The first 40 years of the 20th century saw simultaneous advances in biochemistry and engineering that rapidly and profoundly changed farming. The introduction of the gasoline-powered internal combustion engine ushered in the era of the tractor and made possible hundreds of mechanized farm implements. Research in plant breeding led to the commercialization of hybrid seed. And a new manufacturing process made nitrogen fertilizer — first synthesized in the mid-19th century — affordably abundant. These factors changed the labour equation: there were almost no tractors in the US around 1910, but over 3,000,000 by 1950; in 1900, it took one farmer to feed 2.5 people, but currently the ratio is 1 to well over 100. Fields grew bigger and cropping more specialized to make more efficient use of machinery. The reduced need for manual labour and animal labour that machinery, herbicides, and fertilizers made possible created an era in which the mechanization of agriculture evolved rapidly.