A time series was constructed of farm fields that were converted from conventional to organic farming. This time series covered more than 20 years of land-use transition to organic farming. In this time series, organic fields on clay had more labile organic carbon than conventional fields, especially in soils with low iron oxide content. Fungal diversity increases, and protist diversity decreases. In a long-term experiment, microbial activity in organic fields was higher than in conventional fields, especially in mid-summer months. The multi-functionality decreased with increasing land-use intensity.