Trees impact the chemical properties of the soil in which they grow. When planting forests, the choice of tree species and species combinations is thus expected to partly determine the spatial variation in soil properties, even in the early stages of forest development. We made use of a biodiversity‐ecosystem functioning experiment in Belgium, FORBIO (FORest BIOdiversity and Ecosystem Functioning). The studied site is situated on former agricultural land on loamy sandy soil and replicates tree species richness (1–4 species) and composition. Soil samples (0–10 cm) were taken at five locations in 1, 2 and 4 species plots. We measured the total C and N concentration, the Olsen P, the pH‐H2O and the available Ca, Mg and K concentration. We hypothesised that the within‐plot spatial variability would increase with species richness and that the between‐plot variability would decrease with species richness. We found little evidence to support our hypothesis. We only found a smaller within‐plot variability in monocultures than mixtures for Ca, pH and C:N. Potential reasons for the little evidence for the effect of tree species richness may include the fact that the forest is only 9 years old, the agricultural land‐use history and the soil texture. Further research in sites with different soil properties or different land‐use history is needed to generalise these results. Highlights: Variation in soil chemistry was studied in young forest plots varying in tree species richness.Larger within‐ and a smaller between‐plot variation in mixtures than monocultures was expected.Larger within‐plot variation in mixtures was only found for for Ca, pH and C:N.Little effect of tree species richness may be due to young age, agricultural land‐use history or soil texture. [ABSTRACT FROM AUTHOR]