Hardening of the soil, whether due to man-made or natural causes, frequently restricts root growth through mechanical resistance to root extension. As a result, it is common to find a normally deep-rooted plant having a shallow root system. When this happens, the plant cannot effectively use moisture and nutrients stored in the sub-soil, and drought hazard to the plant is greater. Compacted subsurface layers, termed pans, are present almost without exception in cultivated land and are formed by the passage of tillage implements and heavy machinery over the soil. Subsequent tillage loosens the ploughed layer but leaves an abrupt boundary between the ploughed soil and the compact layer beneath it. The strength of compacted zones is influenced as much by the moisture content of soil as by its degree of compaction. The force required to penetrate wet soil is often increased fivefold or more after drying. While high-strength soil layers can stop root growth completely, it is more common to find root proliferation retarded by intermediate levels of resistance.