Changes in soil organic matter (SOM) content are presented for a 25 ha research farm, based on measurements in 1952, 1976, 1986 and 2002, and for 291 arable fields located throughout southeast Norway, which were sampled in around 1991 and again in 2001. Large declines were found in most cases at the research farm, especially for soils with high initial levels of SOM. The declines between 1952 and 1976 were thought to be partly due to increased depth of ploughing and improved drainage, while those in the latter period may in part be related to climatic change. Declines were also found since 1990 in the majority of the arable fields sampled, and the declines were proportional to the initial SOM level in this case also. No single causative factor was isolated for these declines, but the proportions of autumn tillage and of row crops in the rotation may be involved. A mean relative decline rate of around 1% per year of the initial SOM level was observed both at the research farm and in the arable fields. There was, however, also an indication that the percentage relative decline rate was proportional to one-tenth of the initial SOM. These two alternative scenarios suggest that soils with current SOM levels of 4%, 6% and 10%, will reach a possibly ‘critical’ level of 3% in 30–80, 70–165 and 120–230 years time, respectively.