wings supported by individual wheels and hinge joints to allow flexing of the machine over uneven ground. The wider models usually have a wheel each side to control working depth. Three-point hitch-mounted units are made in widths from about . Cultivators are often similar in form to chisel ploughs, but their goals are different. Cultivator teeth work near the surface, usually for weed control, whereas chisel plough shanks work deep under the surface; therefore, cultivation takes much less power per shank than does chisel ploughing. Country plough The country plough is a slanted plough. The most common plough in India, it is recommended for crops like groundnut after the use of a tractor. Ridging plough A ridging plough is used for crops such as potatoes or scallions grown buried in ridges of soil, using a technique called ridging or hilling. A ridging plough has two back-to-back mould boards cutting a deep furrow on each pass with high ridges either side. The same plough may be used to split the ridges to harvest the crop. Scots hand plough This variety of ridge plough is notable for having a blade pointing towards the operator. It is used solely by human effort rather than with animal or machine assistance and pulled backwards by the operator, requiring great physical effort. It is particularly used for second breaking of ground and for potato planting. It is found in Shetland, some western crofts, and more rarely Central Scotland, typically on holdings too small or poor to merit the use of animals. Mole plough The mole plough allows under-drainage to be installed without trenches, or breaks up the deep impermeable soil layers that impede it. It is a deep plough with a torpedo or wedge-shaped tip and a narrow blade connecting it to the body. When dragged over ground, it leaves a channel deep under it that acts as a drain. Modern mole ploughs may also bury a flexible perforated plastic drain pipe as they go, making a more permanent drainor may be used to lay pipes for water supply or other purposes. Similar machines, so-called pipe-and-cable-laying ploughs, are even used under the sea for laying cables or for preparing the earth for side-scan sonar in a process used in oil exploration. Compacting a tennis ball-sized sample from moling depth by hand, then pushing a pencil through is a simple check to find if the subsoil is