Early agricultural steam engines simply provided belt power for threshing machines and similar stationary work but had to be moved from place to place by horses or oxen. During the 1850s successful self-propelled steam plowing engines were developed, and they were steadily improved over the next 50 years. These steam tractors and stationary engines required several men to haul fuel and water, fire the boiler, and operate the controls.
The first gasoline-powered tractors, whose development was stimulated by the need to reduce the size of the operation crew, closely resembled steam tractors. The advance of the gasoline tractor was slow, however, until expiration of the basic patents of the Otto engine -the common automobile engine- in 1890. Tractors using internal-combustion engines were steadily improved from that time on, with sharp reductions in weight and in the size of operating crew compared to early steam-powered units. Some early steam tractors weighed more than 1,000 pounds per horsepower (448 kg per metric hp). The average weight of contemporary agricultural tractors is about 90 to 100 pounds per horsepower (40–45 kg per metric hp).
The number of low-powered tractors has decreased steadily. In the United States, for instance, 90.8% of the tractors built in 1950 had less than 35 horsepower. By 1960 only 17% were under 35 horsepower, and this dropped to 8.4% in 1968. On the other hand, almost 33% of the tractors built in 1968 had 90 or more horsepower.
Diesel engines are capable of producing power efficiently over a wider range of loads and engine speeds than gasoline engines, and diesel fuel generally costs less than gasoline. Since the power and economy advantages of diesel engines are particularly important in larger tractors, the percentage of diesel engines has climbed rapidly as tractor size has increased. In 1955 only 12.5% of American-built tractors had diesel engines, but by 1968 more than 69% were diesel powered. Tractors burning liquid petroleum (lp) gases were introduced in the 1940s.
