Soils differ in ecologically significant properties (drainage, nutrients content etc.) within a plant community. It seems obvious that soil diversity should be connected with plant species diversity. Nevertheless, methods to estimate this connection are not worked out yet. The objective of our study was to evaluate surface cover plant diversity and soil diversity for two small catchments in Finland, previously mapped in FAO-Unesco soil classification system, and to find out correct criteria for their connection evaluation.

Simpson's and Shannon's diversity coefficients, relative abundance curves proposed by Whittaker (1965), and cumulative profiles of comparative abundance (Swindel, Grosenbauch, 1988) were used to estimate biodiversity of plant communities of the two catchments. The same methods were used for "pedodiversity" evaluation, as proposed by Ibanes (1996). Soil diversity was also qualitatively estimated in classical soil cover structure terms (Fridland, 1972).

The results obtained show no connection between biodiversity and "pedodiversity", evaluated using diversity coefficients and abundance curves. It seems that (-diversity (Whittaker, 1975), i.e. richness in species, reflects more climate and soil fertility, than soil cover structure. Soil diversity really affects (-diversity, i.e. non-uniformity of the plant cover, which is not reflected by diversity coefficients.

When soils were divided into ecological groups according to their drainage, the Shannon's coefficient appeared to be equal for two cathments, although the rate of moderately drained ( intermediate) soils was much higher for more diverse catchment. The rate of extremely dry, wet, and intermediate soils seems to be a reliable criteria for potential diversity of surface vegetation in North European forests.