Final answer:
The drastic reduction in the proportion of farmers by 1890 was primarily caused by the Industrial Revolution's introduction of efficient farming technology, the allure of urban factory jobs, and economic changes post-World War I that encouraged a significant rural-to-urban population shift.
Step-by-step explanation:
In 1790, almost 10 out of every 11 workers in the United States were farmers. By 1890, only about one worker out of every eight was a farmer. The most responsible factor for this dramatic shift from farming to non-farming occupations was the Industrial Revolution. The introduction of improved farm equipment and technology, such as larger tractors and advanced farming methods, meant that farms required fewer workers to operate. This mechanization led to a decrease in the need for manual labor on farms.
At the same time, there was significant urbanization due to the draw of factory jobs in cities. The development of steam engines and, later, commercial electricity allowed industries to concentrate in urban areas, which in turn attracted newly arrived immigrants looking for work. This coincidence of immigration, urbanization, and industrialization transformed American society from a predominantly rural one to a significantly urban one. Between 1880 and 1920, the number of industrial workers in the United States quadrupled. Despite less than 1 percent of Americans being employed in farming today, agricultural output has continued to grow due to continuous advancements in agronomy and farming technology.
The rural-to-urban shift in the population was further exacerbated by the changing economic conditions following World War I, with declining commodity prices and farm foreclosures pushing more people towards urban centers in search of employment. Thus, the cumulative effect of the technological advances in agriculture, the growth of the industrial sector, and the economic changes post-World War I accounted for the reduction of the farming workforce and the rise of the urban worker.