A plough or plow (US; both ) is a farm tool for loosening or turning the soil before sowing seed or planting. Ploughs were traditionally drawn by oxen and horses but in modern farms are drawn by tractors. A plough may have a wooden, iron or steel frame, with a blade attached to cut and loosen the soil. It has been fundamental to farming for most of history. The earliest ploughs had no wheels; such a plough was known to the Romans as an aratrum. Celtic peoples first came to use wheeled ploughs in the Roman era. The prime purpose of ploughing is to turn over the uppermost soil, bringing fresh nutrients to the surface while burying weeds and crop remains to decay. Trenches cut by the plough are called furrows. In modern use, a ploughed field is normally left to dry and then harrowed before planting. Ploughing and cultivating soil evens the content of the upper layer of soil, where most plant feeder roots grow. Ploughs were initially powered by humans, but the use of farm animals is considerably more efficient. The earliest animals worked were oxen. Later, horses and mules were used in many areas. With the Industrial Revolution came the possibility of steam engines to pull ploughs. These in turn were superseded by internal-combustion-powered tractors in the early 20th century. Use of the traditional plough has decreased in some areas threatened by soil damage and erosion. Used instead is shallower ploughing or other less-invasive conservation tillage. Etymology In older English, as in other Germanic languages, the plough was traditionally known by other names, e.g. Old English (modern dialectal ), Old High German , , , Old Norse (Swedish ), and Gothic , all presumably referring to the ard (scratch plough). The modern word comes from the Old Norse , and is therefore Germanic, but it appears relatively late (it is not attested in Gothic) and is thought to be a loan from one of the north Italic languages. The German cognate is "pflug", the Dutch "ploeg" and the Swedish "plog". In many Slavic languages and in Romanian the word is "plug". Words with the same root appeared with related meanings: in Raetic "wheeled heavy plough" (Pliny, Nat. Hist. 18, 172), and in Latin "farm cart", "cart", and "cart box". The word must have originally referred to the wheeled heavy plough, common in Roman north-western Europe by the