We investigate adsorption isotherms of nitrogen at 80 K on several peat soils. In addition to natural soil samples we also study samples thermally treated at 50, 100, and 150 degrees C. The experimental adsorption isotherms are used to evaluate the surface fractal dimension and the energy distribution functions. Moreover, for some samples we have also determined the pore size distributions from mercury intrusion data. We compare the surface fractal dimensions evaluated from the mercury intrusion data and from adsorption isotherms and discuss how the thermal treatment changes the energetic heterogeneity of the samples.