Soil compaction occurs when soil particles are pressed together, reducing pore space between them. Large pores are more effective at transporting water downward through the soil than smaller pores. Heavily compacted soils contain few large pores and less total pore volume resulting in greater density. Compacted soil has a reduced rate of both water infiltration and drainage. In addition, the exchange of gases slows down in compacted soils, causing an increase in the likelihood of aeration-related problems. Finally, while soil compaction increases soil strength, (the ability of soil to resist being moved by an applied force), a compacted soil also means roots must exert greater force to penetrate the compacted layer.