A network of 137 cultivated fields covering the wide diversity of soils, crop rotations and cropping practices throughout the region of Brittany (France) was monitored to collect data on soil organic nitrogen (SON) mineralization and to identify the factors that explain the observed variability. The dataset presented in this article contains all of the information about the soils, which were subjected to pedological description and in-depth analysis of their topsoil properties. The topsoil (0-30 cm) was sampled by mixing 30 samples to obtain one composite per field, which was divided into one sub-sample sieved at 5 mm to analyze soil microbial biomass (SMB) and SON mineralization via anaerobic incubation, and one subsample dried at 40°C and sieved at 2 mm. The physico-chemical analyses included the particle-size distribution of five fractions; organic matter (OM); organic C; organic N; pH; pH KCl; CEC (Metson); CEC (cobaltihexammine); Al, Ca, Fe, K, Mg, Mn, Na échangeables à la cobaltihexammine; Olsen P; Dyer P; and total Al, Ca, Fe, K, Mg, Mn, Na and P. Physical OM fractionation was used to characterize the 200-2000 µm and 50-200 µm fractions of particulate organic matter (POM). Finally, three chemical methods were used to determine extractable organic nitrogen (EON): hot KCl, hot water and phosphate buffer tests. This dataset covers a wide range of pedological situations and cropping systems, and is of great interest to scientists searching for soil properties that can explain SON mineralization. It provides original data on EON indices, SMB and multiple forms of P