The effect on soil porosity of tractor compaction of soil in the spring was studied by taking cylindrical core soil samples. The profile samples showed that the tractor most seriously compacts the soil below the harrowed layer at the depth of 10-25 cm. Soil was compacted most severely when till age and drilling were performed under wet conditions about one week be for enormal sowing time. The sub soil at the depth of 35-40 cm was compacted only under very wet conditions. The grain yield of wheat was significantly reduced when the volume of large pores was reduced to about 10 % or less. Porosity measurements showed that the severely compacted soil almost completely recovered from one spring to the next. Theoretical calculations suggested that compaction by normal traffic does not cause a shortage of oxygen at least in the inter-crumb pores of soil if the soil surface structure is not dispersed and encrusted. The decrease in crop growth by compaction is primarily due to mechanical impedance, which slows down development of the root system.