Assessment of forest land in general, and in Republic of Karelia in particular, is a challenging task because its soil cover is extremely motley and patchy due to the highly dissected terrain, frequent alternation of parent rocks and the tree stand structure.

The soil cover in Karelia is made up of a wide assortment of soils of different genesis. Most wide-spread are podzolic soils – they occupy about a half of the territory. Under automorphic conditions of middle taiga, there occur soils of the brown-earth soil formation type, which are azonal for the territory, and form under the influence of the specific mineralogical composition of parent rocks. A fifth of the territory is under peatland soils, raised-bog and transitional types prevailing. Edge zones between mires and dry areas are occupied by podzolic peatland soils. Unique for the republic are alluvial marsh soils on the White Sea coast, as well as dark coloured brown earths over shungites on the north-western shore of Lake Onego. Forest soils of Karelia are mostly coarse-textured, highly acidic, with low biological activity and low content of nitrogen compounds. Phosphorus and potassium availability in the soils is highly variable, depending on the mineralogical composition of parent rocks.

Data on soil characteristics and the productivity of pine and spruce stands were statistically treated to determine soil diagnostic horizons and their parameters. The closest correlation with the tree stand productivity in middle taiga has been found for gross nitrogen content in the forest floor, and in northern taiga – also for potassium availability in the solum. Multifactor models have been built to describe relationships between tree stand productivity and highly significant parameters of soil fertility.

Comprehensive information about the genesis and fertility of soils, the soil cover structure in forests, in situ studies of correlations between forest site conditions and stand productivity were used as the basis for developing the system for typological assessment of forest soils. All soils have been divided into 20 classes by stand productivity and clustered into 5 groups by forest site conditions. The highest fertility scores were given to dark coloured brown earths over shungite-bearing rocks (100), the lowest – to primitive soils over acidic crystalline bedrock outcrops (10). The average score for the most wide-spread podzolic soils is 55, with variations from 30 to 80.

The typological scale of soil fertility and the 1:500 000 genetic soil map developed in our studies permitted a first map of forest soil fertility to be compiled for the republic and a first zoning of the territory by forest growth conditions to be carried out. As the result, 26 districts differing in the soil cover fertility were singled out.