Farm machines include a great variety of devices with a wide range of complexity: from simple hand-held implements used since prehistoric times to the complex harvesters of modern mechanized agriculture.
The operations of farming for which machines are used are diverse. For crop production they include handling of residues from previous crops; primary and secondary tillage of the soil; fertilizer distribution and application; seeding, planting, and transplanting; cultivation; pest control; harvesting; transportation; storage; premarketing processing; drainage; irrigation and erosion control; and water conservation. Livestock production, which not so long ago depended primarily on the pitchfork and scoop shovel, now uses many complicated and highly sophisticated machines for handling water, feed, bedding, and manure, as well as for the many special operations involved in producing milk and eggs.
In the early 19th century, animals were the chief source of power in farming. Later in the century, steam power gained in importance. During World War I gasoline- (petrol-) powered tractors became common, and diesel engines later became prevalent. In the developed countries, the number of farm workers has steadily declined in the 20th century, while farm production has increased because of the use of machinery.