Harrow (tool)
Disc harrows are typically used for heavy work, such as following plowing to break up the sod. The discs have notched edges and are slightly curved to help them catch the soil and rotate. This helps mix the soil, instead of leaving ridges and trails behind.  In addition, there are various types of power harrow, in which the cultivators are power-driven from the tractor rather than working by being dragged.  Giant disc harrows can be used in place of a plow in rough areas with tree stumps and rocks.