, 5th century AD In cooler climates, the most common types are the
disc harrow, the
chain harrow, the
tine harrow or
spike harrow and the
spring tine harrow. Chain harrows are often used for lighter work, such as leveling the tilth or covering the seed, while disc harrows are typically used for heavy work, such as following ploughing to break up the
sod. In addition, there are various types of
power harrow, in which the cultivators are power-driven from the tractor rather than depending on its forward motion. Tine harrows are used to refine seed-bed conditions before planting, remove small weeds in growing crops, and loosen the inter-row soils to allow water to soak into the
subsoil. The fourth is a chain disk harrow. Disks attached to chains are pulled at an angle over the ground. These harrows move rapidly across the surface. The chain and disk rotate to stay clean while breaking up the top surface to about deep. A smooth seedbed is prepared for planting with one pass. Chain harrowing can be used on pasture land to spread dung and break up dead material (
thatch) in the sward. Similarly, in sports-ground maintenance, light chain harrowing is often used to level off the ground after heavy use to remove and smooth out boot marks and indentations. Used on tilled land in combination with the other two types, chain harrowing rolls remaining larger soil clumps to the surface, where weather breaks them down and prevents interference with seed germination. All four harrow types can be used in one pass to prepare soil for seeding. Using any combination of two harrows for various
tilling processes is also common. Where harrowing provides a very fine tilth or the soil is very light so that it might easily be wind-blown, a
roller is often added as the last of the set. Harrows may be of several types and weights, depending on their purpose. They almost always consist of a rigid frame that holds discs, teeth, linked chains, or other means of moving soil—but tine and chain harrows are often only supported by a rigid towing bar at the front of the set. In the southern hemisphere, so-called
giant discs are a specialised kind of disc harrows that can stand in for a plough in rough country where a mouldboard plough cannot handle tree stumps and rocks, and a disc-plough is too slow (because of its limited number of discs). Giant scalloped-edged discs operate in a set, or frame, that is often weighted with concrete or steel blocks to improve penetration of the cutting edges. This cultivation is usually followed by broadcast fertilisation and seeding rather than drilled or row seeding. A
drag is a heavy harrow.
Power harrow A rotary power harrow, or simply a power harrow, has multiple sets of vertical tines. Each set of tines is rotated on a vertical axis and tills the soil horizontally. The result is that, unlike a
rotary tiller, soil layers are not turned over or inverted, which is useful in preventing dormant weed seeds from being brought to the surface, and there is no horizontal slicing of the subsurface soil that can lead to hardpan formation. ==Historical reference==