Studies that have carried out made it possible to objectively and with an excellent spatial resolution to assess the agroecological state of automorphic soils in several districts of Yaroslavl oblast from 1986 to 1990 and identify its changes by 2019–2020. In general, agro–sod–podzolic soils were characterized by low humus content and a low or medium supply of nutrients and were medium to weakly acidic. The ranking of soils according to the points of the soil ecological index (SEI) score showed that, in the groups with a lower score, the climatic (C) component made a more significant contribution. In contrast, in the groups with a higher score, the shares of SEI agrochemical (A) and soil (S) components increase. In the Poshekhonsky and Breitovsky raions, the proportion of soils with an average SEI score from 20 to 39 was higher, and in the Uglichsky raion, located in the south, it was from 30 to 49. Based on the principal component analysis of an extensive array of empirical data, it was discovered that the set of initial variables used in the SEI fertility model is statistically valid, allowing it to cover regional pedo- and litho-diversity. The variability in the group of variables used to calculate the A and S components of SEI is better explained by the lithological factor than by the study area and the degree of water-logging. Transparent degradation processes in arable–arable chronosequences were identified and expressed dehumification, an increase in potential acidity, a decrease in the content of mobile forms of phosphorus and potassium, and the appearance of gleyey differences, which led to a decrease in the SEI values. There was no unified trend regarding humus content in the arable-soil sequence on loamy deposits. Still, a clear tendency towards acidification, depletion of mobile forms of phosphorus and potassium, and the development of hydromorphism was diagnosed. In the vast majority of cases, this caused a decrease in the final score of SEI. At the same time, an increase in humus content, a reduction in the range of nutrients, and the absence of a trend concerning potential acidity were noted in the chronosequences of arable land-fallow land on sandy loamy varieties. The specifics of postagrogenic evolution and the emergence of new soil properties should be considered when performing a soil-agroecological assessment and forecasting the agroproductive potential against the background of changed climatic conditions.