The largest area of land in Russia is located in the fallow state, there is a change in plant communities, physico-chemical parameters of soils and changes in the enzymatic activity of soils. To analyze the condition of fallow and undisturbed soils, we studied different-aged changes in the main physico-chemical parameters of soils, analyzed the features of morphological structure of soils, and also studied the enzymatic activity of soils of such classes of enzymes as hydrolase and oxidoreductase. Sampling was carried out from the upper humus-accumulative horizons of 13 soil sections of the Leningrad and Novgorod regions of Russia. As a result of research, it was revealed that transition of lands to fallow state leads to transformation of soils towards zonal series of soils. Soil transformation is accompanied by a decrease in pH value, content of biogenic elements, with an increase in the content of carbon and biogenic elements in old-age plots. The study of enzyme activity in soils showed that the activity of the studied enzymes at different sites varies differently, depending on land use. Significantly higher activity of oxidoreductases class was noted for soils in which transformation of wood residues takes place and O horizon is formed. A comparative assessment of the biological activity of the studied soils was given using the indicator of total relative enzymatic activity (indicator representing the total biochemical activity of soil based on enzyme analysis). According to the comparative assessment of soil biological activity, it was found that the biological activity increases with increasing time of soils being in fallow state. Thus, to restore soil biochemical activity and agroecosystem stability, long (30-year) fallow periods with secondary forest formation should be maintained, which provides neutral pH, organic carbon accumulation, and maximum enzymatic activity superior to both recently abandoned and arable lands.