Loads applied to soil by agricultural machines can increase soil bulk density and reduce crop, production. Over 80% of a field may be trafficked at least once each year when producing corn or soybeans. When most field operations are performed, soil has a moisture content near field capacity and, therefore, is in a compactable condition. Changes in bulk density, cone index, and surface elevation caused by traffic were found to depend upon the tillage system used. Combine wheels depressed the soil surface from 1 to 4 cm. Reductions in growth rate and yield of corn indicate that tillage for seedbed preparation does not remove the effect of wheel tracks made during that tillage. Lighter machines, lower ground pressures, appropriate tillage, and controlled traffic can minimize soil compaction effects on crop production.