The recent increase in conservation tillage offers the possible benefit of decreasing soil compaction risk by wheeling. Excessive compaction has damaging consequences for agriculture and the environment. Direct drilling is likely to change soil porosity and soil carbon content in the long term. This paper analyses the effect of both of these factors on soil mechanical strength through measurements of the compression index C-c, the swelling index C-s, and the pre-compression stress p(c)*. Oedometer tests were performed on remoulded soils taken from an experiment with a wheat-maize rotation, where three soil tillage systems were compared over 31 years: annual mouldboard ploughing, superficial tillage (10 cm depth), and no tillage. The results show that initial structural porosity (i.e. macro-porosity) increased C-c slightly and decreased p(c)* exponentially. Consequently, the risk of compaction in the direct drilling systems should decrease when a decrease in soil porosity is found, compared with conventional systems. However, this change in soil porosity in direct drilling is not always observed. The other variables that can be influenced by tillage system, i.e. carbon content or soil moisture, have also been examined to assess the possible benefit of direct drilling on soil compaction risk. For soils with similar initial structural porosity, our results show that the increase in carbon content of the superficial soil layers in direct drilling systems tends to increase the soil's susceptibility to compaction by increasing C-c in wet conditions and by decreasing p(c)* in dry conditions. Moisture conditions at wheeling determine the degree of soil compaction as a function of tillage system.