, has continued, accompanied by the breakdown of skeletal carbonates, partial leaching of products, formation and accumulation of amorphous and crystalline nonsiliceous products of weathering, progress of argillization in situ, and slight lessivage of fine silt and clay within the thin top of enriched humus solum. Net accumulation of organic carbon and nitrogen was obtained by nearly equivalent amounts of humifiable issues of the production process. As these are temporally dynamic, the temporal periodicity of mineralization and humification relationships is also characteristic of synchronous pedogenesis. The low C : N ratio indicates an excellent quality of the humus formed since the beginning of primary soil formation. Against the background of the decadewise dynamic fulvicity of the humus and evident decrease in its total solubility, the transformation of Ca-humates into humins and the formation of R2O3-humic-fulvic complexes at the expense of RO-humic-fulvic complexes already during the third decade were ascertained. Intensification of the bonds of the humic-fulvic complexes with inactive sesquioxides and clay minerals and decrease in the amount of fulvic acids in the interlayeral structure of clay progressed during the fourth decade. Due to the weathering of sand fractions, an accumulation of silty-clayey particles, accompanied by an increase in the cation exchange capacity and in the specific surface area, progressed in the formed soils. The progress of Calcaric Cambisol (Rendollic Eutrochrept) can be diagnosed on the basis of data obtained during 40 years.
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