When determining the stability of slopes, engineers use the parameters required by the calculation model. The most important properties of soil are its fric­tion angle, density and cohesion. lf a steep slope is covered with vegetation or even a century-old forest on an embankment, the values of these properties will change, due to the climate, relief and many organisms associated with the vegetation. lt is these factors that make soils as described in soil science, which examines the processes of soil formation and descrbes many parameters other than its physical properties. Soil' s physical properties mainly apply to fine soil, with a particle size of less than 2 mm. Larger components are referred to as skeleton. In geotechnics, fractions smaller than 2 mm are described as sand, silt or clay, whereas larger components are called gravel. The differences between them mainly have to do with their quantitative characteri­sation: soil scientists use volume-based percentages and geotechnicians mass-based percentages. These two values cannot be compared directly, but also depend on the soil's porosity and density as well as on the density of its solid substance. This article provides practical examples of the relationships between the various parameters and highlights the major differences between results obtained from a soil science point of view and from a geotechnical perspective. The physical properties of the forest soils presented here mainly relate to their density, porosity, water content and particle size distribution in the Swiss Prealps.